CYP21A2

UniProt ID: P08686
Organism: Homo sapiens
Review Status: INITIALIZED
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Gene Description

Cytochrome P450 21A2 (P450c21, steroid 21-hydroxylase; EC 1.14.14.16) is a heme-thiolate cytochrome P450 monooxygenase anchored to the endoplasmic reticulum (microsomal) membrane, expressed predominantly in the adrenal cortex. It catalyses hydroxylation at carbon 21 of C21 steroids using molecular oxygen and electrons supplied by NADPH via cytochrome P450 reductase (POR). It acts at the branch point of adrenal corticosteroid biosynthesis, converting progesterone to 11-deoxycorticosterone (DOC) in the mineralocorticoid branch (feeding aldosterone synthesis) and 17alpha-hydroxyprogesterone to 11-deoxycortisol in the glucocorticoid branch (feeding cortisol synthesis), thereby supplying both the mineralocorticoid and glucocorticoid arms of steroidogenesis. Loss-of-function variants cause 21-hydroxylase deficiency, the commonest cause of congenital adrenal hyperplasia (>90-95% of cases), producing cortisol and aldosterone deficiency with adrenal androgen excess across a spectrum from salt-wasting and simple-virilizing classic forms to non-classic late-onset disease; deleterious alleles arise largely by gene conversion with the adjacent CYP21A1P pseudogene.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0004509 steroid 21-monooxygenase activity
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic (IBA) assignment of the core catalytic activity, steroid 21-monooxygenase (EC 1.14.14.16), the defining molecular function of CYP21A2. This is well supported by direct enzymology and structure, and represents the core molecular function of the gene.
Reason: This is the correct, appropriately specific core molecular-function term for P450c21. UniProt records it as steroid 21-hydroxylase (EC 1.14.14.16) with the two C21-hydroxylation reactions, and direct assays confirm the activity.
Supporting Evidence:
PMID:25855791
Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase, and deficiency of this enzyme is involved in ∼95% of cases of human congenital adrenal hyperplasia
file:human/CYP21A2/CYP21A2-uniprot.txt
Catalyzes the hydroxylation at C-21 of
GO:0006704 glucocorticoid biosynthetic process
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic (IBA) assignment placing CYP21A2 in glucocorticoid biosynthesis. 21-hydroxylation of 17alpha-hydroxyprogesterone yields 11-deoxycortisol, the immediate precursor of cortisol, so CYP21A2 is a committed step in the glucocorticoid branch. Core biological process.
Reason: Directly supported by the enzyme's role converting 17alpha-hydroxyprogesterone to 11-deoxycortisol in the cortisol pathway; a core process for this gene.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
the biosynthetic pathway of mineralocorticoids and glucocorticoids
PMID:16984992
the conversion of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to deoxycorticosterone
GO:0020037 heme binding
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetic (IBA) assignment of heme binding. CYP21A2 is a heme-thiolate P450; the crystal structure resolves heme b with Cys429 as the axial iron ligand. A correct, essential cofactor-binding function, secondary to the catalytic MF.
Reason: The enzyme binds heme b as an obligate cofactor (structure PMID:25855791; UniProt BINDING residues 92/121/366/427/429). Correct but a supporting molecular function rather than the defining catalytic activity.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Name=heme b
file:human/CYP21A2/CYP21A2-uniprot.txt
axial binding residue
GO:0004497 monooxygenase activity
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: InterPro2GO (IEA) mapping from cytochrome P450 domains to the generic parent term monooxygenase activity. Correct in essence but far less informative than the specific steroid 21-monooxygenase activity already annotated.
Reason: True but over-general; the specific activity GO:0004509 (steroid 21-monooxygenase) captures the actual function. Retained as a broad IEA parent, marked as over-annotated relative to the core MF.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
A cytochrome P450 monooxygenase that plays a major role in
GO:0004509 steroid 21-monooxygenase activity
IEA
GO_REF:0000120 		ACCEPT
Summary: Electronic assignment (ARBA/EC 1.14.14.16) of the core catalytic activity, identical to the IBA/experimental term. Correct and appropriately specific.
Reason: EC 1.14.14.16 maps exactly to steroid 21-monooxygenase activity, the core molecular function; independently supported experimentally.
Supporting Evidence:
PMID:22014889
More than 90% of all cases of congenital adrenal hyperplasia (CAH) result from steroid 21-hydroxylase gene (CYP21A2) mutations
GO:0005506 iron ion binding
IEA
GO_REF:0000002 		ACCEPT
Summary: InterPro2GO (IEA) mapping of iron ion binding, reflecting the catalytic heme iron coordinated by the axial cysteine thiolate. Correct supporting cofactor function.
Reason: The heme b iron is coordinated by Cys429 (axial ligand) at the catalytic centre; iron binding is a valid, if generic, supporting molecular function.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
axial binding residue
GO:0005789 endoplasmic reticulum membrane
IEA
GO_REF:0000120 		ACCEPT
Summary: Electronic (ARBA / UniProt SubCell SL-0097) assignment of ER-membrane localization. CYP21A2 is a microsomal P450, a peripheral ER-membrane protein anchored by its N-terminal hydrophobic segment. Correct core localization.
Reason: Matches the experimentally determined microsomal/ER-membrane localization; correct cellular component.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral
GO:0006705 mineralocorticoid biosynthetic process
IEA
GO_REF:0000117 		ACCEPT
Summary: Electronic (ARBA) assignment placing CYP21A2 in mineralocorticoid biosynthesis. 21-hydroxylation of progesterone yields 11-deoxycorticosterone (DOC), the precursor of the mineralocorticoid branch leading to aldosterone. Core biological process.
Reason: Directly supported by the progesterone-to-DOC reaction feeding aldosterone synthesis; a core process for this gene (also independently annotated TAS by Reactome).
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
the biosynthetic pathway of mineralocorticoids and glucocorticoids
PMID:16984992
progesterone to deoxycorticosterone
GO:0016705 oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen
IEA
GO_REF:0000002 		MARK AS OVER ANNOTATED
Summary: InterPro2GO (IEA) mapping to the broad oxidoreductase/monooxygenase parent. Chemically accurate (the enzyme inserts one oxygen atom and reduces the other to water) but far less specific than steroid 21-monooxygenase activity.
Reason: Correct high-level chemistry but over-general relative to the annotated core MF GO:0004509; retained as a broad IEA parent.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
with two electrons provided by NADPH via cytochrome P450
GO:0020037 heme binding
IEA
GO_REF:0000002 		ACCEPT
Summary: InterPro2GO (IEA) assignment of heme binding, duplicating the IBA/IDA heme-binding annotation. Correct essential cofactor function.
Reason: Consistent with the P450 heme-thiolate architecture and the resolved heme b in the structure; correct supporting molecular function.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Name=heme b
GO:0034308 primary alcohol metabolic process
IEA
GO_REF:0000117 		MARK AS OVER ANNOTATED
Summary: ARBA (IEA) process term derived from the chemical class of the products (21-hydroxy steroids bear a primary alcohol). This is a chemistry-generic over-annotation that does not describe the biological role of CYP21A2.
Reason: The product hydroxyl is a primary alcohol, but the biologically meaningful process is corticosteroid biosynthesis, already captured by the specific glucocorticoid/mineralocorticoid/cortisol terms. This substrate-chemistry parent adds no functional insight.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Catalyzes the hydroxylation at C-21 of
