is being developed for molecular mechanisms for activity of carboxypeptidase A, and of both activity and allosteric regulation in aspartate transcarbamylase. On carboxypeptidase A, we hope to obtain a single crystal at minus 40 degrees C of the anhydride intermediate in ester hydrolysis, to obtain X-ray diffraction data on this and many other derivatives associated with activation, inhibition, metal substitution for Zn ion 2 at the active site, and binding modes of substrates and their analogues. Further studies of the binding of the 39 amino acid inhibitor of the potato to carboxypeptidase A are under way. On asparate transcarbamylase, we expect to solve the three-dimensional structure to 2.8 A resolution of the complex of this enzyme with PALA (N-phosphonacetyl-L-aspartate), a substrate analogue in use for treatment of human tumors. Comparison of this R(active) structure with our newly refined structures of the T(inactive) form should indicate the nature of the allosteric conformational change. Chemical modifications and mutants will be studied by X-ray diffraction and biochemical methods to elucidate the structure functional relationships. Theoretical studies of enzyme reaction mechanisms, and X-ray diffraction studies of smaller biochemically important molecules (beta-amanitin, Li chelates, etc.) will also be carried out.
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