[Effect of temperature and pH on the ribulose-1,5-diphosphate carboxylase activity of the thermophilic hydrogen bacterium, Pseudomonas thermophila].

The activity of ribulose 1,5-diphosphate (RDP) carboxylase was found in the soluble fraction of the cytoplasm from sonicated Pseudomonas thermophila K-2 cells. The enzyme is relatively thermolabile and completely loses its activity at 80 degrees C. The activity of RDP carboxylase at 60 degrees C increases by 40% during the first 10 min of heating in the presence of Mg2+ ions, bicarbonate and dithiothreitol, and again decreases if the enzyme is heated over 20 min. The optimum temperature of the enzyme is 50--55 degrees C. The specific activity of the enzyme in fresh preparations under these conditions reaches 0.22 unit per 1 mg of protein in the extract. The calculated value of the activation energy for RDP carboxylase is 6.4 kcal-mole-1, but 11.6 kcal-mole-1 in frozen preparations. The optimal pH is 7.0--7.3 depending on the buffer. The temperature optimum for the enzyme action does not depend on pH within the range of 7.3 to 8.8. Therefore, RDP carboxylase of Ps. thermophila K-2 differs from RDP carboxylases of mesophilic cultures studied earlier by a higher susceptibility to a decrease in temperature (the enzyme activity is negligible at 30 degrees C), by a lower value of the activation energy at suboptimal temperatures, and by a lower pH optimum of the enzyme action.