Structural basis of the thermostability of monomeric malate synthase from a thermophilic Bacillus

Malate synthases from a thermophilic Bacillus and Escherichia coli have been isolated in a high state of purity. Molecular weights of these two proteins determined in the native state and after denaturation in sodium dodecyl sulfate-mercaptoethanol show that the enzymes are monomeric. This conclusion is supported, for the thermophile enzyme, by the result of an electrophoretic analysis of that protein after treatment with dimethylsuberimidate and denaturation. The thermophilic Bacillus malate synthase is considerably more thermostable than its mesophilic counterparts from E. coli, Bacillus licheniformis, and Pseudomonas indigofera. It is, however, markedly labilized by an increase in the ionic strength of the medium brought about by the addition of 0.2 M potassium chloride or in pH above 9. Increased ionic strength has little effect on the thermostability of the mesophilic bacterial malate synthases. These observations provide strong support for the idea that monomeric proteins in thermophiles owe their unusual heat stability to the presence of salt bridges in their tertiary structure.