Enthalpy–entropy compensation of oxamate binding by homologous lactate dehydrogenases

Abstract

STUDIES^1,2 of the amino acid sequence and crystal structure of M₄ lactate dehydrogenase (EC 1.1.1.27) indicate that all binding interactions are of the weak, non-covalent type (proceeding with only minor free energy changes). Because critical regions of the substrate and coenzyme-binding site seem highly conserved³, and because non-covalent interactions are highly, if differentially, sensitive to the physical environment⁴, we wondered how organisms living in widely different temperatures and pressures stabilise enzyme–ligand interactions. We have therefore compared pyruvate binding site functions of lactate dehydrogenases obtained from organisms living in different temperatures and pressures. We used a placental mammal, the ox, with a cell temperature of 37 °C; a marsupial, the possum (Trichosurus vulpecula), with an average cell temperature of 36 °C (ref. 5), two monotremes, the platypus (Ornithorhynchus anatinus), with a cell temperature of about 31 °C (ref. 6), and the echidna (Tachyglossus aculeatus), with a body temperature of 23–32 °C (ref. 7); the goanna (Varanus gouldi), an Australian lizard with a cell temperature ranging from about 20 up to nearly 40 °C (ref. 8); the Pacific coast dogfish (Squalus acanthias), with an average body temperature of about 15 °C; an intertidal sculpin (Oligocottus maculosus), living in varying temperatures between about 5 and 15 °C, and an abyssal teleost fish, Antimora rostrata, with a highly constant body temperature of 2–3 °C. All these, except the dogfish and Antimora, can be viewed as species adapted to 1 atm; by contrast the abyssal species must be able to tolerate pressures of up to several hundred atmospheres, while the dogfish may encounter pressures from 1 to about 100 atm (N. J. Wilimovsky, personal communication). We found that adjustments in enthalpic and entropic contributions to lactate dehydrogenase binding allowed each homologue to bind ligand with similar avidity at its respective “normal” temperature and pressure.