Effect of Adenosine Monophosphate, Adenosine Diphosphate, and Reduced Nicotinamide Adenine Dinucleotide on Adenosine Triphosphate-dependent Carbon Dioxide Fixation in the Autotroph Thiobacillus neapolitanus

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Abstract
The observation that adenosine triphosphate (ATP)-dependent CO 2 fixation in extracts of chemosynthetic and photosynthetic autotrophs may be regulated in part by adenosine monophosphate (AMP) was extended to the strict autotroph Thiobacillus neapolitanus (X). In addition, this report presents data which include adenosine diphosphate (ADP) in the regulatory role. When the primary CO 2 acceptor, ribose-5-phosphate, was replaced by ribulose-1,5-diphosphate, no inhibition of CO 2 fixation occurred unless the Mg ++ concentration was limiting. A molar ratio of 5:1 AMP or ADP to ATP reduced the specific activity (micromoles of CO 2 fixed per milligram of protein per minute) of the extracts from 0.22 to 0.12 and 0.11, respectively. The reported stimulation of the carboxylative phase of ATP-dependent CO 2 fixation by reduced nicotinamide adenine dinucleotide (NADH 2 ) was investigated. Adding NADH 2 to the extracts did not stimulate CO 2 fixation, even at carbonate levels from 0.05 to 30 μmoles, except in the absence of ribose-5-phosphate. Slight increases in CO 2 fixation were noted when the assay system was incubated in air instead of the usual helium atmosphere.

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