Catalyzes the first reaction in the catabolism of the essential branched chain amino acids leucine, isoleucine, and valine (PubMed:17050531, PubMed:25653144, PubMed:8702755). Branched chain amino acid catabolism plays a role in adipocyte differentiation by providing lipogenic acetyl-CoA pools in differentiated adipocytes (By similarity). Mechanistically, acetyl-CoA derived from branched chain amino acid catabolism is used by EP300/p300 to acetylate and inhibit PRDM16, thereby preventing adipose tissue browning (By similarity). May also function as a transporter of branched chain alpha-keto acids (By similarity). {ECO:0000250|UniProtKB:O35854, ECO:0000250|UniProtKB:O35855, ECO:0000269|PubMed:17050531, ECO:0000269|PubMed:25653144, ECO:0000269|PubMed:8702755}. This is the function of ENSG00000105552 (BCAT2, branched chain amino acid transaminase 2).