GO:0042180 ketone metabolic process
IEA
GO_REF:0000117 		MARK AS OVER ANNOTATED
Summary: ARBA (IEA) process term derived from the ketone-containing steroid substrates/products. A chemistry-generic over-annotation that does not describe the enzyme's biological role.
Reason: The steroid substrates carry ketone groups, but the biologically relevant process is corticosteroid biosynthesis; this generic parent adds no functional insight.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Catalyzes the hydroxylation at C-21 of
GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity
IEA
GO_REF:0000116 		ACCEPT
Summary: Rhea-based (IEA) assignment of the specific reaction 17alpha-hydroxyprogesterone + O2 -> 11-deoxycortisol (RHEA:50308), the glucocorticoid-branch activity of CYP21A2. Correct and appropriately specific; experimentally confirmed.
Reason: This is one of the two physiological reactions catalysed by P450c21 and is directly demonstrated by enzyme assays; a core specific molecular function.
Supporting Evidence:
PMID:16984992
the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
GO:0106309 progesterone 21-hydroxylase activity
IEA
GO_REF:0000116 		ACCEPT
Summary: Rhea-based (IEA) assignment of the specific reaction progesterone + O2 -> 11-deoxycorticosterone (RHEA:50304), the mineralocorticoid-branch activity of CYP21A2. Correct and appropriately specific; experimentally confirmed.
Reason: This is one of the two physiological reactions catalysed by P450c21 and is directly demonstrated by enzyme assays; a core specific molecular function.
Supporting Evidence:
PMID:16984992
progesterone to deoxycorticosterone
GO:0120254 olefinic compound metabolic process
IEA
GO_REF:0000117 		MARK AS OVER ANNOTATED
Summary: ARBA (IEA) process term derived from the C=C (olefinic) double bond present in the steroid substrates. A chemistry-generic over-annotation unrelated to the enzyme's biological role.
Reason: The steroid substrates contain a carbon-carbon double bond, but CYP21A2's biological role is corticosteroid biosynthesis, not olefin metabolism; this parent adds no functional insight.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Catalyzes the hydroxylation at C-21 of
GO:0005789 endoplasmic reticulum membrane
IDA
PMID:10198222
Effects of missense mutations and deletions on membrane anch... 		ACCEPT
Summary: Direct experimental (IDA) localization to the ER/microsomal membrane. Lajic et al. studied membrane binding of P450c21 and its N-terminal anchoring segments, showing the protein integrates into microsomal membranes via its first hydrophobic domain. Core cellular location.
Reason: Experimentally supports the microsomal/ER-membrane localization where the enzyme is active; consistent with UniProt subcellular location. Core component.
Supporting Evidence:
PMID:10198222
P450c21 spans the membrane through its first hydrophobic domain only
file:human/CYP21A2/CYP21A2-uniprot.txt
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral
GO:0006704 glucocorticoid biosynthetic process
TAS
Reactome:R-HSA-194002 		ACCEPT
Summary: Reactome (TAS) placement of CYP21A2 in glucocorticoid biosynthesis, consistent with the IBA annotation and the enzyme's role generating 11-deoxycortisol en route to cortisol. Core biological process.
Reason: Authoritative pathway (Reactome) annotation matching the demonstrated function; core process.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
the biosynthetic pathway of mineralocorticoids and glucocorticoids
GO:0006705 mineralocorticoid biosynthetic process
TAS
Reactome:R-HSA-193993 		ACCEPT
Summary: Reactome (TAS) placement of CYP21A2 in mineralocorticoid biosynthesis, consistent with the ARBA IEA and the progesterone-to-DOC reaction feeding aldosterone. Core biological process.
Reason: Authoritative pathway (Reactome) annotation matching the demonstrated function; core process.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
the biosynthetic pathway of mineralocorticoids and glucocorticoids
GO:0016125 sterol metabolic process
TAS
Reactome:R-HSA-211976 		KEEP AS NON CORE
Summary: Reactome (TAS) assignment to the broad sterol metabolic process ("Endogenous sterols"). Steroids are derived from sterols, but this is a broad parent of the specific corticosteroid-biosynthesis processes; not the core term.
Reason: Correct at a general level (corticosteroids derive from the sterol cholesterol), but the specific glucocorticoid/mineralocorticoid/cortisol biosynthesis terms are the informative core processes; retain as broad non-core context.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
adrenal steroidogenesis
GO:0004509 steroid 21-monooxygenase activity
TAS
Reactome:R-HSA-5601976 		ACCEPT
Summary: Reactome (TAS) assignment of the core catalytic activity, in the context of defective CYP21A2 failing to 21-hydroxylate progesterone. Correct core molecular function.
Reason: Same appropriately specific core MF as the IBA/EC/experimental annotations.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Catalyzes the hydroxylation at C-21 of
GO:0106309 progesterone 21-hydroxylase activity
TAS
Reactome:R-HSA-193964 		ACCEPT
Summary: Reactome (TAS) assignment of the specific progesterone 21-hydroxylase reaction (CYP21A2 21-hydroxylates PROG). Correct and appropriately specific.
Reason: Matches the demonstrated progesterone-to-DOC reaction; core specific MF.
Supporting Evidence:
PMID:16984992
progesterone to deoxycorticosterone
GO:0106309 progesterone 21-hydroxylase activity
TAS
Reactome:R-HSA-193981 		ACCEPT
Summary: Reactome (TAS) assignment of progesterone 21-hydroxylase activity (duplicate of the reaction annotation via a different Reactome event). Correct and appropriately specific.
Reason: Same core specific MF as the other progesterone 21-hydroxylase annotations; experimentally supported.
Supporting Evidence:
PMID:16984992
progesterone to deoxycorticosterone
GO:0005789 endoplasmic reticulum membrane
TAS
Reactome:R-HSA-5601976 		ACCEPT
Summary: Reactome (TAS) ER-membrane localization, consistent with the IDA and IEA localizations. Correct core cellular component.
Reason: Matches the experimentally determined microsomal/ER-membrane localization.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral
GO:0034651 cortisol biosynthetic process
IDA
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase... 		ACCEPT
Summary: Direct experimental (IDA) assignment to cortisol biosynthesis, based on the structural/enzymatic characterization of P450c21 as the major steroid 21-hydroxylase producing 11-deoxycortisol, the cortisol precursor. Specific, core biological process.
Reason: The 17alpha-hydroxyprogesterone-to-11-deoxycortisol reaction is the committed 21-hydroxylation step of the cortisol pathway; this is the most specific and informative BP term and is directly supported.
Supporting Evidence:
PMID:25855791
Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
PMID:16984992
the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity
EXP
PMID:16984992
Four novel missense mutations in the CYP21A2 gene detected i... 		ACCEPT
Summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase activity. Grischuk et al. measured conversion of 17-hydroxyprogesterone to 11-deoxycortisol for wild-type and mutant CYP21A2 in vitro. Core specific molecular function.
Reason: Directly measured physiological reaction of P450c21; appropriately specific core MF.
Supporting Evidence:
PMID:16984992
the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity
EXP
PMID:22014889
p.H282N and p.Y191H: 2 novel CYP21A2 mutations in Italian co... 		ACCEPT
Summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase activity. Concolino et al. determined residual enzyme activity and apparent kinetic values of CYP21A2 variants toward 17-hydroxyprogesterone. Core specific molecular function.
Reason: Directly measured physiological reaction; appropriately specific core MF.
