Dr. Honzatko proposes to carry out x-ray crystallographic investigations which will reveal the structural basis for the mechanisms of catalysis and inhibition of adenylosuccinate synthetase from Escherichia coli, the enzyme which governs the first committed step in the de novo synthesis of adenine nucleotides. The synthetase from E. coli may be the first enzyme of the purine biosynthetic pathway for which a high resolution crystal structure will be available. Furthermore, the synthetase is a prime example of an important class of enzme involved in ternary reactions. Indeed, adenylosuccinate synthetase is a guanosine triphosphate (GTP) hydrolyase which is either distantly related to GTP binding proteins of mammals or represents an altogehter distinct class of GTP hydrolyses. Two different crystal forms of adenlylosuccinate synthetase have been obtained and are under investigation . The complex of the enzyme with succinate and the complex with succinate, inosine monophosphate and magnesium guanosine diphosphate. Studies of these two crystal forms will reveal the details of substrate-enzyme interactions and the conformational changes induced in the synthetase by the binding of substrates and substrate analogues. Additional structural investigations are planned as well, specifically of the complex of the antibiotic hadacidin with the first crystalline form and several mutant forms of the enzyme.
