Temperature is a dominant factor in governing the distribution patterns of ectothermic marine organisms (Fields

distribution patterns of ectothermic marine organisms (Fields et al., 1993; Barry et al., 1995; Hughes, 2000). Wholeorganism thermal tolerance limits closely reflect differences in habitat temperature that result from different latitudinal and vertical distribution patterns (Vernberg and Tashian, 1959; Edney, 1961; Stillman and Somero, 1996; Stillman and Somero, 2000; Tomanek and Somero, 2000). Thermal tolerance limits of organisms are established by a combination of morphological, physiological and biochemical traits, including thermal sensitivities of enzymatic and structural proteins (Alexandrov, 1977; Hochachka and Somero, 1984; Cossins and Bowler, 1987; Somero, 1995; Somero, 1997). The need for proteins to undergo conformational changes during function, e.g. during rate-limiting steps in catalysis (Dunn et al., 1991), requires that proteins evolve a high degree of structural flexibility. Thus, net stabilization free energies of proteins are typically of the order of the energy contained in a few non-covalent bonds, which render them highly sensitive to thermal perturbation (Jaenicke, 1991). Studies of orthologous homologs (orthologs) of proteins have shown that functional properties, e.g. Michaelis–Menten constants (Km=binding ability) and catalytic rate constants (kcat=rate of function), are conserved within a narrow range at normal body temperatures in differently adapted species (for reviews, see Somero, 1995; Somero, 1997). Because the values of these kinetic properties can be set by energy barriers to catalytic conformational changes (Dunn et al., 1991), it has been conjectured that temperature adaptation of protein function will involve adjustments in protein stability (Arpigny et al., 1994; Somero, 1995). Cold adaptation is predicted to lead to more flexible proteins, i.e. to lower energy barriers to conformational changes, whereas adaptation to high temperatures is predicted to lead to more rigid proteins that 767 The Journal of Experimental Biology 204, 767–776 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3247