Quantum Chemical Studies of the Hydrolysis of 8-6-Phosphogluconolactone to 6-Phosphogluconate

Abstract

The pentose phosphate pathway has an essential role for metabolic amino acid and nucleic acid synthesis across species. Glucose-6-phopshate dehydrogenase is the first enzyme of the pentose phosphate pathway that produces 8-6-phosphogluconolactone. The 5-6-phosphogluconolactone undergoes hydrolysis reaction to produce 6-phosphogluconate. The hydrolysis reaction of 8-6- phosphogluconolactone is either catalyzed by the 6-phosphogluconolactonase, the second metabolic enzyme of the oxidative pentose phosphate pathway or it can also occur spontaneously. The mechanism of hydrolysis by which 8-6-phosphogluconolactone is hydrolyzed into 6- phosphogluconate is only conjecturally understood. Here, by using quantum chemical computations at different levels of density functional theory, the reaction mechanisms of the spontaneous hydrolysis of 8-6-phosphogluconolactone to 6-phopshogluconate have been studied. Three different pathways of the hydrolysis mechanism of 8-6-phosphogluconolactone were found, i.e. a) concurrent mechanism in which the breaking of O-H bond of the attacking water molecule and the breaking of the C-O bond of the -6-phopshogluconolactone occur concurrently b) a sequential mechanism in which two event are separate and a single water molecule is involved during hydrolysis and c) a sequential mechanism in which two water molecules are involved. The rate-limiting energy barriers of the un-catalyzed hydrolysis via these mechanisms are 52.4, 45.7 and 31.6 kcal mol¹, respectively. For the sequential mechanism with two water molecules, the inclusion of solvation effects at the RB3LYP/SDD(pol)//RB3LYP/6-31+G*+ZPE level of theory results in the rate limiting energy barrier of 26.5 kcal mol-¹. This rate-limiting barrier is consistent with the experimental free energy barrier of 21.1 kcal mol-¹. A comparison of the mechanisms of 6-phosphogluconolactone with the most recent quantum chemical studies of hydrolysis of adenosine triphosphate in myosin and DNA cleavage in the restriction enzyme EcoRV is also carried out. Results clearly indicate that the sequential mechanism of hydrolysis that involves two water molecules is energetically more favorable as compared to the concurrent mechanism.