Supporting Evidence:
PMID:22014889
Functional in vitro assay for mutagenized CYP21A2 enzymes was performed in transiently transfected mammalian cells to test the residual enzyme activity and the apparent kinetic values
GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity
EXP
PMID:27721825
Functional and Structural Consequences of Nine CYP21A2 Mutat... 		ACCEPT
Summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase activity. de Paula Michelatto et al. expressed wild-type and mutant CYP21A2 in COS-1 cells and measured enzyme activities toward 17-hydroxyprogesterone and progesterone. Core specific molecular function.
Reason: Directly measured physiological reaction; appropriately specific core MF.
Supporting Evidence:
PMID:27721825
enzyme activities towards 17-hydroxyprogesterone and progesterone were determined
GO:0106309 progesterone 21-hydroxylase activity
EXP
PMID:16984992
Four novel missense mutations in the CYP21A2 gene detected i... 		ACCEPT
Summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity. Grischuk et al. measured conversion of progesterone to deoxycorticosterone for wild-type and mutant CYP21A2 in vitro. Core specific molecular function.
Reason: Directly measured physiological reaction of P450c21; appropriately specific core MF.
Supporting Evidence:
PMID:16984992
progesterone to deoxycorticosterone
GO:0106309 progesterone 21-hydroxylase activity
EXP
PMID:22014889
p.H282N and p.Y191H: 2 novel CYP21A2 mutations in Italian co... 		ACCEPT
Summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity. Concolino et al. assayed residual activity and kinetics of CYP21A2 variants, which include the progesterone 21-hydroxylation reaction. Core specific molecular function.
Reason: Directly measured physiological reaction; appropriately specific core MF.
Supporting Evidence:
PMID:22014889
Functional in vitro assay for mutagenized CYP21A2 enzymes was performed in transiently transfected mammalian cells to test the residual enzyme activity and the apparent kinetic values
GO:0106309 progesterone 21-hydroxylase activity
EXP
PMID:27721825
Functional and Structural Consequences of Nine CYP21A2 Mutat... 		ACCEPT
Summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity. de Paula Michelatto et al. measured CYP21A2 enzyme activities toward progesterone (and 17-hydroxyprogesterone) for wild-type and mutant proteins in COS-1 cells. Core specific molecular function.
Reason: Directly measured physiological reaction; appropriately specific core MF.
Supporting Evidence:
PMID:27721825
enzyme activities towards 17-hydroxyprogesterone and progesterone were determined
GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity
IDA
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase... 		ACCEPT
Summary: Direct experimental (IDA) assignment of 17-hydroxyprogesterone 21-hydroxylase activity from the structural/kinetic study of the human P450 21A2 enzyme, which characterized 21-hydroxylation with 17alpha-hydroxyprogesterone as a substrate. Core specific molecular function.
Reason: Directly demonstrated physiological reaction; appropriately specific core MF.
Supporting Evidence:
PMID:25855791
Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
GO:0106309 progesterone 21-hydroxylase activity
IDA
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase... 		ACCEPT
Summary: Direct experimental (IDA) assignment of progesterone 21-hydroxylase activity. Pallan et al. solved the human P450 21A2-progesterone complex and characterized 21-hydroxylation of progesterone (kinetics, rate-limiting C-H cleavage). Core specific molecular function.
Reason: Directly demonstrated physiological reaction with the enzyme-progesterone structure and kinetics; appropriately specific core MF.
Supporting Evidence:
PMID:25855791
progesterone, a substrate in adrenal 21-hydroxylation
GO:0020037 heme binding
IDA
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase... 		ACCEPT
Summary: Direct experimental (IDA) assignment of heme binding. The crystal structure of human P450 21A2 (Pallan et al.) resolves the heme b prosthetic group with the axial cysteine iron ligand, directly demonstrating heme binding. Correct essential cofactor function, secondary to the catalytic MF.
Reason: The enzyme binds heme b as an obligate cofactor, resolved in the structure; correct supporting molecular function.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
Name=heme b
file:human/CYP21A2/CYP21A2-uniprot.txt
axial binding residue
GO:0006694 steroid biosynthetic process
IDA
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase... 		KEEP AS NON CORE
Summary: Direct experimental (IDA) assignment to the broad steroid biosynthetic process. Correct but a general parent of the specific corticosteroid-biosynthesis processes that better describe CYP21A2's role.
Reason: Accurate but less informative than the specific glucocorticoid/mineralocorticoid/cortisol biosynthesis terms; retain as a broad non-core process.
Supporting Evidence:
PMID:25855791
Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
GO:0008202 steroid metabolic process
IMP
PMID:16984992
Four novel missense mutations in the CYP21A2 gene detected i... 		KEEP AS NON CORE
Summary: Mutant-phenotype (IMP) assignment to the broad steroid metabolic process, based on loss of 21-hydroxylation activity in CYP21A2 disease variants. Correct but a general parent of the specific corticosteroid-biosynthesis processes.
Reason: Accurate at a general level; the specific corticosteroid biosynthesis terms are the informative core processes. Retain as broad non-core.
Supporting Evidence:
PMID:16984992
almost absent or negligible CYP21 activity for the conversion of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to deoxycorticosterone
GO:0008395 steroid hydroxylase activity
IMP
PMID:16984992
Four novel missense mutations in the CYP21A2 gene detected i... 		MODIFY
Summary: Mutant-phenotype (IMP) assignment of the generic steroid hydroxylase activity, inferred from loss of 21-hydroxylation in disease variants. Correct but less specific than steroid 21-monooxygenase activity (GO:0004509).
Reason: The essence is correct, but the specific catalytic term steroid 21-monooxygenase activity (GO:0004509) precisely captures the demonstrated function; replace the generic parent with the specific term.
Supporting Evidence:
PMID:16984992
almost absent or negligible CYP21 activity for the conversion of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to deoxycorticosterone
GO:0005789 endoplasmic reticulum membrane
TAS
Reactome:R-HSA-193964 		ACCEPT
Summary: Reactome (TAS) ER-membrane localization (progesterone 21-hydroxylation event), consistent with the IDA and other localizations. Correct core cellular component.
Reason: Matches the experimentally determined microsomal/ER-membrane localization.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral
GO:0005789 endoplasmic reticulum membrane
TAS
Reactome:R-HSA-193981 		ACCEPT
Summary: Reactome (TAS) ER-membrane localization (17-hydroxyprogesterone oxidation event), consistent with the IDA and other localizations. Correct core cellular component.
Reason: Matches the experimentally determined microsomal/ER-membrane localization.
Supporting Evidence:
file:human/CYP21A2/CYP21A2-uniprot.txt
SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral
GO:0008395 steroid hydroxylase activity
TAS
PMID:1406709
R339H and P453S: CYP21 mutations associated with nonclassic ... 		MODIFY
Summary: Traceable-author (TAS) assignment of the generic steroid hydroxylase activity from an early CYP21 mutation study. Correct but less specific than steroid 21-monooxygenase activity (GO:0004509).
Reason: The essence is correct, but the specific catalytic term steroid 21-monooxygenase activity (GO:0004509) captures the actual demonstrated function; replace the generic parent with the specific term.
Supporting Evidence:
PMID:1406709
R339H and P453S CYP21 mutations associated with nonclassic steroid 21-hydroxylase deficiency

