The catalytic effect of molybdate on the hydrolysis of organic phosphate bonds

Abstract

The catalytic effect of molybdate on the hydrolysis of a number of phosphate compounds was investigated. The effect increased with increasing concn. of molybdate, though not in a strictly linear fashion, and also exponentially with temp. Evaluation of the thermal data indicated a lowering of the activation energy and of the temp.-independent factor by molybdate. The molybdate effect might have a well pronounced pH optimum varying from compound to compound. In 2 compounds hydrolysis was inhibited by molybdate at the acid end of the pH scale. For the substances investigated,the ratios of the hydrolysis constants in presence and in absence of molybdate, fell into 2 groups. The 1st consisted of acetyl phosphate, creatine phosphate and the terminal phosphate group of adenosinetriphosphate (ATP), which showed ratios 20 to 100 times higher than those of the 2d group, which comprised the other substances studied. The differences of the ratios within these 2 groups were relatively small. A catalytic effect of molybdate was demonstrated in all the compounds tested. Although the terminal and intermediate phosphate groups of ATP were hydrolyzed at the same rate by acid, their hydrolysis constants in the presence of molybdate differed by a factor of 10. No catalytic activity was found when molybdate was replaced by tungstate or vanadate. It was suggested that the catalytic effect of molybdate involved the formation of an unstable transition complex between catalyst and substrate. The biological and analytical implications of the effect were discussed.