Branched-chain amino acid (BCAA) catabolism — transamination and the BCKDH complex

The committed, shared initial steps of the catabolism of the three branched-chain amino acids (BCAAs: L-leucine, L-isoleucine, L-valine), operating in the mitochondrial matrix. Branched-chain aminotransferase (BCAT2, the mitochondrial isozyme; BCAT1 is cytosolic) first reversibly transaminates each BCAA with 2-oxoglutarate to the corresponding branched-chain 2-oxo (alpha-keto) acid (BCKA: alpha-ketoisocaproate from Leu, alpha-keto-beta- methylvalerate from Ile, alpha-ketoisovalerate from Val) plus L-glutamate. The branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex — a large mitochondrial multienzyme complex homologous to the pyruvate and 2-oxoglutarate dehydrogenase complexes — then catalyses the irreversible, rate-limiting, committed oxidative decarboxylation of the BCKAs to the corresponding branched-chain acyl-CoA thioesters + CO2 + NADH. The complex has three catalytic components: E1 (a thiamine-pyrophosphate-dependent decarboxylase, an alpha2-beta2 heterotetramer of BCKDHA/E1-alpha and BCKDHB/E1-beta) which decarboxylates the BCKA and reductively acylates the lipoyl arm; E2 (DBT, dihydrolipoyllysine-residue (2-methylpropanoyl)transferase) which forms the structural core and transfers the acyl group to CoA; and E3 (DLD, dihydrolipoyl dehydrogenase, an FAD flavoenzyme) which reoxidises the dihydrolipoyl arm and passes electrons to NAD+. E3/DLD is not BCKDH-specific: it is the common E3 shared with the pyruvate dehydrogenase, 2-oxoglutarate dehydrogenase and 2-oxoadipate dehydrogenase complexes and the glycine cleavage system. BCKDH flux is regulated by reversible phosphorylation of E1-alpha (inhibitory kinase BCKDK; activating phosphatase PPM1K). Inherited deficiency of BCKDHA, BCKDHB or DBT causes maple syrup urine disease (MSUD, from accumulation of BCAAs and their neurotoxic BCKAs); DLD deficiency causes a combined E3-deficiency phenotype (MSUD-like plus lactic acidosis / 2-oxoglutaric aciduria because PDH and OGDH are simultaneously impaired). Downstream, the branched-chain acyl-CoAs are degraded by substrate-specific dehydrogenases and hydratases (out of scope here) to acetyl-CoA, acetoacetate and/or succinyl-CoA (via propionyl-CoA/methylmalonyl-CoA for Ile and Val).

MODULE:branched_chain_amino_acid_catabolismDRAFTMetabolic Pathwaymodules/branched_chain_amino_acid_catabolism.yaml
branched-chain amino acid catabolic processGO:0009083
GO:0009083
branched-chain amino acid catabolic process
The module is grounded in the GO branched-chain amino acid catabolic process (GO:0009083) and the specific BCKDH step (GO:0120552, branched-chain alpha-keto acid decarboxylation to branched-chain acyl-CoA).
Reactome:R-HSA-70895
Branched-chain amino acid catabolism
Step order and reactions follow the human Reactome "Branched-chain amino acid catabolism" pathway (R-HSA-70895), including the BCAT2 transamination (R-HSA-70724).
file:human/BCAT2/BCAT2-ai-review.yaml
BCAT2 gene review (human)
The transamination step grounding (UniProtKB:O15382, GO:0004084 branched-chain-amino-acid:2-oxoglutarate transaminase activity) matches the completed human BCAT2 review.
file:human/BCKDHA/BCKDHA-ai-review.yaml
BCKDHA gene review (human)
The E1-alpha grounding (UniProtKB:P12694, GO:0003863 branched-chain 2-oxo acid dehydrogenase activity, complex GO:0160157) matches the completed human BCKDHA review.
file:human/BCKDHB/BCKDHB-ai-review.yaml
BCKDHB gene review (human)
The E1-beta grounding (UniProtKB:P21953, GO:0003863, complex GO:0160157) matches the completed human BCKDHB review.
file:human/DBT/DBT-ai-review.yaml
DBT gene review (human)
The E2 grounding (UniProtKB:P11182, GO:0043754 dihydrolipoamide branched chain acyltransferase activity, complex GO:0160157) matches the completed human DBT review.
file:human/DLD/DLD-ai-review.yaml
DLD gene review (human)
The E3 grounding (UniProtKB:P09622, GO:0004148 dihydrolipoyl dehydrogenase (NADH) activity) matches the completed human DLD review, which documents DLD as the shared E3 of PDH/OGDH/BCKDH and the glycine cleavage system.
3Nodes
2Parts
0Variant Sets
0Variants
5Annotons
1Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:branched_chain_amino_acid_catabolism deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Function consistency and core coverage

