Crystal structure of the phosphomimetic variant of the brain-specific cytosolic branched chain aminotransferase

This study presents the crystal structure of the phosphomimetic variant of human cytosolic branched-chain aminotransferase protein (BCATc) phosphorylated by PKC⍺. Here, we identify S123 as a key phosphorylation site, located near the pyridoxal 5′-phosphate (PLP) cofactor that is important for optimal transamination. The phosphomimetic variant BCAT, S123D, exhibited a significant reduction in transaminase activity compared with the wild type protein. Structural analysis of the S123D variant revealed a conformational change in the interdomain loop, resulting in a positional shift that promotes additional hydrogen bond formation and alters substrate binding. This structural rearrangement leads to a decreased kcat supporting a functional role for phosphorylation in regulating BCATc transamination activity and its associated moonlighting functions.