THREE-DIMENSIONAL STRUCTURE OF O-ACETYLSERINE SULFHYDRYLASE FROM SALMONELLA TYPHIMURIUM. P. Burkhard*, E. Hohenester*, G.S.J. Rao#, P.F. Cook# and J.N. Jansonius*. *Department of Structural Biology, Biozentrum, University of Basel, Switzerland. #Department of Biochemistry, The University of Texas Southwestern Medical Center, Forth Worth, Texas, U.S.A

The A-isozyme of O-acetylserine sulihydrylase (OASS), an [[alpha]]-dimeric pyridoxal 5'-phosphate-dependent enzyme isolated from Salmonella typhimurium catalyses the synthesis of L-cysteine from O-acetyl-L-serine l and sulfide. The pyridoxal form of the enzyme has been crystallized in the ortho-rhombic space group P2_l2₁2₁ with cell constants a=54.3 Å, b=96.9 Å and c=144.4 Ź). The crystals diffract to 2.3 Å and contain one dimer per asymmetric unit. The subunit molecular weight is 34000.

The structure has been solved by MIRAS-phasing of six heavy atom derivatives and refinement is underway (current R-factor is 22% at 2.7 Å) OASS has a sequence similarity of about 30% to tryptophan synthase-ß (TRPSß) but less than 20% of the residues are identical. Both enzymes have the same fold, but there are some major differences: The interface to the [[alpha]]- subunit in TRPSß (residues ß9-ß22 and ß275-ß295) is missing in OASS. There are two additional surface helices in TRPSß (residues ß23-ß54) and an additional loop (ß260-ß266). The active site cleft of OASS is wider and therefore more exposed to the solvent. The hydrophobic channel for indole transport from the [[alpha]] to the ß active site is, not unexpectedly, missing in OASS. The dimer interface, however, is more or less conserved in the two enzymes.

The only cysteine residue of OASS (which is the residue following the active site Iysine in the sequence) cannot be directly involved in the reaction mechanism, since it is completely buried and more than 10 A away from the PLP cofactor. Further investigations on substrate binding and possible reaction mechanisms are planned.

1) Crystallization and Preliminary X-ray Data for the A-Isozyme of O-Acetylserine Sulfhydrylase from Salmonella typhimurium, Rao et al., JMB 231, 1130-1132 (1993)