Functional determinants of temperature adaptation in enzymes of cold- versus warm-adapted mussels (Genus Mytilus).

Temperature is a strong selective force on the evolution of proteins due to its effects on higher orders of protein structure and, thereby, on critical protein functions like ligand binding and catalysis. Comparisons among orthologous proteins from differently thermally adapted species show consistent patterns of adaptive variation in function, but few studies have examined functional adaptation among multiple structural families of proteins. Thus, with our present state of knowledge, it is difficult to predict what fraction of the proteome will exhibit adaptive variation in the face of temperature increases of a few to several degrees Celsius, that is, temperature increases of the magnitude predicted by models of global warming. Here, we compared orthologous enzymes of the warm-adapted Mediterranean mussel Mytilus galloprovincialis and the cold-adapted Mytilus trossulus, a native of the North Pacific Ocean, species whose physiologies exhibit significantly different responses to temperature. We measured the effects of temperature on the kinetics (Michaelis-Menten constant-K(m)) of five enzymes that are important for ATP generation and that represent distinct protein structural families. Among phosphoglucomutase (PGM), phosphoglucose isomerase (PGI), pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (GTP) (PEPCK), and isocitrate dehydrogenase (NADP) (IDH), only IDH orthologs showed significantly different thermal responses of K(m) between the two species. The K(m) of isocitrate of M. galloprovincialis-IDH was intrinsically lower and more thermally stable than that of M. trossulus-IDH and thus had higher substrate affinity at high temperatures. Two amino acid substitutions account for the functional differences between IDH orthologs, one of which allows for more hydrogen bonds to form near the mobile region of the active site in M. galloprovincialis-IDH. Taken together, our findings cast light on the targets of adaptive evolution in the context of climate change; only a minority of proteins might adapt to small changes in temperature, and these adaptations may involve only small changes in sequence.