© 1990 by The Society for Integrative and Comparative Biology
Life at Low Volume Change: Hydrostatic Pressure as a Selective Factor in the Aquatic Environment1
Scripps Institution of Oceanography, University of California San Diego, Lajolla, California 92093-0202
The changes in system (biomolecules plus solvent) volume that accompany most biochemical reactions are large enough to cause strong perturbations of the reactions by hydrostatic pressure. Therefore, below a certain depth, adaptations are required to reduce volume changes and, thereby, the effects of pressure on biochemical structures and functions. These adaptations play important roles in establishing the depth distribution patterns of aquatic species. The pressures at which perturbation becomes strong enough to favor selection for pressure-adapted proteins differ among classes of proteins. Dehydrogenase enzymes, e.g., lactate and malate dehydrogenases, are especially pressure-sensitive. Species occurring below approximately 500 m (corresponding to 51 atm pressure) have dehydrogenases with reduced pressure sensitivities relative to shallower-living species. Thus, selection for pressure-adapted proteins may characterize organisms found in over 80 percent of the biosphere, by volume. The reduced sensitivities to pressure of dehydrogenases from deep-living species are linked to reductions in the catalytic efficiencies (kcat values) of the enzymes, suggesting that adaptation to pressure exacts a price in enzymatic performance. For skeletal muscle actins, pressure adaptations of subunit assembly reactions are observed only in species living below 2,000–3,000 m. Reductions in volume changes may be achieved by controlling the changes in water structure, i.e., solvent volume, that accompany catalysis, ligand binding, and protein assembly. These reductions in solvent volume changes may be effected by reducing the reliance on hydrophobic effects and by closely regulating the shifts in exposure to solvent of water-structuring protein groups over the entire protein surface. Adaptation to pressure may involve amino acid substitutions throughout the protein, not only in the active sites.