Core Functions

Steroid 21-monooxygenase (P450c21) that 21-hydroxylates C21 steroids in adrenal corticosteroid biosynthesis, converting progesterone to 11-deoxycorticosterone and 17alpha-hydroxyprogesterone to 11-deoxycortisol, using heme b and electrons from cytochrome P450 reductase, at the endoplasmic reticulum membrane.

Molecular Function:
steroid 21-monooxygenase activity
Directly Involved In:
glucocorticoid biosynthetic process
Cellular Locations:
endoplasmic reticulum membrane
Supporting Evidence:
  • PMID:25855791
    Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
  • file:human/CYP21A2/CYP21A2-uniprot.txt
    Catalyzes the hydroxylation at C-21 of

Same 21-hydroxylase activity supplying the mineralocorticoid branch of steroidogenesis, converting progesterone to 11-deoxycorticosterone (DOC), the precursor of aldosterone.

Molecular Function:
steroid 21-monooxygenase activity
Directly Involved In:
mineralocorticoid biosynthetic process
Cellular Locations:
endoplasmic reticulum membrane
Supporting Evidence:
  • file:human/CYP21A2/CYP21A2-uniprot.txt
    the biosynthetic pathway of mineralocorticoids and glucocorticoids
  • PMID:16984992
    progesterone to deoxycorticosterone

Heme-thiolate cofactor binding required for catalysis; the enzyme binds heme b with an axial cysteine iron ligand at the catalytic centre.

Molecular Function:
heme binding
Cellular Locations:
endoplasmic reticulum membrane
Supporting Evidence:
  • file:human/CYP21A2/CYP21A2-uniprot.txt
    Name=heme b
  • file:human/CYP21A2/CYP21A2-uniprot.txt
    axial binding residue

References

GO_REF:0000002
Gene Ontology annotation through association of InterPro records with GO terms
GO_REF:0000033
Annotation inferences using phylogenetic trees
GO_REF:0000116
Automatic Gene Ontology annotation based on Rhea mapping
GO_REF:0000117
Electronic Gene Ontology annotations created by ARBA machine learning models
GO_REF:0000120
Combined Automated Annotation using Multiple IEA Methods
file:human/CYP21A2/CYP21A2-uniprot.txt
UniProtKB entry P08686 (CP21A_HUMAN), Steroid 21-hydroxylase
PMID:10198222
Effects of missense mutations and deletions on membrane anchoring and enzyme function of human steroid 21-hydroxylase (P450c21).
PMID:1406709
R339H and P453S: CYP21 mutations associated with nonclassic steroid 21-hydroxylase deficiency that are not apparent gene conversions.
PMID:16984992
Four novel missense mutations in the CYP21A2 gene detected in Russian patients suffering from the classical form of congenital adrenal hyperplasia: identification, functional characterization, and structural analysis.
PMID:22014889
p.H282N and p.Y191H: 2 novel CYP21A2 mutations in Italian congenital adrenal hyperplasia patients.
PMID:25855791
Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase: STRUCTURE OF THE ENZYME·PROGESTERONE SUBSTRATE COMPLEX AND RATE-LIMITING C-H BOND CLEAVAGE.
PMID:27721825
Functional and Structural Consequences of Nine CYP21A2 Mutations Ranging from Very Mild to Severe Effects.
Reactome:R-HSA-193964
CYP21A2 21-hydroxylates PROG
Reactome:R-HSA-193981
CYP21A2 oxidises 17HPROG
Reactome:R-HSA-193993
Mineralocorticoid biosynthesis
Reactome:R-HSA-194002
Glucocorticoid biosynthesis
Reactome:R-HSA-211976
Endogenous sterols
Reactome:R-HSA-5601976
Defective CYP21A2 does not 21-hydroxylate PROG

📚 Additional Documentation

Notes

(CYP21A2-notes.md)

CYP21A2 (Steroid 21-hydroxylase, P450c21) — review notes

UniProtKB: P08686 (CP21A_HUMAN); gene CYP21A2 (syn. CYP21, CYP21B); HGNC:2600; EC 1.14.14.16.

Core biology (from UniProt P08686 + cached publications)

  • Molecular function. A microsomal (ER-membrane) cytochrome P450 monooxygenase.
    Heme-thiolate enzyme; heme b cofactor with Cys429 as axial (proximal) iron ligand
    (BINDING 92/121/366/427; 429 = "axial binding residue" for Fe). Electrons delivered
    by NADPH via cytochrome P450 reductase (POR/CPR).
  • Reactions (both C21 hydroxylations):
  • progesterone + NADPH-hemoprotein reductase(red) + O2 -> 21-hydroxyprogesterone
    (= 11-deoxycorticosterone, DOC) + reductase(ox) + H2O + H+ (RHEA:50304; GO:0106309
    progesterone 21-hydroxylase activity) — mineralocorticoid branch.
  • 17alpha-hydroxyprogesterone + reductase(red) + O2 -> 11-deoxycortisol + reductase(ox)
    • H2O + H+ (RHEA:50308; GO:0103069 17-hydroxyprogesterone 21-hydroxylase activity)
      — glucocorticoid branch.
      These two specific MFs are subsumed by GO:0004509 steroid 21-monooxygenase activity
      (EC 1.14.14.16), which is the appropriate CORE MF term the GOA carries.
  • Location. ER membrane / microsome membrane; peripheral membrane protein,
    anchored by the N-terminal leucine-rich hydrophobic segment (spans the membrane through
    its first hydrophobic domain only, per PMID:10198222). GO:0005789 ER membrane.
  • Biological process. Adrenal steroidogenesis: supplies BOTH the mineralocorticoid
    (aldosterone via DOC) and glucocorticoid (cortisol via 11-deoxycortisol) branches of
    corticosteroid biosynthesis. GO:0006704 glucocorticoid biosynthetic process,
    GO:0006705 mineralocorticoid biosynthetic process, GO:0034651 cortisol biosynthetic
    process.
  • Disease. Deficiency (AH3, MIM:201910) is the cause of ~95% of congenital adrenal
    hyperplasia (21-hydroxylase deficiency), spectrum salt-wasting -> simple virilizing ->
    non-classic; cortisol/aldosterone deficiency with androgen excess. Deleterious alleles
    arise mostly by gene conversion with the pseudogene CYP21A1P.

Key supporting quotes (verbatim, verified against cached files)

  • UniProt FUNCTION: "Catalyzes the hydroxylation at C-21 of progesterone and
    17alpha-hydroxyprogesterone to respectively form 11-deoxycorticosterone and
    11-deoxycortisol, intermediate metabolites in the biosynthetic pathway of
    mineralocorticoids and glucocorticoids" and "two electrons provided by NADPH via
    cytochrome P450 reductase". [file:human/CYP21A2/CYP21A2-uniprot.txt]
  • UniProt SUBCELLULAR LOCATION: "Endoplasmic reticulum membrane; Peripheral membrane
    protein ... Microsome membrane". COFACTOR "Name=heme b". [file: uniprot]
  • PMID:25855791 (abstract): "Cytochrome P450 (P450) 21A2 is the major steroid
    21-hydroxylase, and deficiency of this enzyme is involved in ∼95% of cases of human
    congenital adrenal hyperplasia". Structure with progesterone; "progesterone, a substrate
    in adrenal 21-hydroxylation."
  • PMID:16984992 (abstract): mutations gave "almost absent or negligible CYP21 activity for
    the conversion of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to
    deoxycorticosterone." Localization not affected by mutants (immunofluorescence).
  • PMID:22014889 (abstract): "More than 90% of all cases of congenital adrenal hyperplasia
    (CAH) result from steroid 21-hydroxylase gene (CYP21A2) mutations"; residual activity +
    kinetic values measured for both substrates.
  • PMID:27721825 (full text): assayed "enzyme activities towards 17-hydroxyprogesterone and
    progesterone"; "The most common cause of CAH is 21-hydroxylase deficiency (21OHD)".
  • PMID:10198222 (abstract): membrane anchoring study; "P450c21 spans the membrane through
    its first hydrophobic domain only"; N-terminal hydrophobic segment for membrane
    integration. Supports ER/microsome membrane peripheral localization (is_active_in).
  • PMID:1406709 (abstract): "R339H and P453S: CYP21 mutations associated with nonclassic
    steroid 21-hydroxylase deficiency" — TAS for steroid hydroxylase activity.

