ENZYMATIC MECHANISMS OF SULFUR-NUCLEOSIDE METABOLISM

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The sulfur containing adenine nucleoside derivatives S-adenosylmethionine and adenosine-5'-phosphosulfate play essential roles in the metabolsim of both eukaryotic and prokaryotic cells. The objectives of this research are to characterize the catalytic mechanisms and active site structures of enzymes which catalyze unique biosynthetic reactions involving these metabolites. Studies of S-adenosylmethionine (AdoMet) synthetase (ATP:L-methionine S-adenosyltransferase) will determine which amino acid residues are important in substrate binding and catalysis. Photoaffinity labelling of the enzyme and in vitro mutagenesis of the cloned structural gene for the enzyme will be used. The ligands to the protein-bound divalent metal ion and the structures of the complexes with the two metal ions bound in enzyme-substrate complexes will be determined by EPR spectroscopy. The studies of S-adenosylmethionine decarboxylase will determine the partitioning of enzyme among complexes with substrate and product in the steady state of the reaction. The protonation states of the Schiff bases formed with substrate and product will be characterized by 15 N NMR. The molecular basis for inhibition by the chemotherapeutic agent methylglyoxal bis (guanylhydrazone) will be investigated. Whether the required divalent metal ion is an allosteric activator or if it binds at the active site will be determined. The studies of ATP sulfurylase (ATP:sulfate adenylyltransferase) will determine whether the protein binds a divalent metal ion activator, in addition to the metal-ATP complex. The equilibrium constant and interconversion rates for enzyme-bound reactants will be measured in order to determine whether the equilibrium constant is displaced toward unity from the value of 10 to the minus 8 for free reactants. The studies of adenosine-5'-phosphosulfate kinase (ATP:adenylylsulfate-3'-phosphotransferase) will investigate whether a divalent metal-adenosine-5'-phosphosulfate complex, as well as a divalent metal-ATP complex, is substrate. The possible requirement for a monovalent cation activator will be assessed.