Acetate biosynthesis by acetogenic bacteria. Evidence that carbon monoxide dehydrogenase is the condensing enzyme that catalyzes the final steps of the synthesis.

S W Ragsdale,H G Wood

doi:10.1016/s0021-9258(18)89217-1

S W Ragsdale, H G Wood

Open Access

https://doi.org/10.1016/s0021-9258(18)89217-1

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Abstract

The purified carbon monoxide dehydrogenase from Clostridium thermoaceticum is the only protein required to catalyze an exchange reaction between carbon monoxide and the carbonyl group of acetyl-CoA. This exchange requires that the CO dehydrogenase bind the methyl, the carbonyl, and the CoA groups of acetyl-CoA, then equilibrate the carbonyl with CO in the solution and re-form acetyl-CoA. CoA is not necessary for the exchange and, in fact, inhibits the reaction. These studies support the view that CO dehydrogenase is the condensing enzyme that forms acetyl-CoA from its component parts. Carbon dioxide also exchanges with the C-1 of acetyl-CoA, but at a much lower rate than does CO. At 50 degrees C and pH 5.3, the optimal pH, the turnover number is 70 mol of CO exchanged per min/mol of enzyme. Low potential electron carriers are stimulatory. The Km app for stimulation by ferredoxin is 50-fold less than the value for flavodoxin. Neither ATP or Pi stimulate the exchange. The EPR spectrum of the CO-reacted enzyme is markedly changed by binding of CoA or acetyl-CoA. Arginine residues of the CO dehydrogenase appear to be involved in the active site, possibly by binding acetyl-CoA. Mersalyl acid, methyl iodide, 5,5-dithiobis-(2-nitrobenzoate), and sodium dithionite inhibit the exchange reaction. A scheme is presented to account for the role of CO dehydrogenase in the exchange reaction and in the synthesis of acetate.

Highlights

EVIDENCE THAT CARBON MONOXIDE DEHYDROGENASE IS THE CONDENSING ENZYME THAT CATALYZES THE FINAL STEPS OF THE SYNTHESIS*
Pe-Clostridiumthermoaceticum is the only protein re- zacka and Wood [7] demonstrated that CO dehydrogenase quired to catalyze an exchange reaction between car- was required for the synthesis of acetyl-coA from CO, and bon monoxide and the carbonyl group of acetyl-coA
Our studies demonstrate that the purified CO dehydrogenase catalyzes the exchange reaction and based on these results, we present

Summary

Introduction

EVIDENCE THAT CARBON MONOXIDE DEHYDROGENASE IS THE CONDENSING ENZYME THAT CATALYZES THE FINAL STEPS OF THE SYNTHESIS*. Methyl iodide, S,S-dithiobis-(2- into acetyl-coA, and, third, because CO dehydrogenase had nitrobenmate), and sodium dithionite inhibit the exchange reaction.

Results

Conclusion