Coupling of Phosphorylation and Carbon Dioxide Fixation in Extracts of Thiobacillus thioparus

Abstract

A cell-free system from T. thioparus which fixes large quantities of C14O2 in the presence of ribose-5-phosphate, adenosine-triphosphate (ATP) and Mg++ has been described. The specific activity (0.041 jxmole of ribulose-1,5-diphosphate min-1 mg-1 protein) of the CO2-fixing system approaches that of green plants, and is further evidence for the importance of the role of carboxydismutase in the thiobacilli. In addition to ATP, adensine-diphosphate (ADP) and other nucleoside triphosphates served with varying degrees of effectiveness for the fixation of C14O2. The ATP requirement for CO2 fixation was partially replaced under aerobic conditions by a combination of SO3=, PO4= and adenosine-monophosphate (AMP). Phos-phorylation and CO2 fixation were separated in time by first incubating SO3= and AMP aerobically, and then anaerobically introducing C14O3= and ribose-5-phosphate into the reaction mixture. During the first incubation, P32O4= was esterified into nucleotides, mainly ADP, and the second incubation C14O2 was fixed, with the concomitant utilization of almost equal amounts of the esterified phosphate. These data provide the first in-vitro evidence for the mechanism of the coupling of CO2 fixation and phosphorylation of T. thioparus. The fixation of C14O2 was shown to be almost completely inhibited by AMP. This inhibition was not due to the conversion of ATP to ADP by adenylic kinase, or to the binding of magnesium by the nucleotide. The inhibition was specific for AMP, since other mononucleotides, adenosine, and adenine did not inhibit. The AM P regulation of CO2 fixation may represent a basic control mechanism in autotrophic metabolism.