CYB5R4 (NADH-cytochrome b5 reductase 4; also known as NCB5OR or b5+b5R) is a multidomain, soluble flavohemoprotein and NAD(P)H-dependent oxidoreductase. It contains an N-terminal CS/Hsp20 (p23-like) domain, a cytochrome b5-like heme-binding domain (with axial His-89 and His-112 ligating a six-coordinate low-spin heme) joined by a long hinge to a C-terminal FAD-dependent cytochrome-b5-reductase (FNR-type FAD- and NAD-binding) module. It binds stoichiometric heme and FAD and catalyzes EC 1.6.2.2 (NADH:cytochrome b5 oxidoreductase), reducing electron acceptors including cytochrome b5, cytochrome c, methemoglobin and ferricyanide; unlike the classical single-domain cytochrome b5 reductase it lacks a membrane anchor. The protein localizes to the endoplasmic reticulum and perinuclear cytoplasm. It functions in the response to oxidative and endoplasmic reticulum stress, protecting cells (notably pancreatic beta cells) from excess reactive oxygen species; loss of the orthologous gene in mice causes diabetes and lipoatrophy. Although early work proposed it as an NAD(P)H oxidase / candidate oxygen sensor, subsequent enzymology showed it preferentially reduces substrates rather than transferring electrons to molecular oxygen and is not an efficient superoxide-generating oxidase.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment of the core cytochrome-b5 reductase activity. This matches the experimentally demonstrated EC 1.6.2.2 activity and is the core molecular function. Reason: The cytochrome-b5 reductase activity acting on NAD(P)H is directly supported by experimental characterization of NCB5OR, which contains a cytochrome-b5-reductase domain and reduces cytochrome b5-type acceptors using NAD(P)H. Supporting Evidence: PMID:10611283 We have identified a cytosolic cytochrome b-type NAD(P)H oxidoreductase in mammals, a flavohemoprotein (b5+b5R) containing cytochrome b5 (b5) and b5 reductase (b5R) domains. |
| GO:0020037 heme binding | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assignment of heme binding, supported by experimental evidence that NCB5OR binds stoichiometric heme via its cytochrome b5-like domain. Reason: NCB5OR contains a cytochrome b5-like heme-binding domain with axial His ligands and binds stoichiometric heme experimentally. Supporting Evidence: PMID:15131110 Recombinant NCB5OR is soluble and has stoichiometric amounts of heme and flavin adenine dinucleotide. |
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assignment of ER localization, consistent with experimental colocalization of NCB5OR with the ER marker calreticulin. Reason: NCB5OR colocalizes with the ER marker calreticulin and is annotated by UniProt to the endoplasmic reticulum. Supporting Evidence: PMID:15131110 we show that NCB5OR colocalizes with calreticulin, a marker for endoplasmic reticulum. |
| GO:0006801 superoxide metabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic assignment of superoxide metabolic process. NCB5OR can produce superoxide in vitro, but only with very slow turnover, and later enzymology concluded it preferentially reduces substrates rather than transferring electrons to oxygen, so this process is not a core in vivo function. Reason: Superoxide production by NCB5OR occurs only at very low turnover rates in vitro; the protein is concluded to be a substrate reductase rather than an oxidase, so superoxide metabolism is at most a minor/non-core activity. Supporting Evidence: PMID:15131110 both full-length and truncated NCB5OR produce superoxide from oxygen with slow turnover rates |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) ER localization, redundant with and supported by the experimental IDA ER annotation. Reason: ER localization is experimentally established by colocalization with calreticulin. Supporting Evidence: PMID:15131110 we show that NCB5OR colocalizes with calreticulin, a marker for endoplasmic reticulum. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: Very general InterPro-based oxidoreductase activity. Correct but uninformative given the more specific cytochrome-b5 reductase activity annotations. Reason: This is a high-level parent of the specific cytochrome-b5 reductase activity that is already annotated; it is correct but too general to be informative. Supporting Evidence: PMID:10611283 Human b5+b5R flavohemoprotein is a NAD(P)H oxidoreductase |
| GO:0016653 oxidoreductase activity, acting on NAD(P)H, heme protein as acceptor | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning annotation of oxidoreductase activity acting on NAD(P)H with a heme protein as acceptor, redundant with the experimental IDA annotation of the same term. Reason: This activity is directly supported by experimental demonstration that NCB5OR uses NAD(P)H to reduce heme-protein acceptors such as cytochrome b5/cytochrome c and methemoglobin. Supporting Evidence: PMID:15131110 reduces cytochrome c, methemoglobin, ferricyanide, and molecular oxygen in vitro |
