RAMAN AND INFRARED-SPECTROSCOPY OF CYANIDE-INHIBITED CO DEHYDROGENASEACETYL-COA SYNTHASE FROM CLOSTRIDIUM-THERMOACETICUM - EVIDENCE FOR BIMETALLIC ENZYMATIC CO OXIDATION/
D. Qiu et al., RAMAN AND INFRARED-SPECTROSCOPY OF CYANIDE-INHIBITED CO DEHYDROGENASEACETYL-COA SYNTHASE FROM CLOSTRIDIUM-THERMOACETICUM - EVIDENCE FOR BIMETALLIC ENZYMATIC CO OXIDATION/, Journal of the American Chemical Society, 118(43), 1996, pp. 10429-10435
Clostridium thermoaceticum and other autotrophic anaerobic bacteria co
ntain a bifunctional enzyme, carbon monoxide dehydrogenase/acetyl-CoA
synthase (CODH/ACS), that catalyzes two reactions of CO at two separat
e Ni-FeS clusters. Oxidation of CO to CO2 is catalyzed by Cluster C, w
hile incorporation of CO into acetyl-CoA occurs at Cluster A. In this
study, resonance Raman [RR] and infrared [IR] spectroscopy are applied
to the adduct of Cluster C with cyanide, a selective inhibitor of CO
oxidation. The RR spectra reveal that CN- binds simultaneously to Fe a
nd Ni, because bands whose C-13 and N-15 shifts identify them as cyani
de-metal stretching and bending modes are sensitive to incorporation o
f both Fe-54 and Ni-64 into the enzyme. The IR spectrum reveals a low
frequency, 2037 cm(-1), for the C-N stretch, indicative of Fe-II bindi
ng via the C end. Vibrational modeling of the frequencies and isotope
shifts indicates a bent Fe-CN-Ni bridging geometry, with a similar to
140 degrees C-N-Ni angle. This geometry of the inhibitory adduct sugge
sts that CO oxidation involves a bimetallic mechanism. It is proposed
that pre-organization of the metal ions by the enzyme promotes CO oxid
ation by Ni-II-OH- attack on Fe-II-CO, followed by Ni-FeS reduction as
CO2 is released. This chemistry is analogous to the metal-catalyzed w
ater-gas shift reaction.