Mechanisms of phosphoryl and acyl transfer

Acyl and phosphoryl transfer are important biochemical reactions. We have been using isotope effects caused by O-18, N-15, C-13, and deuterium substitution to examine the mechanisms and transition-state structures for enzymatic and nonenzymatic transfers of phosphoryl and acyl groups. Phosphoryl transfers from phosphate monoesters are highly dissociative, although not truly stepwise in protic solvents or in enzymatic reactions. Phosphodiesters show ANDN (SN2) reactions, whereas triester hydrolyses involve more associative transition states. Except under acidic conditions, true phosphorane intermediates likely form only when geometry requires (i.e., when the leaving group cannot be axial until pseudorotation of the phosphorane). Enzymatic phosphoryl transfers appear similar to nonenzymatic ones. The reactions of oxygen or sulfur nucleophiles with p-nitrophenyl acetate are concerted with a tetrahedral transition state, which is more dissociative with sulfur than with oxygen. Enzymatic hydrolyses of p-nitrophenyl acetate are also concerted reactions.