| GO:0020037 heme binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based heme binding annotation, redundant with and supported by the IBA heme binding annotation and experimental evidence of stoichiometric heme. Reason: Heme binding is experimentally established and the protein has a cytochrome b5-like heme-binding domain with axial His ligands. Supporting Evidence: PMID:15131110 Recombinant NCB5OR is soluble and has stoichiometric amounts of heme and flavin adenine dinucleotide. |
| GO:0090524 cytochrome-b5 reductase activity, acting on NADH | IEA GO_REF:0000120 | ACCEPT | Summary: EC/Rhea-based annotation of NADH-specific cytochrome-b5 reductase activity (EC 1.6.2.2). Supported by experimental catalytic activity; closely related to the NAD(P)H form. Reason: UniProt records the catalytic reaction 2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + NAD+ + H+ (EC 1.6.2.2), demonstrated experimentally. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt EC=1.6.2.2 {ECO:0000305|PubMed:10611283}; Reaction=2 Fe(III)-[cytochrome b5] + NADH = 2 Fe(II)-[cytochrome b5] + NAD(+) + H(+) |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IEA GO_REF:0000107 | ACCEPT | Summary: Ensembl Compara ortholog transfer of the core cytochrome-b5 reductase activity, redundant with the experimental IDA annotation of the same term. Reason: The core cytochrome-b5 reductase activity acting on NAD(P)H is directly demonstrated experimentally. Supporting Evidence: PMID:10611283 The recombinant b5+b5R protein can be reduced by NAD(P)H ... and by cytochrome c reduction in vitro. |
| GO:0015701 bicarbonate transport | TAS Reactome:R-HSA-1237044 | MARK AS OVER ANNOTATED | Summary: Reactome pathway-context annotation ("Erythrocytes take up carbon dioxide and release oxygen"). CYB5R4 is a redox enzyme, not a bicarbonate transporter; this is a pathway-membership artifact rather than a true molecular role. Reason: CYB5R4 has no bicarbonate transport activity; the term derives from inclusion in a broad erythrocyte gas-exchange Reactome pathway and does not reflect the protein's function. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt NADH-cytochrome b5 reductase involved in endoplasmic reticulum stress response pathway. |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | TAS Reactome:R-HSA-6806831 | ACCEPT | Summary: Reactome TAS annotation of the core cytochrome-b5 reductase activity (in the context of methemoglobin reduction), consistent with experimental data. Reason: The cytochrome-b5 reductase activity is experimentally demonstrated and NCB5OR reduces methemoglobin in vitro. Supporting Evidence: PMID:15131110 reduces cytochrome c, methemoglobin, ferricyanide, and molecular oxygen in vitro |
| GO:0072593 reactive oxygen species metabolic process | IDA PMID:15131110 NCB5OR is a novel soluble NAD(P)H reductase localized in the... | KEEP AS NON CORE | Summary: ROS metabolic process supported by NCB5OR's redox chemistry and role in protecting cells against oxidant stress; consistent with the slow superoxide-producing activity and substrate-reducing behavior. Reason: NCB5OR participates in cellular ROS handling/oxidant-stress protection, but this is a downstream physiological consequence of its redox activity rather than the core catalytic function. Supporting Evidence: PMID:15131110 both full-length and truncated NCB5OR produce superoxide from oxygen with slow turnover rates |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-6806831 | MARK AS OVER ANNOTATED | Summary: Reactome annotation to ER membrane. NCB5OR is explicitly a soluble protein lacking a membrane anchor, so an ER membrane localization is not accurate; it is associated with the ER but as a soluble protein. Reason: Unlike classical single-domain cytochrome b5 reductase, NCB5OR has no membrane anchor and is soluble; the appropriate localization is the ER compartment, not the ER membrane. Supporting Evidence: PMID:10611283 b5+b5R also has binding motifs for heme, FAD, and NAD(P)H prosthetic groups but no membrane anchor. |
| GO:0030073 insulin secretion | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS transfer from the mouse ortholog (Q3TDX8). The Ncb5or-null mouse has a diabetes/beta-cell phenotype, so a role in insulin secretion is plausible but indirect and phenotype-driven, not a core molecular function. Reason: A role in insulin secretion reflects the physiological consequence of beta-cell dysfunction in Ncb5or-deficient mice rather than a direct biochemical function of the enzyme. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt Plays a critical role in protecting pancreatic beta-cells against oxidant stress |