3 gene-function check(s) reflected in core functions · 0 conflict(s) · 7 evidence or coverage gap(s) (11 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
bcat2_activity BCAT2 (human, mitochondrial)
UniProtKB:O15382
Family Representative 		branched-chain-amino-acid:2-oxoglutarate transaminase activity
GO:0004084 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
bcat2_activity BCAT1 (human, cytosolic paralog)
UniProtKB:P54687
Family Representative 		branched-chain-amino-acid:2-oxoglutarate transaminase activity
GO:0004084 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
bckdha_e1alpha BCKDHA (human)
UniProtKB:P12694
Family Representative 		branched-chain 2-oxo acid dehydrogenase activity
GO:0003863 		Annotation Supported
A retained annotation or curated replacement supports this function, but core_functions does not yet reflect it.
bckdhb_e1beta BCKDHB (human)
UniProtKB:P21953
Family Representative 		branched-chain 2-oxo acid dehydrogenase activity
GO:0003863 		Annotation Supported
A retained annotation or curated replacement supports this function, but core_functions does not yet reflect it.
dbt_e2 DBT (human)
UniProtKB:P11182
Family Representative 		dihydrolipoamide branched chain acyltransferase activity
GO:0043754 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
dld_e3 DLD (human)
UniProtKB:P09622
Family Representative 		dihydrolipoyl dehydrogenase (NADH) activity
GO:0004148 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
bcat2_activity Branched-chain aminotransferase family (BCAT1/BCAT2)
PANTHER:PTHR11825
Family 		branched-chain-amino-acid:2-oxoglutarate transaminase activity
GO:0004084 		Family Supported
Family review supports GO:0004084 FAMILY_WIDE. The PANTHER subfamily inventory comprises branched-chain aminotransferase, BCAT1 and BCAT2 groups. Rat BCAT2 expression directly produces branched-chain aminotransferase activity with processing of its mitochondrial targeting sequence (PMID:9165094); the human ortholog and reviewed substrate chemistry support the same broad reaction. This supports the reaction across intact members, while the shared class-IV PLP fold alone would not: mitochondrial location, catabolic direction and physiological substrate flux differ among lineages.
bckdha_e1alpha BCKDH E1 alpha subunit family (BCKDHA)
PANTHER:PTHR43380
Family 		branched-chain 2-oxo acid dehydrogenase activity
GO:0003863 		Family Function Unreviewed
Family review leaves GO:0003863 UNRESOLVED. The gap this family review exists to record. Human, bovine and rat BCKDHA all carry experimental annotations to GO:0003863 (for human: IDA PMID:10745006 and PMID:9582350 with contributes_to, IDA PMID:7883996 with enables), yet PAINT asserts no molecular function anywhere on the branched-chain side of this family - only the BP (PTN000162347) and CC (PTN000162283) terms. Human BCKDHA therefore receives no MF IBA even though the experimental grounding for one is stronger than for either term it does inherit. It is left UNRESOLVED rather than granted because the scope cannot be safely expressed at subfamily level. On the InterPro slice's reading, PTHR43380:SF1 contains both branched-chain and pyruvate E1-alpha subunits, so a SUBFAMILY_ONLY grant naming SF1 would hand the term to the pdhA proteins while denying it at SF1 would take it from BCKDHA. On PANTHER's member-index reading the pdhA proteins are in PTHR11516 and SF1 would be safe - but the two sources are in open conflict (see coherence_reason), and a grant that is only correct under one of them is not a grant worth making. The correct vehicle is the PTN node: an IBD for GO:0003863 placed at PTN000162347 (or at the eukaryotic PTN000162283) would land exactly on the clade that has the evidence, and would be sound for human BCKDHA. Note the qualifier question travels with the term - the activity belongs to the alpha2-beta2 heterotetramer, so contributes_to is the more accurate qualifier for a subunit.
bckdhb_e1beta BCKDH E1 beta subunit family (BCKDHB)
PANTHER:PTHR42980
Family 		branched-chain 2-oxo acid dehydrogenase activity
GO:0003863 		Family Scope Restricted
GO:0003863 is SUBFAMILY_ONLY; this family grounding does not establish the allowed subfamily scope. The molecular function is the most conspicuous gap in this family's PAINT curation: human BCKDHB has three experimental annotations to GO:0003863 (IDA PMID:10745006, IDA PMID:9582350, IMP PMID:2022752, all with contributes_to), and the bovine and rat orthologs are characterised too, yet PAINT asserts only the two BP terms and the CC term. Nothing in the family argues against the activity: all reviewed members are branched-chain E1-beta chains. Scoped to SF1 rather than FAMILY_WIDE because SF1 is where every reviewed member actually sits, and because the family name ("...BETA-RELATED") leaves open that unclassified members may be beta subunits of a related, non-branched-chain 2-oxo acid dehydrogenase - which would take a different substrate-specific term. A future IBD would be soundest placed on PTN000178891, the node that already carries GO:0009083 and covers the same clade. One qualifier caveat travels with the term: the activity is a property of the alpha2-beta2 heterotetramer and the beta subunit has no independent active site, so contributes_to (as every manual annotation on BCKDHB uses) is the right qualifier - an IBA propagating it as enables would overstate what the subunit does.
dbt_e2 2-oxoacid dehydrogenase E2 (dihydrolipoyl acyltransferase) family (DBT)
PANTHER:PTHR43178
Family 		dihydrolipoamide branched chain acyltransferase activity
GO:0043754 		Family Scope Restricted
GO:0043754 is SUBFAMILY_ONLY; this family grounding does not establish the allowed subfamily scope. The BCKDH E2 activity of mitochondrial DBT (SF5) and plant BCE2 (SF14). It does not apply to the PDH E2 subfamilies (SF2, SF10, SF13), nor to B. subtilis AcoC, the acetoin-system acetyl transferase that UniProt places in SF5.
dld_e3 Dihydrolipoyl dehydrogenase / pyridine-nucleotide-disulphide oxidoreductase family (DLD)
PANTHER:PTHR22912
Family 		dihydrolipoyl dehydrogenase (NADH) activity
GO:0004148 		Family Scope Restricted
GO:0004148 is SUBFAMILY_ONLY; this family grounding does not establish the allowed subfamily scope. The E3 subfamilies, not the divergent SthA transhydrogenase. SF219 is included on the basis of its independently mapped AcoL representative, the native sequence-based E3 assignment in PMID:10368162 and the conserved catalytic architecture. This extends chemistry scope, not a requirement that all E3 paralogs serve the same physiological complexes. No new PAINT node is inferred.

