CRYSTAL-STRUCTURE OF THE REDUCED SCHIFF-BASE INTERMEDIATE COMPLEX OF TRANSALDOLASE-B FROM ESCHERICHIA-COLI - MECHANISTIC IMPLICATIONS FOR CLASS-I ALDOLASES

Transaldolase catalyzes transfer of a dihydroxyacetone moiety from a k etose donor to an aldose acceptor. During catalysis, a Schiff-base int ermediate between dihydroxyacetone and the epsilon-amino group of a ly sine residue at the active site of the enzyme is formed. This Schiff-b ase intermediate has been trapped by reduction with potassium borohydr ide, and the crystal structure of this complex has been determined at 2.2 Angstrom resolution. The overall structures of the complex and the native enzyme are very similar, formation of the intermediate induces no large conformational changes. The dihydroxyacetone moiety is coval ently linked to the side chain of Lys 132 at the active site of the en zyme. The C1 hydroxyl group of the dihydroxyacetone moiety forms hydro gen bonds to the side chains of residues Asn 154 and Ser 176. The C3 h ydroxyl group interacts with the side chain of Asp 17 and Asn 35. Base d on the crystal structure of this complex a reaction mechanism for tr ansaldolase is proposed.