Section of Catalysis: MODEL REACTIONS FOR CATALYSIS OF PHOSPHATE AND SULFATE TRANSFER*

Abstract

Phosphate-and sulfate-transfer reactions are found in a great variety of metabolic and biosynthetic pathways essential for the survival of the organism. Our knowledge of the detailed molecular mechanisms for these enzymecatalyzed group transfers in most cases is superficial, although a more complete visualization of the molecular events for several of these enzymes, e.g., pancreatic ribonuclease' and Escherichiu coli alkaline phosphatase,? is beginning to emerge. In these latter cases, much of the proposed mechanism is founded on results obtained in model systems, i.e., reactions designed to simulate the potential pathways available to a reactive enzyme-substrate complex. This paper documents some of our efforts aimed at elucidating possible operative mechanism €or enzymic catalysis of phosphate-and sulfate-group transfer.Numerous physical-organic studies of the hydrolytic lability of phosphate monoester monoanions have led to the postulation of a unique mechanism involving pre-equilibrium zwitterion formation followed by rate-determining expulsion of a hypothetical species, monomeric metaphosphate.2 Only in the case of a superior leaving group such as 2,4-dinitrophenol does proton transfer to form the zwitterion become partially rate-determining, as suggested by the incursion of a deuterium solvent isotope effect (klrlo/k,,20 = 1.4) and the negative deviation of the rate of hydrolysis from a linear free-energy relationship (log k,. vs. pK, of the alcohol).:' The highly reactive monomeric metaphosphate that is generated ( EQ. l ) undergoes rapid hydrolysis to phosphoric acid, with the overall result being a net transfer of a phosphoryl moiety [PO,-].One can anticipate from the mechanism in EQ. I , which requires the conversion of a potential alkoxide to the neutral-alcohol leaving group, that certain phosphate monoestcr dianions also will be hydrolytically labile provided a more acidic leaving alkoxide moiety is present. Nitro group stabilization of the *This paper was presented at a meeting of the Section on November 13, 1969. 330

Enzyme Catalyzed Hydrolysis of Inorganic Pyrophosphate

“…Nonenzymatic hydrolysis is catalyzed by acid [16] via an unstable intermediate, metaphosphate [48] ; the process is thus similar to the mechanism suggested for nonenzymatic hydrolysis of phosphomonoester anions [11], Above pH 4, in neutral and in alkaline solution, PPi is stable [116].…”