Malate dehydrogenase from thermophilic and mesophilic bacteria. Molecular size, subunit structure, amino acid composition, immunochemical homology, and catalytic activity

Abstract

Malate dehydrogenases isolated from a number of mesophilic, moderately thermophilic and extremely thermophilic bacteria yield upon denaturation subunits of MW 32,000-36,000. Determination of their native MW shows that some of the enzymes are dimeric and others are tetrameric; the 2 types are distributed in each of the 3 classes of bacteria. The amino acid compositions of the enzymes show no consistent trend that can be related to the progression of thermostability from the mesophile through the moderate thermophile, to the extreme thermophile species. The tetrameric enzyme species all exhibit a high level of structural homology as judged by the criterion of immunological cross-reaction. Little cross-reaction occurs between the tetramers and the dimers. The dimeric enzyme from the extreme thermophile, Thermus aquaticus, cross-reacts only weakly, if at all, even with dimeric malate dehydrogenases. The catalytic activities of the malate dehydrogenases vary over a wide range. KCl, organic solvents such as acetone, and the protein denaturants urea and guanidine hydrochloride, activate a number of the malate dehydrogenases under the assay conditions employed. The diversity among the bacterial malate dehydrogenases, manifested not only in molecular size and subunit structure but also in properties such as catalytic activity and the dependnece of this activity on electrolytes, organic solvents and denaturants, indicates significant structural differences between several of these cognate enzyme species.