1. Pyruvate kinase of Alaskan king-crab leg muscle exists in two kinetically distinct forms, each of which displays a different temperature-dependence in the Km for phosphoenolpyruvate. 2. A ‘cold’ variant of the enzyme has hyperbolic kinetics and exhibits a minimal Km for substrate at 5°. At physiological concentrations of phosphoenolpyruvate the ‘cold’ enzyme is active only below 10°. A ‘warm’ pyruvate kinase has a minimal Km for substrate at about 12°. This enzyme displays sigmoidal kinetics and is likely to be inactive, at physiological substrate concentrations, at temperatures below 9°. 3. The combined activities of these two pyruvate kinases yield highly temperature-independent rates of catalysis, at physiological substrate concentrations, over the range of habitat temperatures encountered by the organism, namely 4–12°. 4. The two variants of pyruvate kinase do not appear to be isoenzymes in the conventional sense. Electrophoretic and electrofocus analyses revealed only single peaks of activity. 5. The results suggest that the ‘warm’ pyruvate kinase and the ‘cold’ pyruvate kinase are formed by a temperature-dependent interconversion of one protein species. This interconversion has major adaptive significance: as the temperature is lowered the ‘warm’ enzyme is converted into the ‘cold’ enzyme; the opposite situation obtains when the temperature is raised. Temperature changes thus mimic the effects noted for fructose 1,6-diphosphate on certain mammalian pyruvate kinases.

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