| Step number | Enzyme | Reaction | Substrates | Products | Compartment | Regulatory mechanism |
|---|---|---|---|---|---|---|
| 1 | **BCAT2 (branched-chain amino acid aminotransferase, mitochondrial)** | Reversible PLP-dependent transamination of branched-chain amino acids to branched-chain α-keto acids (BCKAs) | Leucine, isoleucine, valine + α-ketoglutarate | 2-ketoisocaproate (KIC), 2-keto-3-methylvalerate (KMV), 2-ketoisovalerate (KIV) + glutamate | Mitochondria, especially in extrahepatic tissues such as skeletal muscle | Requires **PLP** cofactor; activity is redox-sensitive via the **CXXC motif (C315/C318)**, where oxidation decreases activity and alters substrate affinity; BCAT2 can physically associate with BCKDH to support substrate channeling; BCAT2 stability is regulated by **K44 acetylation**—**CBP** promotes acetylation and ubiquitin-proteasome degradation, whereas **SIRT4** deacetylates and stabilizes BCAT2 (pqac-00000000, pqac-00000001, pqac-00000012, pqac-00000014, pqac-00000033, pqac-00000034, pqac-00000036) |
| 2 | **BCKDH complex** (branched-chain α-ketoacid dehydrogenase; E1/E2/E3) | Oxidative decarboxylation of BCKAs to branched-chain acyl-CoA derivatives; committed/rate-limiting step of BCAA oxidation | KIC, KMV, KIV | Isovaleryl-CoA, 2-methylbutyryl-CoA, isobutyryl-CoA + reducing equivalents | Mitochondrial matrix | Activity is inhibited by **BCKDK-mediated phosphorylation** and activated by **PPM1K-mediated dephosphorylation**; BCAT2 interaction with BCKDH supports metabolon formation and efficient transfer of BCKA products; BCAT2 loss can abolish effective BCKDH function in this pathway context (pqac-00000015, pqac-00000016, pqac-00000018, pqac-00000019, pqac-00000020, pqac-00000058) |
| 3 | **Downstream BCAA oxidation enzymes** (multiple acyl-CoA dehydrogenation/hydration/cleavage steps) | Conversion of branched-chain acyl-CoA intermediates to central carbon metabolites that feed the TCA cycle | Isovaleryl-CoA, 2-methylbutyryl-CoA, isobutyryl-CoA and subsequent intermediates | Acetyl-CoA and/or succinyl-CoA (depending on BCAA species), which enter the TCA cycle for energy metabolism and biosynthesis | Mitochondria | Flux through this stage depends on upstream BCAT2/BCKDH activity and whole-pathway control of BCAA catabolism; impaired upstream catabolism contributes to BCAA/BCKA accumulation linked to metabolic disease, while intact downstream oxidation supports mitochondrial energy production and metabolic flexibility (pqac-00000016, pqac-00000018, pqac-00000020, pqac-00000051, pqac-00000054) |


*Table: This table summarizes the core mitochondrial steps of branched-chain amino acid catabolism involving BCAT2, from transamination through BCKDH-mediated oxidation to TCA cycle entry. It also highlights the main regulatory layers controlling pathway flux, including redox regulation, post-translational control of BCAT2, and BCKDH phosphorylation status.*