title: Temperature: a "shaping force' in protein evolution.
abstract: 1. Comparisons of homologous enzymes from species adapted to widely different temperatures reveal that ligand-binding affinities are rigorously conserved. This is interpreted to mean that a critical relationship between ligand-binding ability and intracellular ligand concentrations must be maintained for proper enzymic regulation. 2. The catalytic efficiencies of enzyme homologues differ in temperature-compensatory manners. Activation free energies are proportional to adaptation temperature and, consequently, low-temperature-adapted enzymes have the highest substrate turnover numbers. 3. Temperature compensatory adjustments in catalytic efficiency may be achieved by altering the number of weak bonds that form or break during a catalytic conformational change. Support for this hypothesis comes from the finding that activation enthalpy and activation entropy values co-vary in a regular manner and by magnitudes consistent with different amounts of weak-bond formation/rupture during catalytic activation in differnt enzyme homologues. 4. Adaptive adjustments in ligand-binding energetics may also involve utilization of the energy changes that occur during conformational changes. This mechanism would permit enzymes with identical binding-site chemistries to display adaptively different ligand affinities. 5. The greater heat-stabilities of enzymes from warm-adapted species may cause these enzymes to be less efficient catalysts than cold-adapted heat-labile enzymes. Heat-stable enzymes may have to break more weak bonds during a catalytic conformational change than do cold-adapted enzymes. The requirements for thermal stability and high catalytic efficiency thus appear to force an adaptational 'compromise'.
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001665:Binding Sites; D005075:Biological Evolution; D004798:Enzymes / Q000378:metabolism*; D005399:Fishes; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D011487:Protein Conformation; D011770:Pyruvate Kinase / Q000378:metabolism; D011817:Rabbits; D013045:Species Specificity; D013696:Temperature
chemical_list: D004798:Enzymes; D007527:Isoenzymes; D011770:Pyruvate Kinase