Pyruvate Synthetase Pathway Reductive Carboxylic Acid Cycle

Pyruvate Synthetase Pathway Reductive Carboxylic Acid Cycle

Pyruvate Synthetase Pathway Reductive Carboxylic Acid Cycle - In the green bacterium Chlorobium thiosulfatophilum Evans et. al (1966) described the pyruvate synthetase pathway for CO₂ fixation.CO₂ is used to form pyruvate by means of the pyruvate synthetase reaction the ultimate reductant is hydrogen sulphide.

The light dependent oxidation of HIS provides the reducing power for the reduction of ferredoxin (Fd). Acetyl CoA then accepts CO₂ and is reduced by ferredoxin to yield pyruvate,

1) Formation of pyruvate by pyruvate synthetase is dependent on reduced ferredoxin.

Acetyl-CoA+Co₂+Ferredoxin (reduced)--->

Pyruvate+CoA+Ferredoxin (oxidized)

(2) Conversion of pyruvate into oxaloacetate.
Pyruvate + ATP + CO₂--> Oxaloacetate + ADP + Pi
Oxaloacetate then enters a reversed tricarboxylic acid cycle.

(3) Carboxylation of succinyl CoA to yield d-Ketoglutarate (involving reduced ferredoxin).
Succinyl CoA + co₂ + Ferredoxin (reduced)-->
α-Ketoglutarate + CoA + Ferredoxin (oxidized)

(4) α -Ketoglutarate is converted into citrate through oxalosuccinate. Citrate then splits into oxaloaceticacid and acetate.
α -Ketoglutarate -->Oxalossuccinate -->Citrate --> Oxaloacetic acid + Acetate.

The net result of each cycle is that a molecules of CO₂ are fixed (reductive fixation) and one equivalent of oxaloacetate is produced.

Three molecules of ATP are required:

(I) for activation of acetate,
(2) for carboxylation of pyruvate and
(3) for activation of succinate.
The reductive carboxylic acid cycle appears to be particularly suited to provide the carbon skeletons for the main products of bacterial photosynthesis, which are mainly amino acids.

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