Environmental adaptation of proteins: Strategies for the conservation of critical functional and structural traits

Abstract

1.1. Comparative studies of lactate dehydrogenases (LDHs) and skeletal muscle actins from vertebrates adapted to widely different temperatures and hydrostatic pressures reveal major conservative trends in protein evolution and adaptation. For enzymes, ligand binding, as estimated by apparent Michaelis constant (Km) values, is strongly conserved at physiological temperatures, pressures, intracellular pH values and osmotic compositions of different organisms. The catalytic rate constants (kcatvalues) of enzyme homologues are highest for enzymes of low-body-temperature organisms, a trend that can be interpreted in terms of temperature compensation of metabolism. For skeletal muscle actins. the enthalpy and entropy changes accompanying the assembly of filamentous (F) actin from globular (G) actin are highest in high-body-temperature species and especially low in polar and deep-sea fishes. The thermal stability of G-actin is positively correlated with adaptation temperature, except in the case of actins of deep-sea fishes, which are also highly heat stable. Hydrophobic interactions between actin subunits may be of reduced importance in low-body-temperature animals and, especially, in deep-sea fishes. The differences in enthalpy and entropy changes during the G-to-F transformation favor a close conservation of the equilibrium constant for actin assembly under physiological conditions of temperature and pressure for different species. These adaptive patterns in enzymes and actin are likely to reflect changes in protein primary structure.2.2. The appropriate values for protein traits such as ligand binding abilities and catalytic rates are also shown to be established by the composition of the low molecular weight constituents of the cytosol. For example, the use of a combination of urea and methylamine solutes for osmoregulation by marine elasmobranchs is shown to be a mechanism which permits the conservation of key protein traits at high osmolarities. The methylamine solutes such as trimethylamine-N-oxide have effects on proteins opposite to those of urea, and at the approximately 2:1 concentration ratio of urea to methylamines, these counteracting effects are virtually complete.3.3. Regulation of hydrogen ion activity (pH) also is shown to play a major role in the conservation of critical protein traits. The importance of temperature-dependent pH in ectotherms is discussed in terms of stabilizing binding abilities and maintaining correct regulatory and structural sensitivities of proteins. The buffering capacity of tissues reflects the potential of the tissue for generating acidic end-products during anaerobic metabolism. Skeletal muscle, especially white locomotory muscle of fishes, is highly buffered relative to red locomotory muscle and heart muscle. Much of the difference in buffering may be due to different amounts of histidine-containing dipeptide buffers in the tissues. However, comparisons of muscle-type (M4) and heart-type (H4) LDHs show that the M4 isozyme has approximately twice the number of histidine residues per tetramer of the H4 isozyme. This difference between M4 and H4 isozymes which occur predominantly in anaerobically- and aerobically-poised tissues, respectively, suggests that selection for buffering capacity may be a heretofore unappreciated facet of protein evolution.