Annotation-review reasoning

  • CORE MF = GO:0004509 steroid 21-monooxygenase activity (IBA + IEA/EC + TAS). The two
    Rhea/EXP substrate-specific terms (GO:0106309, GO:0103069) are correct, specific, and
    directly experimentally supported (PMID:16984992/22014889/27721825/25855791) — ACCEPT.
  • CORE BP = corticosteroid biosynthesis, split as glucocorticoid (GO:0006704), cortisol
    (GO:0034651), and mineralocorticoid (GO:0006705) biosynthetic processes — all directly
    supported by function; ACCEPT the specific ones.
  • Heme binding GO:0020037 (IDA PMID:25855791, IBA) ACCEPT (secondary, structural). Iron
    ion binding GO:0005506 IEA ACCEPT (heme-Fe). Monooxygenase GO:0004497 and oxidoreductase
    GO:0016705 IEA are correct but too general vs GO:0004509 -> MARK_AS_OVER_ANNOTATED.
  • Steroid hydroxylase activity GO:0008395 (IMP/TAS) correct but less specific than
    GO:0004509 -> MODIFY to GO:0004509.
  • Broad BP IEAs from ARBA: primary alcohol metabolic (GO:0034308), ketone metabolic
    (GO:0042180), olefinic compound metabolic (GO:0120254) — chemistry-generic
    over-annotations from substrate/product chemical class -> MARK_AS_OVER_ANNOTATED.
    Steroid metabolic (GO:0008202 IMP) / steroid biosynthetic (GO:0006694 IDA) / sterol
    metabolic (GO:0016125 TAS) are correct but broad parents of the specific corticosteroid
    terms -> KEEP_AS_NON_CORE.
  • ER membrane GO:0005789 (IDA PMID:10198222 is_active_in; IEA; several Reactome TAS) —
    ACCEPT the experimental/IDA as core location; ACCEPT the others.

No falcon deep-research file (provider out of credits, HTTP 402); grounded in UniProt +
GOA + cached PMIDs.

