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1.
The final steps in the synthesis of acetyl-CoA by CO dehydrogenase (CODH) have been studied by following the exchange reaction between CoA and the CoA moiety of acetyl-CoA. This reaction had been studied earlier (Pezacka, E., and Wood, H. G. (1986) J. Biol. Chem. 261, 1609-1615 and Ramer, W. E., Raybuck, S. A., Orme-Johnson, W. H., and Walsh, C. T. (1989) Biochemistry 28, 4675-4680). The CoA/acetyl-CoA exchange activity was determined at various controlled redox potentials and was found to be activated by a one-electron reduction with half-maximum activity occurring at -486 mV. There is approximately 2000-fold stimulation of the exchange by performing the reaction at -575 mV relative to the rate at -80 mV. Binding of CoA to CODH is not sensitive to the redox potential; therefore, the reductive activation affects some step other than association/dissociation of CoA. We propose that a metal center on CODH with a midpoint reduction potential of less than or equal to -486 mV is activated by a one-electron reduction to cleave the carbonyl-sulfur bond and/or bind the acetyl group of acetyl-CoA. Based on a comparison of the redox dependence of this reaction with that for methylation of CODH (Lu, W-P., Harder, S. R., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3124-3133) and CO2 reduction and formation of the Ni-Fe-C EPR signal (Lindahl, P. A., Münck, E., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3873-3879), we propose that the assembly of the acetyl group of acetyl-CoA, i.e. binding the methyl group of the methylated corrinoid/iron-sulfur protein, binding CO, and methyl migration to form the acetyl-CODH intermediate, occur at the novel Ni-Fe3-4-containing site in CODH. CO has two effects on the CoA/acetyl-CoA exchange: it activates the reaction due to its reductive capacity and its acts as a noncompetitive inhibitor. We also discovered that the CoA/acetyl-CoA exchange was inhibited by nitrous oxide via an oxidative mechanism. In the presence of a low-potential electron donor, CODH becomes a nitrous oxide reductase which catalytically converts N2O to N2. This study combined with earlier results (Lu, W-P., Harder, S. R., and Ragsdale, S. W. (1990) J. Biol. Chem. 265, 3124-3133) establishes that the two-subunit form of CODH is completely active in all reactions known to be catalyzed by CODH. 相似文献
2.
M?ssbauer, EPR, and optical studies of the corrinoid/iron-sulfur protein involved in the synthesis of acetyl coenzyme A by Clostridium thermoaceticum 总被引:10,自引:0,他引:10
We have purified to homogeneity the 88-kDa corrinoid protein from Clostridium thermoaceticum which acts as a methyl carrier in the synthesis of acetyl-CoA. As shown here, this protein contains a [4Fe-4S]1+/2+ cluster in addition to a corrinoid. The corrinoid is 5-methoxybenzimidazolylcobamide, with an OH- group probably present as the upper axial ligand. Co+ is present in the reduced form, Co2+ in the as-isolated form, and Co3+ in the methylated form of the protein. The as-isolated corrinoid/Fe-S protein exhibits a Co2+ EPR signal lacking nitrogen superhyperfine splittings, indicating that the benzimidazole base is uncoordinated ("base-off") in the Co2+ state. Optical studies suggest that the Co3+-CH3 corrinoid is also base-off. In the as-isolated and methylated forms, the iron-sulfur cluster is diamagnetic, with quadrupole splittings and isomer shifts characteristic of [4Fe-4S]2+ clusters. The protein can be reduced by CO and CO dehydrogenase in the absence of ferredoxin. The EPR spectra of the reduced cluster exhibit two components: one with principal g-values at 2.07, 1.93, and 1.82 and the other at 2.02, 1.94, and 1.86. The M?ssbauer data show that these signals result from [4Fe-4S]1+ clusters. Chemical analysis shows that the iron:cobalt atomic ratio is close to 4:1, suggesting that a single [4Fe-4S]1+ cluster occurs in two distinct S = 1/2 spin states in the reduced state. Treatment with 1-2.5 M urea converts the two cluster forms into a single one, with EPR and M?ssbauer spectra of typical [4Fe-4S]1+ clusters. A 27-kDa corrinoid protein (Ljungdahl, L.G., LeGall, J., and Lee, J.P. (1973) Biochemistry 12, 1802-1808) also was purified and found to be inactive in the synthesis of acetyl-CoA, contrary to the suggestion of Ljungdahl et al. (1973). 相似文献
3.
