Relationships between the osmoadaptation strategy, amino acid composition of bulk protein, and properties of certain enzymes of haloalkaliphilic bacteria

Haloalkaliphilic microorganisms isolated from soda lakes were compared in terms of the amino acid composition of bulk protein and the reaction of a number of key enzymes to salts and pH of the medium. In the extremely haloalkaliphilic bacterium Natroniella acetigena (selt-in osmoadaptation strategy), acidic amino acids (glutamic and aspartic) made up 30.91 mol % of the total of bulk protein amino acids. In the moderate haloalkaliphiles Tindallia magadiensis, Halomonas campisalis, and Halomonas sp. AIR-2 (compatible-solutes osmoadaptation strategy), the proportion of acidic amino acids (24.36, 23.15, and 23.58 mol %, respectively) was lower than in N. acetigena but higher than in the freshwater Acetobacterium paludosum (20.77 mol %). The excess of acidic amino acids over basic amino acids (lysine and arginine) increased with the degree of halophily. The enzymes of haloalkaliphiles proved to be tolerant to salts and high pH values, although the degree of tolerance varied. The activity of N. acetigena CO dehydrogenase was maximum in the presence of 0.7 M NaCl, but it was virtually independent of the NaHCO₃ concentration. The hydrogenase and CO dehydrogenase of T. magadiensis exhibited maximum activity in the absence of NaCl; the Co dehydrogenase was most active at 0.25 M NaHCO₃, and hydrogenase activity was only weakly dependent on NaHCO₃ in the concentration range of 0–1.2 M. The nitrate reductases of H. campisalis and Halomonas sp. AIR-2 were active in broad ranges of NaCl and KCl concentrations; the activity maxima were recorded at moderate concentrations of these salts. The pH optima of most of the studied enzymes of haloalkaliphiles were in the alkaline zone. Thus, it was shown that the amino acid composition of bulk protein is determined by the osmoadaptation strategy employed by the bacterium. A correlation was found between the salt tolerance of enzymes and the proportion of acidic amino acids in the bulk protein. The ability of enzymes to function at high pH values is one of the mechanisms of adaptation of microorganisms to high pH values.