Sunday, November 20, 2011

Reductive TCA Cycle[Co2 Fixation]

One alternative strategy for fixing CO2 in addition to the Calvin-Benson-Bassham cycle and others is to simply reverse the TCA cycle or reverse tricarboxylic acid cycle or reverse Krebs cycle. 

The reductive TCA cycle appears to operate in phylogenetically diverse autotrophic bacteria and archaea, including genera of anoxic phototrophic bacteria (Chlorobium), sulfate-reducing bacteria (Desulfobacter), microaerophilic, hyperthermophilic hydrogen-oxidizing bacteria (Aquifex and Hydrogenobacter), and sulfur-reducing Crenarchaeota (Thermoproteus and Pyrobaculum).

The reductive TCA cycle is essentially the oxidative TCA cycle running in reverse, leading to the fixation of two molecules of CO2 and the production of one molecule of acetyl-CoA. Acetyl-CoA is reductively carboxylated to pyruvate, from which all other central metabolites can be formed. 

Most of the enzymes of the two pathways are shared, with the exception of three key enzymes that allow the cycle to run in reverse: ATP citrate lyase, 2-oxoglutarate:ferredoxin oxidoreductase, and fumarate reductase. 

2-Oxoglutarate:ferredoxin oxidoreductase catalyzes the carboxylation of succinyl-CoA to 2-oxoglutarate, ATP citrate lyase the ATP-dependent cleavage of citrate to acetyl-CoA and oxaloacetate, and fumarate reductase the reduction of fumarate forming succinate. 

The presence of these enzyme activities in autotrophically grown bacteria and archaea is indicative of a functioning reductive TCA cycle .

Enzyme activities: 
1, malate dehydrogenase; 2, fumarate hydratase (fumarase); 3, fumarate reductase; 4, succinyl-CoA synthetase; 5, 2-oxoglutarate:ferredoxin oxidoreductase ; 6, isocitrate dehydrogenase ; 7, aconitate hydratase (aconitase); 8, ATP citrate lyase; and 9, pyruvate:ferredoxin oxidoreductase. Fdred, reduced ferredoxin

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