Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Coenzymes - specialized natural reagents
Lipoic acid and oxidative decarboxylation of α-keto acids
Pyruvate : ferredoxin oxidoreductase

An enzyme similar to lipoyl-dependent keto acid dehydrogenases has been discovered in clostridia and other strict anaerobes: pyruvate:ferredoxin oxidoreductase, which catalyzes the reversible decarboxylation of pyruvate [equation (8-66)]:

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The oxidizing agent ferredoxin is an iron-sulfur protein with a low redox potential (Chapter 10, Section B). Clostridial ferredoxins act as two-electron oxidants. A comparison with equation (8-65) shows that oxidized ferredoxin replaces NAD+. However, the reaction does not require Lipoic Acid. Apparently, the thiamine-bound active acetaldehyde can be oxidized by the iron-sulfur center of the oxidoreductase to form an acetyl-thiamine derivative (Section K, 2), which subsequently reacts with CoA.

A similar enzyme, which appears to be involved in the synthesis of $\alpha$-ketoglutarate from succinyl-CoA and CO2 ($\alpha$-ketoglutarate synthase), has been purified from photosynthetic Bacteria [140a].

It is possible that the Cleavage of pyruvate into CO2 and acetyl-CoA serves in some cases to supply a reductant with a very low potential for other biochemical processes. An example is Biological Nitrogen Fixation (Chapter 14, Section A): reduced ferredoxin or (in Azotobacter) a very low-potential flavodoxin (Section I, 5) may be generated via pyruvate cleavage and subsequently utilized in N2 fixation processes [141]. At the same time, reaction sequence (8-66) may frequently operate in the reverse direction during biosynthetic pathways (Chapter 11, Section B, 1).



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