| GO:0042593 glucose homeostasis | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS transfer from the mouse ortholog. Consistent with the diabetic phenotype of Ncb5or-null mice, but a downstream physiological role rather than a core molecular function. Reason: Glucose homeostasis is a systemic phenotype linked to beta-cell protection in mouse models, not a direct enzymatic function of CYB5R4. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt Plays a critical role in protecting pancreatic beta-cells against oxidant stress |
| GO:0046677 response to antibiotic | ISS GO_REF:0000024 | REMOVE | Summary: ISS transfer from the mouse ortholog. There is no biological evidence linking CYB5R4 to an antibiotic response; this appears to be a spurious/non-specific transferred annotation. Reason: No experimental or mechanistic support connects CYB5R4 to a response to antibiotic; the term is implausible for a soluble ER redox enzyme and is not supported by the literature. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt NADH-cytochrome b5 reductase involved in endoplasmic reticulum stress response pathway. |
| GO:0048468 cell development | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: Very general ISS transfer ("cell development"). Too broad to be informative and not directly supported by mechanistic data on CYB5R4. Reason: Cell development is a high-level, non-specific process term that does not capture the redox/stress-protection function of CYB5R4 and is only indirectly inferred from mouse phenotypes. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt Plays a critical role in protecting pancreatic beta-cells against oxidant stress |
| GO:0003032 detection of oxygen | NAS PMID:10611283 Identification of a cytochrome b-type NAD(P)H oxidoreductase... | REMOVE | Summary: NAS annotation based on the early hypothesis that b5+b5R is a candidate oxygen sensor. Later enzymology showed it is a substrate reductase, not an efficient oxygen-utilizing oxidase, undermining the oxygen-sensing proposal. Reason: The oxygen-sensor/oxygen-detection role was a speculative proposal in the 1999 paper that was superseded by 2004 enzymology showing NCB5OR preferentially reduces substrates rather than reacting with molecular oxygen; there is no direct evidence for oxygen detection. Supporting Evidence: PMID:15131110 endogenous NCB5OR is a soluble NAD(P)H reductase preferentially reducing substrate(s) rather than transferring electrons to molecular oxygen |
| GO:0004128 cytochrome-b5 reductase activity, acting on NAD(P)H | IDA PMID:10611283 Identification of a cytochrome b-type NAD(P)H oxidoreductase... | ACCEPT | Summary: Direct experimental demonstration of the core NAD(P)H-dependent cytochrome-b5 reductase activity. This is the central molecular function of CYB5R4. Reason: The recombinant protein is reduced by NAD(P)H and reduces cytochrome b5-type/cytochrome c acceptors, directly demonstrating cytochrome-b5 reductase activity (EC 1.6.2.2). Supporting Evidence: PMID:10611283 The recombinant b5+b5R protein can be reduced by NAD(P)H ... and by cytochrome c reduction in vitro. |
| GO:0005783 endoplasmic reticulum | IDA PMID:15131110 NCB5OR is a novel soluble NAD(P)H reductase localized in the... | ACCEPT | Summary: Direct experimental ER localization via colocalization with the ER marker calreticulin. This is the well-supported subcellular localization. Reason: Subcellular fractionation and confocal microscopy show colocalization with calreticulin, establishing ER localization. Supporting Evidence: PMID:15131110 we show that NCB5OR colocalizes with calreticulin, a marker for endoplasmic reticulum. |
| GO:0006801 superoxide metabolic process | IDA PMID:10611283 Identification of a cytochrome b-type NAD(P)H oxidoreductase... | KEEP AS NON CORE | Summary: Based on in vitro superoxide production observed with air and excess NAD(P)H. Later work showed this occurs only at very slow turnover and that NCB5OR is not a bona fide oxidase, making superoxide metabolism a non-core activity. Reason: Superoxide production is a slow, minor in vitro activity; NCB5OR is concluded to preferentially reduce substrates rather than generate superoxide, so this is not a core function. Supporting Evidence: PMID:10611283 superoxide production in the presence of air and excess NAD(P)H |
