Intrinsic versus extrinsic stabilization of enzymes: the interaction of solutes and temperature on A4-lactate dehydrogenase orthologs from warm-adapted and cold-adapted marine fishes.

We examined the effects of temperature and stabilizing solutes on A4-lactate dehydrogenase (A4-LDH) from warm- and cold-adapted fishes, to determine how extrinsic stabilizers affect orthologs with different intrinsic stabilities. Conformational changes during substrate binding are rate-limiting for A4-LDH, thus stabilization due to intrinsic or extrinsic factors leads to decreased activity. A4-LDH from a warm-temperate goby (Gillichthys mirabilis), which has lower values for kcat and the Michaelis constant for pyruvate ( K m PYR), was intrinsically more stable than the orthologs of the cold-adapted Antarctic notothenioids Parachaenichthys charcoti and Chionodraco rastrospinosus, as shown by a higher apparent transition ('melting') temperature (Tm(APP)). We used four solutes, glycerol, sucrose, trimethylamine-N-oxide and poly(ethylene glycol) 8000, which stabilize proteins through different modes of preferential exclusion, to study temperature-solute interactions of the three orthologs. Changes in Tm(APP) were similar for all orthologs in each solute tested, but the catalytic rate of G. mirabilis A4-LDH was decreased most by solutes and increased most by temperature. In contrast, the K m PYR values of the Antarctic orthologs were more affected than that of the goby by both solutes and temperature. We conclude that (a) preferential exclusion of solutes functions within the native state of A4-LDH to favor conformational microstates with minimal surface area; (b) the varied effects of the different solutes on the kinetic properties are due to the interaction between this nonspecific stabilization and the differing intrinsic stabilities of the orthologs; (c) the catalytic rates of A4-LDH orthologs are equally affected by stabilizing solutes, if measurements are made at physiologically appropriate temperatures; and (d) global stability and localized flexibility of these A4-LDH orthologs may evolve independently.