Crystal structures of the phosphorylation mimics of human cytosolic branched chain aminotransferase

The phosphorylation sites of the human cytosolic Branched Chain Aminotransferase (hBCATc) mediated by mitogen-activated protein kinase (MAPK)/extracellular-signal-regulated-kinase 2 (ERK2, also known as MAPK1) were mapped. The crystal structures of the phosphorylation mimics at T33 and T36 were determined. The modified transaminase activity of these variants was analyzed. Although there were no major conformational changes in the phosphorylation mimics of hBCAT, a regional conformational change at the interdomain loop was observed mainly in mutant T33E. Consistently, when the catalytic turnovers of the T33E and T36E mutants were comparable to the wild type of hBCATc, the K_M dropped significantly compared to the wild type, indicating a shift of the substrate binding affinity in the mutants. Taken together, this indicated the phosphorylation of hBCATc by ERK2 is affecting the hBCATc's transaminase activity.