The Interaction between carbon monoxide and the CO dehydrogenase from Clostidum thernoaceticum 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 g1I and 7 G in the go 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 g, = 2.062, gy = 2.047, andg = 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.Carbon monoxide dehydrogenase is present in high concentration in the acetogenic bacteria (1, 2 Clearly the ESR spectrum of the paramagnetic CO dehydrogenase-C1 complex is due to nickel and carbon since hyperfine splitting was detected upon isotopic substitution with 61Ni (I = 3/2) and also when 13CO (I = 1/2) reacted with the enzyme (9). The major effects of the isotope substitutions were a splitting of the g = 2.028 (g I) component by "3CO and a broadening ofthe 2.074 (g1) component by the 61Ni nucleus.

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The most direct conceivable route for synthesis of multicarbon compounds from CO2 is to join two molecules of CO2 together to make a 2‐carbon compound and then polymerize the 2‐carbon compound or add CO2 successively to the 2‐carbon compound to make multicarbon compounds. Recently, it has been demonstrated that the bacterium, Clostridium thermoaceticum, grows autotrophically by such a process. The mechanism involves the reduction of one molecule of CO2 to a methyl group and then its combination with a second molecule of CO2 and CoA to form acetyl‐CoA. We have designated this autotrophic pathway the acetyl‐CoA pathway [1]. Evidence is accumulating that this pathway is utilized by other bacteria that grow with CO2 and H2 as the source of carbon and energy. This group includes bacteria which, like C. thermoaceticum, produce acetate as a major end product and are called acetogens or acetogenic bacteria. It also includes the methane‐producing bacteria and sulfate‐reducing bacteria. The purpose of this review is to examine critically the evidence that the acetyl‐CoA pathway occurs in other bacteria by a mechanism that is the same or similar to that found in C. thermoaceticum. For this purpose, the mechanism of the acetyl‐CoA pathway, as found in C. thermoaceticum, is described and hypothetical mechanisms for other organisms are presented based on the acetyl‐CoA pathway of C. thermoaceticum. The available data have been reviewed to determine if the hypothetical schemes are in accord with presently known facts. We conclude that the formation of acetyl‐CoA by other acetogens, the methanogens and sulphate‐reducing bacteria occurs by a mechanism very similar to that of C. thermoaceticum.

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Carbon monoxide dehydrogenase from Clostridium thermoaceticum (Ct-CODH) is a nickel-containing enzyme that catalyzes acetyl-CoA synthesis and CO oxidation at two separate Ni sites, the A-cluster and C-cluster, respectively. Carbon monoxide dehydrogenase from Rhodospirillum rubrum (Rr-CODH) contains only a C-type cluster and catalyzes only CO oxidation. We have used L-edge X-ray absorption spectroscopy to study the Ni electronic structure of these two enzymes. The spectra indicate that most of the Ni in asisolated Ct-CODH is low-spin Ni(II). Upon CO treatment, a fraction of the nickel is converted either to highspin Ni(II) and/or to Ni(I). Ni in dithionite-reduced Rr-CODH also exhibits a clear low spin Ni(II) component, again mixed with either high-spin Ni(II) or Ni(I). The spectrum of Rr-CODH shifts to higher energy upon indigo carmine oxidation, suggesting either that most of the high-spin Ni(II) is converted to low-spin Ni(II) and/or that some Ni is oxidized between these two forms. These results are discussed and compared with recent L-edge spectra for the Ni site in hydrogenase.