Donor Interactions at Phosphorus. Implications Regarding Enzymatic Reaction Intermediates

Abstract

In recent work on the biological relevance of hypervalent phosphorus compounds, a new aspect of active site interactions of phosphoryl transfer enzymes undergoing nucleophilic attack is suggested by structural studies on phosphites, phosphates, and oxyphosphoranes which are shown to interact with donor groups to give higher coordinate geometries. The degree of coordination increases from phosphate to pentaoxyphosphoranes which model substrates and active site transition states, respectively. Thus a rate enhancement effect is anticipated due to stronger enzyme binding in the transition state-enzyme complex. The studies suggest that donor interactions at applicable active sites may assist in nucleophilic attack in causing a general loosening of P-O bonds undergoing cleavage to form products via a hexacoordinate transition state. Previously, only pentacoordinate intermediates have been invoked in nucleophilic displacement reactions of phosphoryl transfer enzymes. The results are illustrated with reference to the tyrosyl-tRNA synthetase system. Earlier, application to the enzymatic hydrolysis of cAMP was made.