Evidence for the Calvin Cycle and Hexose Monophosphate Pathway in Thiobacillus ferrooxidans

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Abstract
The enzymes of the Calvin reductive pentose phosphate cycle and the hexose monophosphate pathway have been demonstrated in cell-free extracts of Thiobacillus ferrooxidans. This, together with analyses of the products of CO2 fixation in cell-free systems, suggests that these pathways are operative in whole cells of this microorganism. Nevertheless, the amount of CO2 fixed in these cell-free systems was limited by the type and amount of compound added as substrate. The inability of cell extracts to regenerate pentose phosphates and to perpetuate the cyclic fixation of CO2 is partially attributable to low activity of triose phosphate dehydrogenase under the experimental conditions found to be optimal for the enzymes involved in the utilization of ribose-5-phosphate or ribulose-1,5-diphosphate as substrate for CO2 incorporation. With the exception of ribulose-1,5-diphosphate, all substrates required the addition of adenosine triphosphate (ATP) or adenosine diphosphate (ADP) for CO2 fixation. Under optimal conditions, with ribose-5-phosphate serving as substrate, each micromole of ATP added resulted in the fixation of 1.5 μmoles of CO2, whereas each micromole of ADP resulted in 0.5 μmole of CO2 fixed. These values reflect the activity of adenylate kinase in the extract preparations. The Km for ATP in the phosphoribulokinase reaction was 0.91 × 10−3m. Kinetic studies conducted with carboxydismutase showed Km values of 1.15 × 10−4m and 5 × 10−2m for ribulose-1,5-diphosphate and bicarbonate, respectively.