📄 View Raw YAML

 
id: P08686
gene_symbol: CYP21A2
product_type: PROTEIN
status: INITIALIZED
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: Cytochrome P450 21A2 (P450c21, steroid 21-hydroxylase; EC 1.14.14.16)
  is a heme-thiolate cytochrome P450 monooxygenase anchored to the endoplasmic reticulum
  (microsomal) membrane, expressed predominantly in the adrenal cortex. It catalyses
  hydroxylation at carbon 21 of C21 steroids using molecular oxygen and electrons
  supplied by NADPH via cytochrome P450 reductase (POR). It acts at the branch point
  of adrenal corticosteroid biosynthesis, converting progesterone to 11-deoxycorticosterone
  (DOC) in the mineralocorticoid branch (feeding aldosterone synthesis) and 17alpha-hydroxyprogesterone
  to 11-deoxycortisol in the glucocorticoid branch (feeding cortisol synthesis), thereby
  supplying both the mineralocorticoid and glucocorticoid arms of steroidogenesis.
  Loss-of-function variants cause 21-hydroxylase deficiency, the commonest cause of
  congenital adrenal hyperplasia (>90-95% of cases), producing cortisol and aldosterone
  deficiency with adrenal androgen excess across a spectrum from salt-wasting and simple-virilizing
  classic forms to non-classic late-onset disease; deleterious alleles arise largely
  by gene conversion with the adjacent CYP21A1P pseudogene.
alternative_products:
- name: '1'
  id: P08686-1
- name: '2'
  id: P08686-2
  sequence_note: VSP_062040
existing_annotations:
- term:
    id: GO:0004509
    label: steroid 21-monooxygenase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Phylogenetic (IBA) assignment of the core catalytic activity, steroid
      21-monooxygenase (EC 1.14.14.16), the defining molecular function of CYP21A2.
      This is well supported by direct enzymology and structure, and represents the
      core molecular function of the gene.
    action: ACCEPT
    reason: This is the correct, appropriately specific core molecular-function term
      for P450c21. UniProt records it as steroid 21-hydroxylase (EC 1.14.14.16) with
      the two C21-hydroxylation reactions, and direct assays confirm the activity.
    supported_by:
    - reference_id: PMID:25855791
      supporting_text: Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase,
        and deficiency of this enzyme is involved in ∼95% of cases of human congenital
        adrenal hyperplasia
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Catalyzes the hydroxylation at C-21 of
- term:
    id: GO:0006704
    label: glucocorticoid biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: Phylogenetic (IBA) assignment placing CYP21A2 in glucocorticoid biosynthesis.
      21-hydroxylation of 17alpha-hydroxyprogesterone yields 11-deoxycortisol, the
      immediate precursor of cortisol, so CYP21A2 is a committed step in the glucocorticoid
      branch. Core biological process.
    action: ACCEPT
    reason: Directly supported by the enzyme's role converting 17alpha-hydroxyprogesterone
      to 11-deoxycortisol in the cortisol pathway; a core process for this gene.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: the biosynthetic pathway of mineralocorticoids and glucocorticoids
    - reference_id: PMID:16984992
      supporting_text: the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
        and progesterone to deoxycorticosterone
- term:
    id: GO:0020037
    label: heme binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Phylogenetic (IBA) assignment of heme binding. CYP21A2 is a heme-thiolate
      P450; the crystal structure resolves heme b with Cys429 as the axial iron ligand.
      A correct, essential cofactor-binding function, secondary to the catalytic MF.
    action: ACCEPT
    reason: The enzyme binds heme b as an obligate cofactor (structure PMID:25855791;
      UniProt BINDING residues 92/121/366/427/429). Correct but a supporting molecular
      function rather than the defining catalytic activity.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Name=heme b
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: axial binding residue
- term:
    id: GO:0004497
    label: monooxygenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO (IEA) mapping from cytochrome P450 domains to the generic
      parent term monooxygenase activity. Correct in essence but far less informative
      than the specific steroid 21-monooxygenase activity already annotated.
    action: MARK_AS_OVER_ANNOTATED
    reason: True but over-general; the specific activity GO:0004509 (steroid 21-monooxygenase)
      captures the actual function. Retained as a broad IEA parent, marked as over-annotated
      relative to the core MF.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: A cytochrome P450 monooxygenase that plays a major role in
- term:
    id: GO:0004509
    label: steroid 21-monooxygenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Electronic assignment (ARBA/EC 1.14.14.16) of the core catalytic activity,
      identical to the IBA/experimental term. Correct and appropriately specific.
    action: ACCEPT
    reason: EC 1.14.14.16 maps exactly to steroid 21-monooxygenase activity, the core
      molecular function; independently supported experimentally.
    supported_by:
    - reference_id: PMID:22014889
      supporting_text: More than 90% of all cases of congenital adrenal hyperplasia
        (CAH) result from steroid 21-hydroxylase gene (CYP21A2) mutations
- term:
    id: GO:0005506
    label: iron ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO (IEA) mapping of iron ion binding, reflecting the catalytic
      heme iron coordinated by the axial cysteine thiolate. Correct supporting cofactor
      function.
    action: ACCEPT
    reason: The heme b iron is coordinated by Cys429 (axial ligand) at the catalytic
      centre; iron binding is a valid, if generic, supporting molecular function.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: axial binding residue
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: Electronic (ARBA / UniProt SubCell SL-0097) assignment of ER-membrane
      localization. CYP21A2 is a microsomal P450, a peripheral ER-membrane protein
      anchored by its N-terminal hydrophobic segment. Correct core localization.
    action: ACCEPT
    reason: Matches the experimentally determined microsomal/ER-membrane localization;
      correct cellular component.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral'
- term:
    id: GO:0006705
    label: mineralocorticoid biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: Electronic (ARBA) assignment placing CYP21A2 in mineralocorticoid biosynthesis.
      21-hydroxylation of progesterone yields 11-deoxycorticosterone (DOC), the precursor
      of the mineralocorticoid branch leading to aldosterone. Core biological process.
    action: ACCEPT
    reason: Directly supported by the progesterone-to-DOC reaction feeding aldosterone
      synthesis; a core process for this gene (also independently annotated TAS by
      Reactome).
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: the biosynthetic pathway of mineralocorticoids and glucocorticoids
    - reference_id: PMID:16984992
      supporting_text: progesterone to deoxycorticosterone
- term:
    id: GO:0016705
    label: oxidoreductase activity, acting on paired donors, with incorporation or
      reduction of molecular oxygen
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO (IEA) mapping to the broad oxidoreductase/monooxygenase parent.
      Chemically accurate (the enzyme inserts one oxygen atom and reduces the other
      to water) but far less specific than steroid 21-monooxygenase activity.
    action: MARK_AS_OVER_ANNOTATED
    reason: Correct high-level chemistry but over-general relative to the annotated
      core MF GO:0004509; retained as a broad IEA parent.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: with two electrons provided by NADPH via cytochrome P450
- term:
    id: GO:0020037
    label: heme binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro2GO (IEA) assignment of heme binding, duplicating the IBA/IDA
      heme-binding annotation. Correct essential cofactor function.
    action: ACCEPT
    reason: Consistent with the P450 heme-thiolate architecture and the resolved heme
      b in the structure; correct supporting molecular function.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Name=heme b
- term:
    id: GO:0034308
    label: primary alcohol metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: ARBA (IEA) process term derived from the chemical class of the products
      (21-hydroxy steroids bear a primary alcohol). This is a chemistry-generic over-annotation
      that does not describe the biological role of CYP21A2.
    action: MARK_AS_OVER_ANNOTATED
    reason: The product hydroxyl is a primary alcohol, but the biologically meaningful
      process is corticosteroid biosynthesis, already captured by the specific glucocorticoid/mineralocorticoid/cortisol
      terms. This substrate-chemistry parent adds no functional insight.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Catalyzes the hydroxylation at C-21 of
- term:
    id: GO:0042180
    label: ketone metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: ARBA (IEA) process term derived from the ketone-containing steroid substrates/products.
      A chemistry-generic over-annotation that does not describe the enzyme's biological
      role.
    action: MARK_AS_OVER_ANNOTATED
    reason: The steroid substrates carry ketone groups, but the biologically relevant
      process is corticosteroid biosynthesis; this generic parent adds no functional
      insight.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Catalyzes the hydroxylation at C-21 of
- term:
    id: GO:0103069
    label: 17-hydroxyprogesterone 21-hydroxylase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000116
  qualifier: enables
  review:
    summary: Rhea-based (IEA) assignment of the specific reaction 17alpha-hydroxyprogesterone
      + O2 -> 11-deoxycortisol (RHEA:50308), the glucocorticoid-branch activity of
      CYP21A2. Correct and appropriately specific; experimentally confirmed.
    action: ACCEPT
    reason: This is one of the two physiological reactions catalysed by P450c21 and
      is directly demonstrated by enzyme assays; a core specific molecular function.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000116
  qualifier: enables
  review:
    summary: Rhea-based (IEA) assignment of the specific reaction progesterone + O2
      -> 11-deoxycorticosterone (RHEA:50304), the mineralocorticoid-branch activity
      of CYP21A2. Correct and appropriately specific; experimentally confirmed.
    action: ACCEPT
    reason: This is one of the two physiological reactions catalysed by P450c21 and
      is directly demonstrated by enzyme assays; a core specific molecular function.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: progesterone to deoxycorticosterone
- term:
    id: GO:0120254
    label: olefinic compound metabolic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: ARBA (IEA) process term derived from the C=C (olefinic) double bond present
      in the steroid substrates. A chemistry-generic over-annotation unrelated to
