Evidence That an Iron--Nickel--Carbon Complex is Formed by Reaction of CO with the CO Dehydrogenase from Clostridium thermoaceticum

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 with57Fe, a broadening of 18 G in the g∥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.