GENERAL MICROBIOLOGY - T.P. Pirog - 2004

16. USE OF INORGANIC HYDROGEN DONORS: AEROBIC CHEMOLITHOTROPHIC BACTERIA

16.5. CO₂ FIXATION

The Calvin–Bassham cycle, also known as the ribulose diphosphate cycle. The ribulose diphosphate cycle is a reductive process in which CO₂ is reduced to the level of CARBOHYDRATES. It Functions in all aerobic chemolithoautotrophs, almost all phototrophic Bacteria, cyanobacteria, and green plants. It is not involved in CO₂ fixation in methanogenic and acetogenic bacteria.

The Key Enzymes of the cycle are phosphoribulokinase and ribulose diphosphate carboxylase. The cycle can be divided into three main stages (Figure): 1) the carboxylation reaction; 2) the reduction reactions; 3) the regeneration of CO₂ acceptor molecules.

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In the carboxylation reaction, CO₂ is added to ribulose-1,5-diphosphate with the participation of the enzyme ribulose diphosphate carboxylase, yielding two molecules of 3-phosphoglycerate.

The reduction reactions involve the reduction of the carboxyl group of 3-phosphoglycerate to an aldehyde group. The glycolytic enzymes phosphoglycerate kinase and glyceraldehyde phosphate dehydrogenase take part in this process. This is the CO₂ assimilation stage, which requires an input of energy and reducing equivalents. The resulting glyceraldehyde-3-phosphate is in equilibrium with dihydroxyacetone, and both triose phosphates are in equilibrium with fructose-1,6-diphosphate.

In the Third Stage, fructose diphosphate is dephosphorylated by the action of the enzyme fructose diphosphatase to form fructose-6-phosphate. Subsequently, three molecules of ribulose-5-phosphate are formed from one molecule of fructose-6-phosphate and three molecules of triose phosphate. The final reaction of the cycle is the phosphorylation of ribulose-5-phosphate at the expense of ATP (catalyzed by phosphoribulokinase) to yield ribulose-1,5-diphosphate, which serves as the CO₇ acceptor.

Cycle balance:

Other pathways of autotrophic CO₂ fixation. Anaerobic autotrophic bacteria utilize two alternative pathways for carbon dioxide assimilation. Methanogenic, acetogenic, and sulfate-reducing bacteria, which are capable of using H₂ and CO₂ as electron Donors, reduce CO₂ via the anaerobic acetyl-CoA pathway to acetyl-CoA and Pyruvate. Through known reactions, pyruvate is then channeled into central biosynthetic pathways.

Green sulfur bacteria fix CO₂ exclusively via the Reactions of the reductive Tricarboxylic Acid Cycle. In this cycle, CO₂ is fixed through the reductive carboxylation of succinyl-CoA.