      the enzyme's biological role.
    action: MARK_AS_OVER_ANNOTATED
    reason: The steroid substrates contain a carbon-carbon double bond, but CYP21A2's
      biological role is corticosteroid biosynthesis, not olefin metabolism; this
      parent adds no functional insight.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Catalyzes the hydroxylation at C-21 of
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: IDA
  original_reference_id: PMID:10198222
  qualifier: is_active_in
  review:
    summary: Direct experimental (IDA) localization to the ER/microsomal membrane.
      Lajic et al. studied membrane binding of P450c21 and its N-terminal anchoring
      segments, showing the protein integrates into microsomal membranes via its first
      hydrophobic domain. Core cellular location.
    action: ACCEPT
    reason: Experimentally supports the microsomal/ER-membrane localization where the
      enzyme is active; consistent with UniProt subcellular location. Core component.
    supported_by:
    - reference_id: PMID:10198222
      supporting_text: P450c21 spans the membrane through its first hydrophobic domain
        only
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral'
- term:
    id: GO:0006704
    label: glucocorticoid biosynthetic process
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-194002
  qualifier: involved_in
  review:
    summary: Reactome (TAS) placement of CYP21A2 in glucocorticoid biosynthesis, consistent
      with the IBA annotation and the enzyme's role generating 11-deoxycortisol en
      route to cortisol. Core biological process.
    action: ACCEPT
    reason: Authoritative pathway (Reactome) annotation matching the demonstrated function;
      core process.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: the biosynthetic pathway of mineralocorticoids and glucocorticoids
- term:
    id: GO:0006705
    label: mineralocorticoid biosynthetic process
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-193993
  qualifier: involved_in
  review:
    summary: Reactome (TAS) placement of CYP21A2 in mineralocorticoid biosynthesis,
      consistent with the ARBA IEA and the progesterone-to-DOC reaction feeding aldosterone.
      Core biological process.
    action: ACCEPT
    reason: Authoritative pathway (Reactome) annotation matching the demonstrated function;
      core process.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: the biosynthetic pathway of mineralocorticoids and glucocorticoids
- term:
    id: GO:0016125
    label: sterol metabolic process
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-211976
  qualifier: involved_in
  review:
    summary: Reactome (TAS) assignment to the broad sterol metabolic process ("Endogenous
      sterols"). Steroids are derived from sterols, but this is a broad parent of
      the specific corticosteroid-biosynthesis processes; not the core term.
    action: KEEP_AS_NON_CORE
    reason: Correct at a general level (corticosteroids derive from the sterol cholesterol),
      but the specific glucocorticoid/mineralocorticoid/cortisol biosynthesis terms
      are the informative core processes; retain as broad non-core context.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: adrenal steroidogenesis
- term:
    id: GO:0004509
    label: steroid 21-monooxygenase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5601976
  qualifier: enables
  review:
    summary: Reactome (TAS) assignment of the core catalytic activity, in the context
      of defective CYP21A2 failing to 21-hydroxylate progesterone. Correct core molecular
      function.
    action: ACCEPT
    reason: Same appropriately specific core MF as the IBA/EC/experimental annotations.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Catalyzes the hydroxylation at C-21 of
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-193964
  qualifier: enables
  review:
    summary: Reactome (TAS) assignment of the specific progesterone 21-hydroxylase
      reaction (CYP21A2 21-hydroxylates PROG). Correct and appropriately specific.
    action: ACCEPT
    reason: Matches the demonstrated progesterone-to-DOC reaction; core specific MF.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: progesterone to deoxycorticosterone
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-193981
  qualifier: enables
  review:
    summary: Reactome (TAS) assignment of progesterone 21-hydroxylase activity (duplicate
      of the reaction annotation via a different Reactome event). Correct and appropriately
      specific.
    action: ACCEPT
    reason: Same core specific MF as the other progesterone 21-hydroxylase annotations;
      experimentally supported.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: progesterone to deoxycorticosterone
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5601976
  qualifier: located_in
  review:
    summary: Reactome (TAS) ER-membrane localization, consistent with the IDA and IEA
      localizations. Correct core cellular component.
    action: ACCEPT
    reason: Matches the experimentally determined microsomal/ER-membrane localization.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral'
- term:
    id: GO:0034651
    label: cortisol biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:25855791
  qualifier: involved_in
  review:
    summary: Direct experimental (IDA) assignment to cortisol biosynthesis, based on
      the structural/enzymatic characterization of P450c21 as the major steroid 21-hydroxylase
      producing 11-deoxycortisol, the cortisol precursor. Specific, core biological
      process.
    action: ACCEPT
    reason: The 17alpha-hydroxyprogesterone-to-11-deoxycortisol reaction is the committed
      21-hydroxylation step of the cortisol pathway; this is the most specific and
      informative BP term and is directly supported.
    supported_by:
    - reference_id: PMID:25855791
      supporting_text: Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
    - reference_id: PMID:16984992
      supporting_text: the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
- term:
    id: GO:0103069
    label: 17-hydroxyprogesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:16984992
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase
      activity. Grischuk et al. measured conversion of 17-hydroxyprogesterone to 11-deoxycortisol
      for wild-type and mutant CYP21A2 in vitro. Core specific molecular function.
    action: ACCEPT
    reason: Directly measured physiological reaction of P450c21; appropriately specific
      core MF.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: the conversion of 17-hydroxyprogesterone to 11-deoxycortisol
- term:
    id: GO:0103069
    label: 17-hydroxyprogesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:22014889
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase
      activity. Concolino et al. determined residual enzyme activity and apparent
      kinetic values of CYP21A2 variants toward 17-hydroxyprogesterone. Core specific
      molecular function.
    action: ACCEPT
    reason: Directly measured physiological reaction; appropriately specific core MF.
    supported_by:
    - reference_id: PMID:22014889
      supporting_text: Functional in vitro assay for mutagenized CYP21A2 enzymes was
        performed in transiently transfected mammalian cells to test the residual
        enzyme activity and the apparent kinetic values
- term:
    id: GO:0103069
    label: 17-hydroxyprogesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:27721825
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of 17-hydroxyprogesterone 21-hydroxylase
      activity. de Paula Michelatto et al. expressed wild-type and mutant CYP21A2
      in COS-1 cells and measured enzyme activities toward 17-hydroxyprogesterone
      and progesterone. Core specific molecular function.
    action: ACCEPT
    reason: Directly measured physiological reaction; appropriately specific core MF.
    supported_by:
    - reference_id: PMID:27721825
      supporting_text: enzyme activities towards 17-hydroxyprogesterone and progesterone
        were determined
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:16984992
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity.
      Grischuk et al. measured conversion of progesterone to deoxycorticosterone for
      wild-type and mutant CYP21A2 in vitro. Core specific molecular function.
    action: ACCEPT
    reason: Directly measured physiological reaction of P450c21; appropriately specific
      core MF.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: progesterone to deoxycorticosterone
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:22014889
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity.
      Concolino et al. assayed residual activity and kinetics of CYP21A2 variants,
      which include the progesterone 21-hydroxylation reaction. Core specific molecular
      function.
    action: ACCEPT
    reason: Directly measured physiological reaction; appropriately specific core MF.
    supported_by:
    - reference_id: PMID:22014889
      supporting_text: Functional in vitro assay for mutagenized CYP21A2 enzymes was
        performed in transiently transfected mammalian cells to test the residual
        enzyme activity and the apparent kinetic values
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: EXP
  original_reference_id: PMID:27721825
  qualifier: enables
  review:
    summary: Experimental (EXP) assignment of progesterone 21-hydroxylase activity.
      de Paula Michelatto et al. measured CYP21A2 enzyme activities toward progesterone
      (and 17-hydroxyprogesterone) for wild-type and mutant proteins in COS-1 cells.
      Core specific molecular function.
    action: ACCEPT
    reason: Directly measured physiological reaction; appropriately specific core MF.
    supported_by:
    - reference_id: PMID:27721825
      supporting_text: enzyme activities towards 17-hydroxyprogesterone and progesterone
        were determined
- term:
    id: GO:0103069
    label: 17-hydroxyprogesterone 21-hydroxylase activity
  evidence_type: IDA
  original_reference_id: PMID:25855791
  qualifier: enables
  review:
    summary: Direct experimental (IDA) assignment of 17-hydroxyprogesterone 21-hydroxylase
      activity from the structural/kinetic study of the human P450 21A2 enzyme, which
      characterized 21-hydroxylation with 17alpha-hydroxyprogesterone as a substrate.
      Core specific molecular function.
    action: ACCEPT
    reason: Directly demonstrated physiological reaction; appropriately specific core
      MF.
    supported_by:
    - reference_id: PMID:25855791
      supporting_text: Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
- term:
    id: GO:0106309
    label: progesterone 21-hydroxylase activity
  evidence_type: IDA
  original_reference_id: PMID:25855791
  qualifier: enables
  review:
    summary: Direct experimental (IDA) assignment of progesterone 21-hydroxylase activity.
      Pallan et al. solved the human P450 21A2-progesterone complex and characterized
      21-hydroxylation of progesterone (kinetics, rate-limiting C-H cleavage). Core
      specific molecular function.
    action: ACCEPT
