Enzymic Mechanisms of Temperature Compensation: Immediate and Evolutionary Effects of Temperature on Enzymes of Aquatic Poikilotherms

Immediate and evolutionary effects of temperature on enzymes from several Arctic marine poikilotherms have been examined. The results of these studies have been compared with existing information on temperature effects on enzymes of warm- and cold-blooded animals. The following conclusions have been drawn about temperature effects on enzymes of aquatic poikilotherms: 1. Immediate compensation to temperature changes is promoted by an inverse relationship between temperature and enzyme-substrate affinity (as measured by the reciprocal of the Km of substrate). Activation energy is not an important parameter in immediate compensation. 2. The sharp decreases in enzyme-substrate affinity which frequently occur at the extremes of an organism's habitat temperature may be important in establishing thermal tolerance limits for the organism. 3. In evolutionary adaptation of enzymes to temperature, minimal Km values (maximal enzyme-substrate affinity) normally occur near the lower extreme of a species' adaptation temperature. However, evolutionary adaptation of enzymic rates does not appear to be effected by low absolute values of Km. The evolutionary Km changes observed are directed primarily toward the maintenance of control function. 4. Rate compensation to temperature, on an evolutionary time scale, is accomplished by activation energy changes, at least in the case of some enzymes.