Abstract
The A-isozyme of O-acetylserine sulfhydrylase, a pyridoxal phosphate-dependent enzyme isolated from Salmonella typhimurium catalyzes the synthesis of l-cysteine from O -acetyl-l-serine and sulfide. The pyridoxal form of the enzyme has been crystallized in two different forms. One form is in the orthorhombic space group P212121 with cell constants a = 144·4 Å, b = 96·9 Å and c = 54·3 Å and contains two monomers each of molecular weight 34,000 per asymmetric unit. The second form is in a hexagonal space group with unit cell dimensions a = b = 115 Å, and c = 348 Å and contains two 68,000 dimers per asymmetric unit. Complete native enzyme data sets have been collected for both crystal forms using an R-Axis II detector. A search for suitable heavy-atom derivatives is underway. Although both crystal forms diffract X-rays to better than 2·5 Å, the orthorhombic form is more suited to a detailed structural analysis due to the extended lifetime in the X-ray beam and the relative size of the unit cell.