Gene-review completeness (5/6 grounded genes reviewed)

4 complete review(s) · 5 with deep research · 1 missing review · 0 reviewed but lacking deep research

Gene Review Complete Deep research
BCAT2 O15382 ✓ ✓ ✓
BCKDHA P12694 ✓ 39/41 ✓
BCKDHB P21953 ✓ ✓ ✓
DBT P11182 ✓ ✓ ✓
DLD P09622 ✓ ✓ ✓
BCAT1 (human, cytosolic paralog) P54687 ✗ — —

Details

Context
mitochondrial matrixGO:0005759
BCAA catabolism (transamination + BCKDH complex)Metabolic Pathwaybranched_chain_amino_acid_catabolism
branched-chain amino acid catabolic processGO:0009083
Context
mitochondrial matrixGO:0005759

Initial committed steps of branched-chain amino acid catabolism grounded to the human enzymes BCAT2 (UniProtKB:O15382, GO:0004084, EC 2.6.1.42) and the four BCKDH-complex subunits: E1-alpha BCKDHA (P12694, GO:0003863), E1-beta BCKDHB (P21953, GO:0003863), E2 DBT (P11182, GO:0043754, EC 2.3.1.168) and the shared E3 DLD (P09622, GO:0004148, EC 1.8.1.4). The BCKDH complex is grounded to GO:0160157 and the reaction to GO:0120552. GO molecular-function terms were taken from the human GOA records; Reactome ids/titles verified against the local cache. Each subunit uses a PANTHER family selector (generic over paralogs and orthologs) plus a concrete human representative member; BCAT2's family also lists the cytosolic paralog BCAT1 (UniProtKB:P54687). DLD is deliberately noted as the shared E3 of multiple 2-oxoacid dehydrogenase complexes, so its gene review and this module both flag that its role is not confined to BCAA catabolism. Downstream substrate-specific degradation of the branched-chain acyl-CoAs (e.g. IVD, MCCC, HMGCL for leucine; the propionyl-CoA route for isoleucine/valine) is out of scope for this module.

