The regulation of glycolysis in the hepatopancreas of the sea mussel Mytilus edulis L.

Temperature-related kinetic differentiation of glucosephosphate isomerase alleloenzymes isolated from the blue mussel,Mytilus edulis

“…First, the Km values for both substrates are close to the apparent in vivo levels, implying that GPI is approximately half-saturated. Second, the concentration ratios of fructose 6-phosphate to glucose 6-phosphate are very similar to the equilibrium constant (Koq) for the reaction (Cameselle et al, 1980;Beis and Newsholme, 1975;Ebberink and deZwaan, 1980)--an indication that GPI catalyzes a reaction that is essentially at equilibrium. Third, acclimation to colder physiological temperatures (4°C) leads to a significant increase (P < 0.05) in substrate concentrations (see Cameselle et al, 1980; Table III).…”

“…This is in agreement with observations of mass action ratios (Table III). Figure 10 shows the results for the physiologically most likely set of conditions, where the intracellular pH varies in accord with the pKa of imidazole (Somero, 1981) and in vivo substrate levels increase with decreasing body temperature (Cameselle et al, 1980). The substrate levels for 4 and 15°C in Table III are extrapolated linearly to obtain estimated in vivo substrate concentrations for other temperatures.…”

“…Fortunately, estimates of in vivo substrate concentrations are available for Mytilus edulis (Table III). The concentrations are calculated from the published data, which were calculated as nanomoles per gram wet weight, by assuming an 85% tissue water content and even distribution of these metabolites within the cell (Cameselle et al, 1980). Several conclusions may be drawn from these data.…”

“…Catalytic efficiencies (y-l) for GPI 1°° and GPI ' 96 are calculated at each experimental pH and temperature condition for specified substrate levels (standard states) as described below by substituting kinetic parameter values Cameselle et al (1980). bData from Beis and Newsholme (1975).…”

“…The enzyme is a dimer of two identical subunits and catalyzes the interconversion of fructose 6-phosphate (F6P) and glucose 6-phosphate (G6P). The fact that mass action ratios (i.e., [F6P]/[G6P]) for virtually every species that has been examined are near the equilibrium constant (K~q = 0.3) for the reaction has led most workers to conclude that GPI provides primarily a coupling function between the glucose 6-phosphate branch point and phosphofructokinase/fructose 1,6-bisphosphatase in glycolysis and gluconeogenesis (e.g., Newsholme and Start, 1973;Beis and Newsholme, 1975;Cameselle et al, 1980;Atkinson, 1977). Although several pentose phosphate shunt intermediates, including 6-phosphogluconate, are known competitive inhibitors of GPI, their role in exerting a regulatory influence upon glycolytic flux has yet to be established (Atkinson, 1977;Palumbi et al, 1980;Eanes, 1984).…”