Solute compatibility with enzyme function and structure: Rationales for the selection of osmotic agents and end‐products of anaerobic metabolism in marine invertebrates

Abstract

The major nitrogenous osmolytes present in the cells of marine invertebrates, notably the free amino acids glycine, alanine and proline, and trimethylamine oxide and betaine, are highly compatible with proper enzyme function and structure. These nitrogenous osmolytes display either nonperturbing or, in some cases, favorable effects on enzyme-substrate and enzymecofactor complex formation, catalytic velocity and protein structural stability. In contrast, inorganic salts (KCl and NaCl) and certain of the free amino acids which play only a minor osmotic role, e.g., arginine and lysine, have galy perturbing effects on one more of these enzymic parameters. The compatible nitrogenous solutes therefore are suitable for use at high (several tenth molar) contcetrations and at widely varying consentrations in osmo conferming species. Certain nitrogenous solutes, especially trimethyline oxide, betaine and glutamate, offset some of the perturbing effects of inorganic ions on enzyme fuction. The selective accumulatin of osmolytes thus unvolves not only the concentration of non-perturbing solutes, but also a balanced accumulation of solutes with opposing effects on enzumes. The selection of end-products of anaerobic metabolism also appears to be based, in part, on considerations of solute compatibility with enzyme fuction. Octopine is a non-perturbing solute, whereas arginine, which is condensed with pyruvate to form octopine, is very strongly perturbing. Succinate has marked stabilizing effects on protein structure. We conclude that the composition of the intracellular fluids of marine invertebrates reflects selection for osmolytes and end-products whose net effects create a cellular microen vironment which is conducive to otimal enzyme fuction and structure. The accumulation of compatible solutes may preclude the necessity for widespread changes in protein structure in adapting to concentrated or highly variable osmotic environments.