Reverse Krebs cycle

The reductive citric acid cycle (also reductive citric acid cycle, the reductive citric acid cycle, tricarboxylic acid cycle or reductive reductive Krebs cycle ) is a cyclic pathway in which the carbon dioxide ( CO2) fixation ( carbon dioxide assimilation ) by decreasing steps of the citric acid cycle '. Through this reversal of the steps of the citric acid cycle ', he is also known as reverse citric acid cycle. The cycle was in 1966 by the work of MC. Evans, Bob B. Buchanan and Daniel I. Arnon discovered.

Occurrence

The reductive citric acid cycle has been demonstrated in various micro-aerobic and obligate anaerobic microorganisms. He was identified non- sulfur bacteria in green sulfur bacteria and in the countryside. It was originally discovered in 1966 in the green sulfur bacterium Chlorobium limicola. It is believed that the deep branching thermophilic bacterium Aquifex aeolicus by the reductive citric acid cycle CO2 fixed.

It has previously been proposed that archaea ( Thermoproteus neutrophilus ) via this pathway can fix carbon dioxide. This was however refuted by more recent findings.

Biochemistry

The reductive citric acid cycle is the inverse of ( oxidative ) TCA cycle represents the most enzymes of the citric acid cycle are also used in this pathway - contrary to its direction. In the oxidative TCA cycle, there are three irreversible steps that are bypassed by three special enzymes in the reductive citric acid cycle:

1 ) The reduction of fumarate to succinate is catalyzed by an fumarate reductase. It replaces the succinate dehydrogenase in the ox. Citric acid cycle.

2 ) to succinyl -CoA condenses a molecule of CO2 under consumption of reduced ferredoxin. This reductive carboxylation is catalyzed by α -ketoglutarate synthase. This bypasses the mediated by the α -ketoglutarate dehydrogenase decarboxylation in ox. Citric acid cycle.

3 ) In the ox. Citric acid cycle condenses acetyl -CoA with oxaloacetate, which the citrate synthase catalyses. In the reductive TCA cycle, this reaction is reversed by an ATP - citrate lyase, which is cleaved citrate into acetyl -CoA and oxaloacetate. This closes the circle. ATP is required for this reaction.

In the overall balance is required (in the form of NAD (P ) H and ferredoxin red. ) And two molecules of ATP for the fixation of two molecules of CO2 to acetyl -CoA eight reducing equivalents:

For the reversal of the citric acid cycle ' ie a molecule of ATP is also required, as in the oxidative TCA cycle freely eight reducing equivalents and one molecule of GTP:

Biological Significance

The reductive citric acid cycle allows above-mentioned microorganisms, the fixation of CO2. Acetyl -CoA is not assimilated via the glyoxylate cycle, but converted by a ferredoxin -dependent pyruvate synthase to pyruvate, while a third molecule of CO2 is fixed:

Pyruvate can be eventually metabolized to hexoses in the course of gluconeogenesis and introduced in the constructive metabolism.

The cycle consumes compared to the Calvin cycle less ATP, but, because of the oxygen sensitivity of enzymes involved only occur under anaerobic or microaerobic conditions.