Ferredoxin dependent synthesis of α-ketoglutarate and pyruvate by extracts of the green photosynthetic bacterium [formula omitted

Abstract

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 CO2 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 CO2: 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, CO2, and an acyl-CoA derivative—one carbon shorter than the original α-keto acid. Each preparation catalyzes, in addition, an exchange between CO2 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.