| Component | Gene(s) | Subunit structure | Function in the complex | Key features |
|---|---|---|---|---|
| E1 | **BCKDHA**, **BCKDHB** | **α2β2 heterotetramer** | ThDP-dependent oxidative decarboxylation of branched-chain α-ketoacids (KIC, KMV, KIV); initiates transfer of the acyl group to the lipoyl domain of E2 | Active sites lie at **α–β′ interfaces**; human structure solved at **2.7 Å**; β subunit contributes to **ThDP binding**, **K+ coordination**, and **E2 interaction** (pqac-00000001, pqac-00000003, pqac-00000004, pqac-00000006, pqac-00000008) |
| E2 | **DBT** | **24-mer core** (branched-chain dihydrolipoamide acyltransferase scaffold) | Accepts the branched-chain acyl group from E1 via its lipoyl domain and catalyzes transacylation to CoA, forming branched-chain acyl-CoAs | Central architectural scaffold of the BCKDH complex; lipoyl domain shuttles reaction intermediates between active sites (pqac-00000001, pqac-00000021) |
| E3 | **DLD** | Homodimeric dihydrolipoamide dehydrogenase shared among multiple 2-oxoacid dehydrogenase complexes | Reoxidizes reduced lipoyl groups on E2 and transfers electrons to NAD+, generating NADH | **Shared with PDH and OGDH** complexes; common E3 module in mitochondrial 2-oxoacid dehydrogenase systems (pqac-00000001, pqac-00000021) |
| BCKDK | **BCKDK** | Regulatory kinase associated with the BCKDH complex | **Phosphorylates E1α** to inhibit BCKDH activity | Major negative regulator of BCAA oxidation; inhibited allosterically by BCKAs such as **KIC**; pharmacologic inhibition activates BCKDH (pqac-00000019, pqac-00000020, pqac-00000022) |
| PPM1K | **PPM1K** | Mitochondrial PP2C-family phosphatase | **Dephosphorylates E1α** to reactivate BCKDH | Also called **PP2Cm**; opposes BCKDK to maintain BCAA/BCKA homeostasis and BCKDH flux (pqac-00000018, pqac-00000019, pqac-00000020, pqac-00000022) |


*Table: This table summarizes the catalytic and regulatory composition of the human BCKDH system, highlighting where BCKDHB fits within the E1 decarboxylase module. It is useful for functional annotation because it distinguishes the core enzyme subunits from the kinase/phosphatase regulators that control pathway activity.*