Solvent isotope effects on the reaction catalyzed by yeast hexokinase

Double enzyme-catalyzed microreactors using capillary electrophoresis

This work evaluates the concept of a double enzyme-catalyzed microreactor using capillary electrophoresis (CE). Migrating in a capillary under electrophoresis conditions, plugs of substrate and two enzymes are injected separately in buffer and allowed to react. Extent of reaction and product ratios were subsequently determined by CE. This concept is demonstrated using two model systems: the conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and adenosine monophosphate (AMP) by hexokinase (HK, EC 2.7.1.1) and apyrase (APY, EC 3.6.1.5), respectively, in the conversion of glucose to glucose-6-phosphate and inorganic phosphate, respectively, and the conversion of nicotinamide adenine dinucleotide, reduced form (NADH), to nicotinamide adenine dinucleotide (NAD) and back to NADH by lactate dehydrogenase (LDH, EC 1.1.1.27) and glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), respectively, in the conversion of pyruvate to lactate and glucose-6-phosphate (glc-6-P) to 6-phosphogluconate, respectively. These procedures illustrate the use of the capillary as a double microreactor and the ease of quantitation of reaction products under conditions of electrophoresis.

Solvent isotope effects on the glucokinase reaction. Negative co-operativity and a large inverse isotope effect in 2H2O

The solvent isotope effects on the reaction catalysed by rat-liver glucokinase have been studied. At low concentrations of glucose and high concentrations of MgATP2- there is an inverse solvent isotope effect of 3.5. At high glucose concentrations there is a normal solvent isotope effect of 1.3. In 1H2O there is positive co-operativity with respect to glucose [ Storer , A.C. and Cornish - Bowden , A. (1976) Biochem. J. 159, 7-14], but this is changed to negative co-operativity in 2H2O. The half-saturation points for both glucose and MgATP2- are decreased in 2H2O compared with those in 1H2O. Explanations of these effects in terms of the mnemonical model proposed by Storer and Cornish - Bowden [Biochem. J. 65, 61-69 (1977)] were considered in computer simulation. Two interpretations could account for the results, either a decrease in the rate of interconversion of the two forms of free enzyme postulated in the model, or an increase in the affinity for glucose of the enzyme form with the lower affinity in 1H2O. The results of a proton-inventory analysis were consistent with either of these interpretations. The solvent isotope effects thus provide additional evidence for the mnemonical model as an explanation of glucokinase co-operativity.