Structure of Thermus thermophilus HB8 aspartate aminotransferase and its complex with maleate

The three-dimensional structures of pyridoxal 5'-phosphate-type aspartate a minotransferase (AspAT) from Thermus thermophilus HB8 and pyridoxamine 5'-p hosphate type one in complex with maleate have been determined by X-ray cry stallography at 1.8 and 2.6 Angstrom resolution, respectively. The enzyme i s a homodimer, and the polypeptide chain of the subunit is folded into one arm, one small domain, and one large domain. AspATs from many species were classified into aminotransferase subgroups Ia and Ib. The enzyme belongs to subgroup Ib, its sequence being less than 16% identical to the primary seq uences of Escherichia coli, pig cytosolic, and chicken mitochondrial AspATs , which belong to subgroup Ia whose sequences are more than 40% identical a nd whose three-dimensional structures are quite similar with the active sit e residues almost completely conserved. The first X-ray analysis of AspAT s ubgroup Ib indicated that the overall and the active site structures are es sentially conserved between the AspATs of subgroup Ia and the enzyme of sub group Ib, but there are two distinct differences between them. (1) In AspAT subgroup Ia, substrate (or inhibitor) binding induces a large movement of the small domain as a whole to close the active site. However, in the enzym e of subgroup Ib, only the N-terminal region (Lys13-Val30) of the small dom ain approaches the active site to interact with the maleate. (2) In AspAT s ubgroup Ia, Arg292 recognizes the side chain carboxylate of the substrate; however, residue 292 of the enzyme in subgroup Ib is not Arg, and in place of Arg292, Lys109 forms a salt bridge with the side chain carboxylate. The thermostability of the enzyme is attained at least in part by the high cont ent of Pro residues in the beta-turns and the marked increase in the number of salt bridges on the molecular surface compared with the mesophilic AspA T.