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J. Biol. Chem., Vol. 275, Issue 37, 28494-28499, September 15, 2000

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The Role of Pyruvate Ferredoxin Oxidoreductase in Pyruvate Synthesis during Autotrophic Growth by the Wood-Ljungdahl Pathway^*

Cristina Furdui and Stephen W. Ragsdale

From the Department of Biochemistry, Beadle Center, University of Nebraska, Lincoln, Nebraska 68588-0664

Pyruvate:ferredoxin oxidoreductase (PFOR) catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA and CO₂. The catalytic proficiency of this enzyme for the reverse reaction, pyruvate synthase, is poorly understood. Conversion of acetyl-CoA to pyruvate links the Wood-Ljungdahl pathway of autotrophic CO₂ fixation to the reductive tricarboxylic acid cycle, which in these autotrophic anaerobes is the stage for biosynthesis of all cellular macromolecules. The results described here demonstrate that the Clostridium thermoaceticum PFOR is a highly efficient pyruvate synthase. The Michaelis-Menten parameters for pyruvate synthesis by PFOR are: V_max = 1.6 unit/mg (k_cat = 3.2 s¹), K_m^Acetyl-CoA = 9 µM, and K_m^CO₂ = 2 mM. The intracellular concentrations of acetyl-CoA, CoASH, and pyruvate have been measured. The predicted rate of pyruvate synthesis at physiological concentrations of substrates clearly is sufficient to support the role of PFOR as a pyruvate synthase in vivo. Measurements of its k_cat/K_m values demonstrate that ferredoxin is a highly efficient electron carrier in both the oxidative and reductive reactions. On the other hand, rubredoxin is a poor substitute in the oxidative direction and is inept in donating electrons for pyruvate synthesis.

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