Abstract
Publisher Summary This chapter discusses the enzymes that catalyze ferredoxin-linked carboxylation reactions. All α-keto acids shown to be synthesized via a ferredoxin-linked carboxylation are important intermediates in the biosynthesis of amino acids. The formation of α-keto acids by ferredoxin-dependent carboxylations also constitutes a mechanism for CO 2 fixation. There is now considerable evidence that carboxylation reactions driven by reduced ferredoxin are essential for the operation of two new cyclic mechanisms for the assimilation of CO 2 : the reductive carboxylic acid cycle of photo synthetic bacteria and the reductive monocarboxylic acid cycle of fermentative bacteria. Each of the ferredoxin-linked carboxylation enzymes tested can oxidize a specific α-keto acid to reduced ferredoxin, CO 2 , and an acyl-CoA derivative—one carbon shorter than the original α-keto acid. Each preparation catalyzes, in addition, an exchange between CO 2 and the α-keto acid formed in the synthetic reaction. The synthetic, oxidative, and exchange reactions appear to be separate activities catalyzed by a single protein that is specific for each α-keto acid.
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