3-DIMENSIONAL STRUCTURE OF ESCHERICHIA-COLI BRANCHED-CHAIN AMINO-ACIDAMINOTRANSFERASE AT 2.5 ANGSTROM RESOLUTION

The X-ray crystallographic structure of the branched-chain amino acid aminotransferase from Escherichia coli was determined by means of isom orphous replacement using the selenomethionyl enzyme as one of the hea vy atom derivatives, The enzyme is a homo hexamer with D-3 symmetry, a nd the polypeptide chain of the subunit is folded into two domains (sm all and large domains), The coenzyme, pyridoxal 5'-phosphate, resides at the domain interface, its re-face facing toward the protein, The ac tive site structure shows that the following sites can recognize branc hed-chain amino acids and glutamate as substrates: (1) a hydrophobic c ore formed by Phe36, Tyr164, Tyr31, and Val109* for a branched-chain; (2) Arg97 for an acidic side chain of glutamate; and (3) Tyr95 and tw o main chain NH groups of Thr257 and Ala258 for the alpha-carboxylate of substrates, Although the main chain conformation of the active site is homologous to that of D-amino acid aminotransferase, many of the a ctive site residues are different between them.