30] The reductive carboxylic acid cycle

Abstract

Publisher Summary This chapter discusses the reductive carboxylic acid cycle in detail. The reductive carboxylic acid cycle is a mechanism for CO 2 assimilation which has recently been found in the photosynthetic bacteria Chlorobium thiosulfatophilum and Rhodospirillum rubrum. The cycle provides a pathway for a synthesis from CO 2 of organic acids, which in turn provide the carbon skeletons for the biosynthesis of amino acids— the main products of bacterial photosynthesis—and other cellular constituents. The key carboxylation reactions in the reductive carboxylic acid cycle are the recently discovered pyruvate and α-ketoglutarate synthases. They each reverse a reaction of the citric acid cycle that is irreversible in aerobic cells: the decarboxylation of pyruvate to acetyl-CoA and CO 2 , and the decarboxylation of α-ketoglutarate to succinyl-CoA and CO 2. The reductive carboxylic acid cycle includes a reversal of the degradation of pyruvate via the citric acid cycle and the resultant liberation of CO 2 . A complete turn of the reductive carboxylic acid cycle results in the fixation of four molecules of CO 2 and the formation of one oxaloacetate.