Evidence that an iron-nickel-carbon complex is formed by reaction of CO with the CO dehydrogenase from Clostridium thermoaceticum.

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Abstract
The interaction between carbon monoxide and the CO dehydrogenase from Clostridium thermoaceticum was studied by electron spin resonance (ESR) techniques. When the enzyme reacts with CO, a paramagnetic complex is formed which previously was shown, by isotope substitution, to be due to a nickel-carbon species. In this paper, we demonstrate that iron is also a component of this ESR-detectable complex. When the iron in the enzyme is replaced with 57Fe, a broadening of 18 G in the G .dblvert., and 7 G in the G .perp. region is seen. This hyperfine interaction is probably due to more than one iron atom in the complex. Coenzyme A influences this ESR spectrum. In the absence of CoA, the ESR spectrum consists of two superimposed signals, which were simulated using the following ESR parameters: signal 1, with g = 2.074 and g = 2.028, and signal 2 with gx = 2.062, gy = 2.047, and gz = 2.028. CoA converts signal 2 into signal 1. Since iron, nickel, and carbon all are part of this ESR-detectable complex, we propose that these atoms exist in a spin-coupled complex with net spin = 1/2, analogous to other iron-sulfur centers in which the metals are bridged by acid-labile sulfide.

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