| GO:0016174 NAD(P)H oxidase H2O2-forming activity | IDA NOT PMID:15131110 NCB5OR is a novel soluble NAD(P)H reductase localized in the... | ACCEPT | Summary: Correctly negated annotation. The 2004 enzymology concluded that NCB5OR is not an NAD(P)H oxidase for superoxide/H2O2 production, refuting the earlier oxidase interpretation. Reason: The NOT annotation accurately reflects the experimental conclusion that NCB5OR is a substrate reductase, not an NAD(P)H oxidase. Supporting Evidence: PMID:15131110 therefore not an NAD(P)H oxidase for superoxide production |
| GO:0016174 NAD(P)H oxidase H2O2-forming activity | IDA PMID:10611283 Identification of a cytochrome b-type NAD(P)H oxidoreductase... | REMOVE | Summary: Positive (non-negated) NAD(P)H oxidase annotation from the original 1999 report. This interpretation was explicitly refuted by the 2004 study, which established the NOT annotation for the same activity. Reason: The proposed NAD(P)H oxidase activity was superseded and refuted; NCB5OR produces superoxide only at negligible rates and is concluded not to be an oxidase, so this positive annotation is incorrect. Supporting Evidence: PMID:15131110 endogenous NCB5OR is a soluble NAD(P)H reductase preferentially reducing substrate(s) rather than transferring electrons to molecular oxygen and therefore not an NAD(P)H oxidase for superoxide production |
| GO:0016653 oxidoreductase activity, acting on NAD(P)H, heme protein as acceptor | IDA PMID:15131110 NCB5OR is a novel soluble NAD(P)H reductase localized in the... | ACCEPT | Summary: Direct experimental demonstration that NCB5OR uses NAD(P)H to reduce heme-protein acceptors (cytochrome c, methemoglobin). This is a core, well-supported molecular function. Reason: NCB5OR actively reduces cytochrome c and methemoglobin using NAD(P)H, directly demonstrating oxidoreductase activity acting on NAD(P)H with a heme protein as acceptor. Supporting Evidence: PMID:15131110 reduces cytochrome c, methemoglobin, ferricyanide, and molecular oxygen in vitro |
| GO:0048471 perinuclear region of cytoplasm | IDA PMID:10611283 Identification of a cytochrome b-type NAD(P)H oxidoreductase... | ACCEPT | Summary: Direct observation of cytosolic perinuclear localization in COS-7 cells. Consistent with the soluble nature of the protein and its ER/perinuclear association. Reason: Confocal microscopy of myc-tagged b5+b5R showed cytosolic localization in the perinuclear space, supporting this localization term. Supporting Evidence: PMID:10611283 confocal microscopy revealed a cytosolic localization at the perinuclear space. |
| GO:0050660 flavin adenine dinucleotide binding | IC PMID:15131110 NCB5OR is a novel soluble NAD(P)H reductase localized in the... | NEW | Summary: Proposed annotation not present in the current GOA for CYB5R4. Reason: NCB5OR contains a defined FAD-binding domain and was shown to contain stoichiometric FAD, yet GO:0050660 is absent from existing annotations. Supporting Evidence: PMID:15131110 Recombinant NCB5OR is soluble and has stoichiometric amounts of heme and flavin adenine dinucleotide. |
| GO:0034976 response to endoplasmic reticulum stress | IC file:human/CYB5R4/CYB5R4-uniprot.txt | NEW | Summary: Proposed annotation not present in the current GOA for CYB5R4. Reason: The protein is annotated by UniProt as functioning in the ER stress response pathway and is ER-localized, supporting a response to ER stress process term. Supporting Evidence: file:human/CYB5R4/CYB5R4-uniprot.txt NADH-cytochrome b5 reductase involved in endoplasmic reticulum stress response pathway. |
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Download this section (compressed HTML)Q: What are the physiological electron acceptor(s) of CYB5R4 in vivo, given that it lacks a membrane anchor and is soluble within the ER/perinuclear compartment?
Q: How does the N-terminal CS/Hsp20 (p23-like) domain contribute to function, folding, or partner interactions of CYB5R4?
Q: By what mechanism does CYB5R4 protect pancreatic beta cells from oxidative and ER stress, and is this a direct antioxidant role or an indirect consequence of its reductase activity?
Experiment: Identify the endogenous electron acceptor(s) and interacting partners of CYB5R4 in beta cells using proximity labeling (BioID/APEX) and reconstituted electron-transfer assays with candidate cytochrome b5 / desaturase systems.
Experiment: Generate beta-cell-specific CYB5R4 knockout or catalytic-dead (heme/FAD-binding mutant) cell lines and measure ER stress markers, ROS levels, and insulin secretion under oxidative challenge to define the mechanism of cytoprotection.
Experiment: Perform steady-state and stopped-flow enzymology comparing reduction of physiological heme-protein acceptors versus molecular oxygen to quantitatively confirm that substrate reduction, not oxidase activity, dominates in vivo.
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