This chapter will review the various mechanisms of temperature-dependent enzyme regulation, focusing on cold ocean invertebrates and crustaceans in particular. Temperature is one of the most important abiotic factors affecting physiological processes in marine poikilotherms (e.g. Bullock, 1955, Prosser, 1967; Hochachka & Somero, 1984). The distribution of species and hence the survival of organisms is often closely related to the ambient temperature regime (e.g. Dunbar, 1968; Somero, 1975). Therefore, complex metabolic processes must be adapted to the specific environment to ensure survival even under adverse cold conditions. Since the polar oceans comprise 20% of the world's ocean surface, and since the deeper ocean waters are usually colder than 5°C (Clarke, 1983), cold water is the norm rather than the exception for marine life. Even in the deep-sea, physiological adaptations are mainly determined by low temperatures associated with declining depth, not by high pressure (e.g. Teal & Carey, 1967; Childress et al., 1990). Low temperatures are disadvantageous to physiological processes due to the lower kinetic energy available, and the maintenance of metabolism at low temperatures requires effective biocatalysts, i.e. enzymes with high activities. Consequently, changes in enzyme characteristics or the production of new isoforms contribute to an appropriate temperature adaptation. In addition, enzymes must be regulated to avoid uncontrolled turnover of metabolites. This can be accomplished through feed-back control by metabolic end products (Fersht, 1985).

Mechanisms of biochemical adaptation

In general, metabolic temperature adaptation can be achieved in two different ways: The organism can either reduce metabolism to save energy in the cold (Clarke, 1983) or metabolic activity is increased at low temperatures, compensating for rate limiting effects and maintaining metabolism at an almost constant level (Hochachka & Somero, 1971,1973).