Influence of pH on the regulatory properties of aerobic and anoxic forms of pyruvate kinase in a marine whelk

Abstract

AbstractThe effects of pH on the kinetic properties of pyruvate kinase were determined for both the aerobic and anoxic (16 hours exposure to N2‐bubbled seawater) enzyme forms from foot muscle and gill of the marine whelk, Busycotypus canaliculatum. The two enzyme forms from each tissue had distinctly different properties; compared to the aerobic enzyme, the anoxic enzyme form had significantly increased values for S0.5 phosphoenolpyruvate and Ka fructose‐1,6‐bisphosphate and greatly reduced I50 values for L‐alanine. Effects of pH on kinetic constants were assessed at pH 6.6, 7.0, and 7.5. Both the aerobic and anoxic forms of pyruvate kinase in whelk foot showed minimal values for S0.5 PEP and Ka FBP at pH 7.0; these rose by up to twofold at the higher or lower pH values. I50 values for alanine and Mg.ATP were strongly affected by low pH; I50 Mg.ATP fell by tenfold at pH 6.6 compared to pH 7.0, whereas I50 ALA dropped eightfold for the aerobic and 2.5‐fold for the anoxic enzyme form over the same pH change. The qualitative effects of pH change were similar for the anoxic enzyme form from gill. PK‐aerobic from gill behaved differently, however; as pH decreased, S0.5 PEP increased, whereas Ka FBP and I50 ALA both decreased. Furthermore, inhibition by Mg‐ATP occurred only at low pH. Although anoxia‐induced phosphorylation of PK is clearly the primary mechanism for changing the activity state of PK during anaerobiosis, the results suggest that the progressive fall in intracellular pH that occurs during anoxia would enhance the inactivation of PK activity in whelk organs. In particular, the effect of low pH in increasing the sensitivity of the anoxic enzyme forms to inhibitors could facilitate the anoxia‐induced changes in carbon flow at the phosphoenolpyruvate branchpoint.