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Effect of Adenosine Monophosphate, Adenosine Diphosphate, and Reduced Nicotinamide Adenine Dinucleotide on Adenosine Triphosphate-dependent Carbon Dioxide Fixation in the Autotroph Thiobacillus neapolitanus¹

^a Exobiology Division, Ames Research Center, National Aeronautics and Space Administration, Moffett Field, California 94035

ABSTRACT

The observation that adenosine triphosphate (ATP)-dependent CO₂ 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 CO₂ acceptor, ribose-5-phosphate, was replaced by ribulose-1,5-diphosphate, no inhibition of CO₂ 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 CO₂ 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 CO₂ fixation by reduced nicotinamide adenine dinucleotide (NADH₂) was investigated. Adding NADH₂ to the extracts did not stimulate CO₂ fixation, even at carbonate levels from 0.05 to 30 µmoles, except in the absence of ribose-5-phosphate. Slight increases in CO₂ fixation were noted when the assay system was incubated in air instead of the usual helium atmosphere.

² National Academy of Sciences—National Research Council Postdoctoral Resident Research Associate at Ames Research Center. Present address: Department of Microbiology, Colorado State University, Fort Collins 80521.

³ Present address: Bruce Lyon Memorial Research Laboratory, Oakland, Calif.

¹ Presented in part at the 66th Annual Meeting of The American Society for Microbiology, Los Angeles, Calif., 1966.