Enzymatic consequences under alphastat regulation

Abstract

The importance of conserving the fractional protonation state of histidyl imidazole groups (alphastat pH regulation) is discussed for key structural and functional properties of proteins. For enzymes like lactate dehydrogenase (LDH) which have active site histidyl residues involved in the binding and catalytic events, the advantages of alphastat regulation are seen in the conservation of binding ability (as estimated by apparent Michaelis constants), of catalytic reserve capacity (the ability to respond effectively to changes in substrate concentration) and of reaction reversibility. For enzymes which depend on histidyl residues for their integrity of subunit assembly, e.g., phosphofructokinase (PFK), alphastat regulation ensures the maintenance of the active, assembled structure under different temperatures, and the regulatory responsiveness of the protein is also conserved. These advantages to enzymes of alphastat regulation during changes in body temperature are seen both in short-term changes in body temperature and in evolutionary adaptation to temperature.From the observed effects of changes in temperature and pH on enzyme systems studied in vitro, predictions are made about the effects of alphastat and pHstat (constant pH at all temperatures) regimens on more complex, physiological systems. These predictions, e.g., of alphastat and pHstat effects on total glycolytic flux and partitioning of glucose carbon between lactate and pyruvate, are compared with existing data from studies of whole organisms or isolated, perfused organs. The predictions from enzymatic studies are largely supported, and the critical importance of alphastat regulation in the maintenance of organ function under hypothermic conditions is apparent.The benefits of alphastat regulation are also discussed in the contexts of maintaining pH gradients across the inner mitochondrial membrane, of keeping the activities of pH-sensitive enzymes near optimal pH values, and of maintaining maximal buffering capacities. Lastly, the activation of several physiological systems by eliminating an acidotic state and restoring an alphastat pH value is presented to show the widespread significance, in both natural and clinical settings, of alphastat regulation for metabolic function.