Temperature modulation of the kinetics of phosphoglucose isomerase genetic variants from the sea anemone Metridium senile

The allozymes involved in a one‐locus, two‐allele polymorphism for phosphoglucose isomerase from populations of the sea anemone Metridium senile from the northeast coast of North America exhibit different heat stabilities. The electrophoretically slow form is more stable than the fast, whether or not the enzyme is protected by added 6‐phosphogluconate. In addition, Km of glucose‐6‐phosphate (G6P) values are significantly higher for the slow than for the fast allozyme, the reverse of the situation in the gluconeogenic direction. Km of G6P values for the mixture of isozymes produced by heterozygotes fall intermediate between those for the two homozygotes. All genotypes produce enzymes that have slightly U‐shaped Km vs. temperature curves; these patterns result in differences in thermal sensitivity of reaction rates at low substrate concentrations between high and low temperature ranges. The fast allozyme has higher Vmax values than the slow at all environmentally realistic temperatures. These kinetic differences in the glycolytic direction result in higher Vmax/Km ratios for the fast than for the slow allozyme at all tested temperatures, with the greatest absolute difference falling at 25°C; again, the heterozygote is intermediate. These kinetic differences may yield increased adaptation to thermal aspects of the environment.