Chemical, physical and enzymatic comparisons of formyltetrahydrofolate synthetases from thermo- and mesophilic Clostridia.

Formyltetrahydrofolate synthetase from the mesophiles Clostridium cylindrosporum, C. acidiurici, and C. formicoaceticum and the thermophile C. thermoaceticum have been purified to homogeneity, the first two by Rabinowitz and Himes and their coworkers and the latter two by ourselves. The physical properties of these proteins are very similar. All four enzymes are tetrameric and all are activated by NH4+ or K+, and the mechanism of this activation always involves a decrease in Km for formate. The enzyme from C. formicoaceticum is more thermostable and has a higher temperature optimum than the C. cylindrosporum or C. acidiurici enzymes and the C. thermoaceticum enzyme is the most thermostable. Comparisons of the amino acid compositions indicate possible correlations between thermostability and an increased content of isoleucine, arginine and tryptophan and a decreased content of phenylalanine and aspartic acid. Both of the less thermostable enzymes dissociative above 37 degrees if NH4+ or K+ are removed, but neither of the more stable enzymes do. In 25% D2O, the C. formicoaceticum enzyme is destabilized at elevated temperatures, relative to the C. thermoaceticum enzyme. It is possible that stronger hydrophobic interactions between subunits are responsible for increased thermostability in these enzymes.