    reason: Directly demonstrated physiological reaction with the enzyme-progesterone
      structure and kinetics; appropriately specific core MF.
    supported_by:
    - reference_id: PMID:25855791
      supporting_text: progesterone, a substrate in adrenal 21-hydroxylation
- term:
    id: GO:0020037
    label: heme binding
  evidence_type: IDA
  original_reference_id: PMID:25855791
  qualifier: enables
  review:
    summary: Direct experimental (IDA) assignment of heme binding. The crystal structure
      of human P450 21A2 (Pallan et al.) resolves the heme b prosthetic group with
      the axial cysteine iron ligand, directly demonstrating heme binding. Correct
      essential cofactor function, secondary to the catalytic MF.
    action: ACCEPT
    reason: The enzyme binds heme b as an obligate cofactor, resolved in the structure;
      correct supporting molecular function.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: Name=heme b
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: axial binding residue
- term:
    id: GO:0006694
    label: steroid biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:25855791
  qualifier: involved_in
  review:
    summary: Direct experimental (IDA) assignment to the broad steroid biosynthetic
      process. Correct but a general parent of the specific corticosteroid-biosynthesis
      processes that better describe CYP21A2's role.
    action: KEEP_AS_NON_CORE
    reason: Accurate but less informative than the specific glucocorticoid/mineralocorticoid/cortisol
      biosynthesis terms; retain as a broad non-core process.
    supported_by:
    - reference_id: PMID:25855791
      supporting_text: Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
- term:
    id: GO:0008202
    label: steroid metabolic process
  evidence_type: IMP
  original_reference_id: PMID:16984992
  qualifier: involved_in
  review:
    summary: Mutant-phenotype (IMP) assignment to the broad steroid metabolic process,
      based on loss of 21-hydroxylation activity in CYP21A2 disease variants. Correct
      but a general parent of the specific corticosteroid-biosynthesis processes.
    action: KEEP_AS_NON_CORE
    reason: Accurate at a general level; the specific corticosteroid biosynthesis terms
      are the informative core processes. Retain as broad non-core.
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: almost absent or negligible CYP21 activity for the conversion
        of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to deoxycorticosterone
- term:
    id: GO:0008395
    label: steroid hydroxylase activity
  evidence_type: IMP
  original_reference_id: PMID:16984992
  qualifier: enables
  review:
    summary: Mutant-phenotype (IMP) assignment of the generic steroid hydroxylase activity,
      inferred from loss of 21-hydroxylation in disease variants. Correct but less
      specific than steroid 21-monooxygenase activity (GO:0004509).
    action: MODIFY
    reason: The essence is correct, but the specific catalytic term steroid 21-monooxygenase
      activity (GO:0004509) precisely captures the demonstrated function; replace the
      generic parent with the specific term.
    proposed_replacement_terms:
    - id: GO:0004509
      label: steroid 21-monooxygenase activity
    supported_by:
    - reference_id: PMID:16984992
      supporting_text: almost absent or negligible CYP21 activity for the conversion
        of 17-hydroxyprogesterone to 11-deoxycortisol and progesterone to deoxycorticosterone
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-193964
  qualifier: located_in
  review:
    summary: Reactome (TAS) ER-membrane localization (progesterone 21-hydroxylation
      event), consistent with the IDA and other localizations. Correct core cellular
      component.
    action: ACCEPT
    reason: Matches the experimentally determined microsomal/ER-membrane localization.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral'
- term:
    id: GO:0005789
    label: endoplasmic reticulum membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-193981
  qualifier: located_in
  review:
    summary: Reactome (TAS) ER-membrane localization (17-hydroxyprogesterone oxidation
      event), consistent with the IDA and other localizations. Correct core cellular
      component.
    action: ACCEPT
    reason: Matches the experimentally determined microsomal/ER-membrane localization.
    supported_by:
    - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
      supporting_text: 'SUBCELLULAR LOCATION: Endoplasmic reticulum membrane; Peripheral'
- term:
    id: GO:0008395
    label: steroid hydroxylase activity
  evidence_type: TAS
  original_reference_id: PMID:1406709
  qualifier: enables
  review:
    summary: Traceable-author (TAS) assignment of the generic steroid hydroxylase activity
      from an early CYP21 mutation study. Correct but less specific than steroid 21-monooxygenase
      activity (GO:0004509).
    action: MODIFY
    reason: The essence is correct, but the specific catalytic term steroid 21-monooxygenase
      activity (GO:0004509) captures the actual demonstrated function; replace the
      generic parent with the specific term.
    proposed_replacement_terms:
    - id: GO:0004509
      label: steroid 21-monooxygenase activity
    supported_by:
    - reference_id: PMID:1406709
      supporting_text: R339H and P453S CYP21 mutations associated with nonclassic
        steroid 21-hydroxylase deficiency
core_functions:
- description: Steroid 21-monooxygenase (P450c21) that 21-hydroxylates C21 steroids
    in adrenal corticosteroid biosynthesis, converting progesterone to 11-deoxycorticosterone
    and 17alpha-hydroxyprogesterone to 11-deoxycortisol, using heme b and electrons
    from cytochrome P450 reductase, at the endoplasmic reticulum membrane.
  molecular_function:
    id: GO:0004509
    label: steroid 21-monooxygenase activity
  directly_involved_in:
  - id: GO:0006704
    label: glucocorticoid biosynthetic process
  locations:
  - id: GO:0005789
    label: endoplasmic reticulum membrane
  supported_by:
  - reference_id: PMID:25855791
    supporting_text: Cytochrome P450 (P450) 21A2 is the major steroid 21-hydroxylase
  - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
    supporting_text: Catalyzes the hydroxylation at C-21 of
- description: Same 21-hydroxylase activity supplying the mineralocorticoid branch
    of steroidogenesis, converting progesterone to 11-deoxycorticosterone (DOC), the
    precursor of aldosterone.
  molecular_function:
    id: GO:0004509
    label: steroid 21-monooxygenase activity
  directly_involved_in:
  - id: GO:0006705
    label: mineralocorticoid biosynthetic process
  locations:
  - id: GO:0005789
    label: endoplasmic reticulum membrane
  supported_by:
  - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
    supporting_text: the biosynthetic pathway of mineralocorticoids and glucocorticoids
  - reference_id: PMID:16984992
    supporting_text: progesterone to deoxycorticosterone
- description: Heme-thiolate cofactor binding required for catalysis; the enzyme binds
    heme b with an axial cysteine iron ligand at the catalytic centre.
  molecular_function:
    id: GO:0020037
    label: heme binding
  locations:
  - id: GO:0005789
    label: endoplasmic reticulum membrane
  supported_by:
  - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
    supporting_text: Name=heme b
  - reference_id: file:human/CYP21A2/CYP21A2-uniprot.txt
    supporting_text: axial binding residue
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000116
  title: Automatic Gene Ontology annotation based on Rhea mapping
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: file:human/CYP21A2/CYP21A2-uniprot.txt
  title: UniProtKB entry P08686 (CP21A_HUMAN), Steroid 21-hydroxylase
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Primary UniProt record; FUNCTION, CATALYTIC ACTIVITY (RHEA:50304/50308),
      COFACTOR (heme b), and SUBCELLULAR LOCATION (ER/microsome membrane, peripheral)
      directly support the core catalytic, cofactor, and localization annotations.
- id: PMID:10198222
  title: Effects of missense mutations and deletions on membrane anchoring and enzyme
    function of human steroid 21-hydroxylase (P450c21).
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Membrane-anchoring study; supports ER/microsomal peripheral-membrane
      localization (is_active_in) and N-terminal hydrophobic anchoring. Abstract-only
      in cache but title/abstract confirm the assay is on human P450c21.
- id: PMID:1406709
  title: 'R339H and P453S: CYP21 mutations associated with nonclassic steroid 21-hydroxylase
    deficiency that are not apparent gene conversions.'
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Early CYP21 mutation study; basis for the TAS steroid hydroxylase
      activity annotation (proposed MODIFY to the specific GO:0004509).
- id: PMID:16984992
  title: 'Four novel missense mutations in the CYP21A2 gene detected in Russian patients
    suffering from the classical form of congenital adrenal hyperplasia: identification,
    functional characterization, and structural analysis.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: In vitro functional assays measuring conversion of both 17-hydroxyprogesterone
      and progesterone; directly supports the two substrate-specific 21-hydroxylase
      activities and the steroid metabolic/hydroxylase annotations.
- id: PMID:22014889
  title: 'p.H282N and p.Y191H: 2 novel CYP21A2 mutations in Italian congenital adrenal
    hyperplasia patients.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Functional in vitro assays of CYP21A2 variants with residual activity
      and kinetics for both substrates; supports the substrate-specific 21-hydroxylase
      activities. Notes >90% of CAH is due to CYP21A2.
- id: PMID:25855791
  title: 'Human Cytochrome P450 21A2, the Major Steroid 21-Hydroxylase: STRUCTURE
    OF THE ENZYME·PROGESTERONE SUBSTRATE COMPLEX AND RATE-LIMITING C-H BOND CLEAVAGE.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Crystal structure of human P450 21A2-progesterone complex with heme;
      kinetics and rate-limiting C-H cleavage. Anchors heme binding, the specific
      21-hydroxylase activities, and cortisol/steroid biosynthesis. States CYP21A2
      deficiency causes ~95% of CAH.
- id: PMID:27721825
  title: Functional and Structural Consequences of Nine CYP21A2 Mutations Ranging
    from Very Mild to Severe Effects.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Full text available; expressed WT and mutant CYP21A2 in COS-1 cells
      and determined enzyme activities toward 17-hydroxyprogesterone and progesterone.
      Directly supports the substrate-specific 21-hydroxylase activities.
- id: Reactome:R-HSA-193964
  title: CYP21A2 21-hydroxylates PROG
  findings: []
- id: Reactome:R-HSA-193981
  title: CYP21A2 oxidises 17HPROG
  findings: []
- id: Reactome:R-HSA-193993
  title: Mineralocorticoid biosynthesis
  findings: []
- id: Reactome:R-HSA-194002
  title: Glucocorticoid biosynthesis
  findings: []
- id: Reactome:R-HSA-211976
  title: Endogenous sterols
  findings: []
- id: Reactome:R-HSA-5601976
  title: Defective CYP21A2 does not 21-hydroxylate PROG
  findings: []