Pressure effects on catalysis and control of catalysis by liver fructose diphosphatase from an off-shore benthic fish

Abstract

SYNOPSIS: At low temperature (2°C), in the absence of FDP and Mg2+, the enzyme fructose disphosphatase (FDPase), extracted from the liver of an off-shore benthic Coryphaenoides species, is inactivated by exposures to relatively low pressures. The substrate, FDP, and the cofactor, Mg2+, protect against this inactivation, so that catalysis per se is not retarded by pressure. In contrast, at alkaline pH, pressure dramatically accelerates the catalytic rate when FDP and Mg2+ are saturating. The volume change of activation, ΔV*, for Coryphaenoides FDPase under these conditions is about -40 cm3/mole. At low concentrations of FDP and saturating concentrations of cofactor, the reaction rate at alkaline pH is pressure-independent. Similarly, at low concentrations of Mg2+ but saturating concentration of FDP, the reaction rate is pressure-independent. The Km for FDP does not change measureably with pressure, while the Ka for Mg2+ increases slightly with pressure. Under conditions of low (probable physiological) FDP and Mg2+ concentrations, it is evident that the reaction rate is determined by the kinetic characteristics of the enzyme and not by its energy-volume relationships, a situation which would appear to be of functional and selective significance to an organism living under constantly high hydrostatic pressure. AMP is a potent specific inhibitor of Coryphaenoides FDPase. The K4 for AMP is essentially pressure-independent both at neutral and alkaline pH, suggesting that efficiency of AMP control of this enzyme is comparable at all pressures likely to be encountered in nature. © 1971 by the American Society of Zoologists.