Enzymes of Intermediary Carbohydrate Metabolism in the Obligate Autotrophs Thiobacillus thioparus and Thiobacillus neapolitanus

Levels of enzymes operative in the Embden-Meyerhof-Parnas (glycolytic) pathway, pentose phosphate cycle, citric acid cycle, and certain other phases of intermediary carbohydrate metabolism have been compared inThiobacillus thioparus andT. neapolitanus . All enzymes of the glycolytic pathway except phosphofructokinase were demonstrated in both organisms. There were some striking quantitative differences between the two organisms with respect to the activities of the individual enzymes of the glycolytic pathway and the citric acid cycle. Qualitative differences were also found: the isocitrate dehydrogenase activity ofT. thioparus is strictly nicotinamide adenine dinucleotide phosphate (NADP)-dependent, whereas that ofT. neapolitanus is primarily nicotinamide adenine dinucleotide-dependent, activity with NADP being low; the glucose-6-phosphate dehydrogenase ofT. thioparus is particulate, whereas that ofT. neapolitanus is partly soluble and partly particulate; the 6-phosphogluconate dehydrogenase ofT. thioparus is soluble, that ofT. neapolitanus is partly soluble and partly particulate. All enzymes which function in the carbon reduction cycle were present at very high levels. In contrast, enzymes which operate exclusively in cycles other than the carbon reduction cycle were present at low levels. Of the enzymes not operative in the carbon reduction cycle that were examined, isocitric dehydrogenase had the highest specific activity. Both organisms possessed reduced nicotinamide adenine dinucleotide dehydrogenase activity. The qualitative and quantitative aspects of the data are discussed in relation to possible biochemical explanations of obligate autotrophy.