Connections

bcat2_step -> bckdh_complex_step Provides Input For
The branched-chain 2-oxo acids produced by BCAT2 transamination are the substrates oxidatively decarboxylated by the BCKDH complex.
Part 1: transamination (reversible entry step)
BCAA + 2-oxoglutarate to branched-chain 2-oxo acid + L-glutamateReactionbcat2_step

Annotons

BCAT2: branched-chain aminotransferase (mitochondrial)
bcat2_activity
Participant: Family: Branched-chain aminotransferase family (BCAT1/BCAT2)
Family:
Branched-chain aminotransferase family (BCAT1/BCAT2)PANTHER:PTHR11825
Representative Members: BCAT2 (human, mitochondrial)UniProtKB:O15382 BCAT1 (human, cytosolic paralog)UniProtKB:P54687

Function

branched-chain-amino-acid:2-oxoglutarate transaminase activityGO:0004084
Substrates: L-leucine / L-isoleucine / L-valine 2-oxoglutarate
Products: branched-chain 2-oxo acid (alpha-ketoisocaproate / alpha-keto-beta-methylvalerate / alpha-ketoisovalerate) L-glutamate

Locations

mitochondrial matrixGO:0005759

PLP-dependent, reversible first step committing BCAAs to catabolism by generating the branched-chain 2-oxo acids consumed by BCKDH. The mitochondrial isozyme BCAT2 is the ubiquitous form; the paralog BCAT1 is cytosolic and more tissue-restricted (e.g. brain, some tumours).

Part 2: committed, rate-limiting oxidative decarboxylation (multienzyme complex)
branched-chain 2-oxo acid + CoA + NAD+ to branched-chain acyl-CoA + CO2 + NADHProtein Complexbckdh_complex_step

Annotons

BCKDHA: BCKDH E1 alpha (decarboxylase)
bckdha_e1alpha
Participant: Family: BCKDH E1 alpha subunit family (BCKDHA)
Family:
BCKDH E1 alpha subunit family (BCKDHA)PANTHER:PTHR43380
Representative Members: BCKDHA (human)UniProtKB:P12694

Function

branched-chain 2-oxo acid dehydrogenase activityGO:0003863
Substrates: branched-chain 2-oxo acid thiamine diphosphate (cofactor)
Products: (decarboxylated, lipoyl-bound branched-chain acyl intermediate) carbon dioxide

Locations

mitochondrial matrixGO:0005759

TPP-dependent decarboxylase; with BCKDHB forms the alpha2-beta2 E1 heterotetramer that decarboxylates the BCKA and reductively transfers the acyl group to the E2 lipoyl arm. E1-alpha is the phosphoregulated subunit (BCKDK inhibits, PPM1K activates). MSUD type Ia.

BCKDHB: BCKDH E1 beta
bckdhb_e1beta
Participant: Family: BCKDH E1 beta subunit family (BCKDHB)
Family:
BCKDH E1 beta subunit family (BCKDHB)PANTHER:PTHR42980
Representative Members: BCKDHB (human)UniProtKB:P21953

Function

branched-chain 2-oxo acid dehydrogenase activityGO:0003863

Locations

mitochondrial matrixGO:0005759

The beta subunit of the E1 heterotetramer; completes the TPP-binding active site with E1-alpha. MSUD type Ib.

DBT: BCKDH E2 (dihydrolipoyl transacylase core)
dbt_e2
Participant: Family: 2-oxoacid dehydrogenase E2 (dihydrolipoyl acyltransferase) family (DBT)
Family:
2-oxoacid dehydrogenase E2 (dihydrolipoyl acyltransferase) family (DBT)PANTHER:PTHR43178
Representative Members: DBT (human)UniProtKB:P11182

Function

dihydrolipoamide branched chain acyltransferase activityGO:0043754
Substrates: S-(2-methylpropanoyl)-dihydrolipoyl intermediate coenzyme A
Products: branched-chain acyl-CoA

Locations

mitochondrial matrixGO:0005759

Lipoyl-bearing E2 subunit forming the structural (24-mer) core of the complex to which E1 and E3 dock; transfers the branched-chain acyl group from the lipoyl arm to CoA. MSUD type II (often thiamine-responsive).

DLD: shared E3 (dihydrolipoyl dehydrogenase)
dld_e3
Participant: Family: Dihydrolipoyl dehydrogenase / pyridine-nucleotide-disulphide oxidoreductase family (DLD)
Family:
Dihydrolipoyl dehydrogenase / pyridine-nucleotide-disulphide oxidoreductase family (DLD)PANTHER:PTHR22912
Representative Members: DLD (human)UniProtKB:P09622

Function

dihydrolipoyl dehydrogenase (NADH) activityGO:0004148
Substrates: dihydrolipoyl-E2 (reduced lipoyl arm) NAD+
Products: lipoyl-E2 (reoxidised) NADH

Locations

mitochondrial matrixGO:0005759

FAD-dependent E3 that reoxidises the dihydrolipoyl arm, transferring electrons to NAD+. NOT BCKDH-specific: the common E3 shared with the pyruvate, 2-oxoglutarate and 2-oxoadipate dehydrogenase complexes and the glycine cleavage system, which is why DLD deficiency produces a combined multi-complex phenotype.