Metal centers in the anaerobic microbial metabolism of CO and CO2

Abstract

Carbon dioxide and carbon monoxide are important components of the carbon cycle. Major research efforts are underway to develop better technologies to utilize the abundant greenhouse gas, CO₂, for harnessing ‘green’ energy and producing biofuels. One strategy is to convert CO₂ into CO, which has been valued for many years as a synthetic feedstock for major industrial processes. Living organisms are masters of CO₂ and CO chemistry and, here, we review the elegant ways that metalloenzymes catalyze reactions involving these simple compounds. After describing the chemical and physical properties of CO and CO₂, we shift focus to the enzymes and the metal clusters in their active sites that catalyze transformations of these two molecules. We cover how the metal centers on CO dehydrogenase catalyze the interconversion of CO and CO₂ and how pyruvate oxidoreductase, which contains thiamin pyrophosphate and multiple Fe₄S₄ clusters, catalyzes the addition and elimination of CO₂ during intermediary metabolism. We also describe how the nickel center at the active site of acetyl-CoA synthase utilizes CO to generate the central metabolite, acetyl-CoA, as part of the Wood-Ljungdahl pathway, and how CO is channelled from the CO dehydrogenase to the acetyl-CoA synthase active site. We cover how the corrinoid iron–sulfur protein interacts with acetyl-CoA synthase. This protein uses vitamin B₁₂ and a Fe₄S₄ cluster to catalyze a key methyltransferase reaction involving an organometallic methyl-Co³⁺ intermediate. Studies of CO and CO₂ enzymology are of practical significance, and offer fundamental insights into important biochemical reactions involving metallocenters that act as nucleophiles to form organometallic intermediates and catalyze C–C and C–S bond formations.