Characterization of the recombinant Clostridium pasteurianum ferredoxin and comparison of its properties with those of the native protein 总被引:2,自引:0,他引:2
Ferredoxins (Fds) constitute an important class of nonheme iron-sulfur proteins. One of the most studied Fds is the [8Fe-8S] Fd from Clostridium pasteurianum. The gene for this Fd has previously been cloned and sequenced. We report the expression of this Fd in Escherichia coli, and the characterization and comparison of this recombinant protein to the native Fd. We have found that the purified recombinant protein has the same enzymatic, redox, magnetic and electronic properties as the native Fd isolated from C. pasteurianum, which indicates that the two [4Fe-4S] clusters present in the Fd were correctly formed in E. coli. 相似文献
4.
The reductive acetyl coenzyme A pathway: sequence and heterologous expression of active methyltetrahydrofolate:corrinoid/iron-sulfur protein methyltransferase from Clostridium thermoaceticum. 总被引:1,自引:1,他引:0 下载免费PDF全文
The methyltransferase (MeTr) from Clostridium thermoaceticum transfers the N5-methyl group of (6S)-methyltetrahydrofolate to the cobalt center of a corrinoid/iron-sulfur protein in the acetyl coenzyme A pathway. MeTr was purified to homogeneity and shown to lack metals. The acsE gene encoding MeTr was sequenced and actively expressed in Escherichia coli at a level of 9% of cell protein. Regions in the sequence of MeTr and the E. coli cobalamin-dependent methionine synthase were found to share significant homology, suggesting that they may represent tetrahydrofolate-binding domains. 相似文献
5.
CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) is one of the four known nickel enzymes. It is a bifunctional protein that
catalyzes the oxidation of CO to CO2 at a nickel iron-sulfur cluster (Cluster C) and a remarkable condensation reaction between a methyl group (donated from a
methylated corrinoid iron-sulfur protein), carbon monoxide, and coenzyme A to form acetyl-CoA at a separate nickel iron-sulfur
cluster (Cluster A). This review focuses on the current understanding of the structure and function of Cluster A and on related
model chemistry. It describes studies that uncovered the first example of a biological organometallic reaction sequence. The
mechanism of acetyl-CoA synthesis includes enzymebound methylnickel, iron-carbonyl, and acylmetal intermediates. Discovery
of the methylnickel species constituted the first example of an alkylnickel species in biology and unveiled a new biological
role for nickel.
Received: 10 April 1996 / Accepted: 4 July 1996 相似文献
6.
Many anaerobic bacteria fix CO2 via the acetyl-CoA pathway. Carbon monoxide dehydrogenase (CODH), a key enzyme in the pathway, condenses a methyl group, a carbonyl group from CO, CO2, or the carboxyl group of pyruvate, and CoA to form acetyl-CoA. When treated with CO, CODH exhibits an EPR signal which results from an organometallic complex containing nickel, at least 3 iron, and CO and has been referred to as the NiFeC signal. Although this EPR signal has been presumed to be the spectroscopic signature of the enzyme-bound C-1 precursor of the carbonyl group of acetyl-CoA, its catalytic relevance had not been rigorously studied. We have demonstrated the catalytic competence of this NiFeC species by showing that the rate of formation of the NiFeC EPR signal is faster than the rate of an isotope exchange reaction between CO and acetyl-CoA, a partial reaction in the overall synthesis. Generation of the NiFeC signal in the absence of CO by acetyl-CoA has been demonstrated and requires a one-electron reduction at a midpoint potential of -541 mV versus the standard hydrogen electrode. In addition, we have observed and characterized an isotope exchange reaction between the carbonyl group of acetyl-CoA and the carbonyl group of the NiFeC complex, indicating that the C in the NiFeC complex is in the form of CO. These combined results demonstrate that the NiFeCO complex exhibits the characteristics expected of the precursor of the carbonyl group of acetyl-CoA. 相似文献
7.
Q-Band ENDOR studies on carbon monoxide dehydrogenase (CODH) from the acetogenic bacterium Clostridium thermoaceticum provided unambiguous evidence that the reaction of CO with CODH produces a novel metal center that includes at least one nickel, at least three iron sites, and the carbon of one CO. The 57Fe hyperfine couplings determined by ENDOR are similar to the values used in simulation of the M?ssbauer spectra [Lindahl et al. (1989) J. Biol. Chem. 265, 3880-3888]. EPR simulation using these AFe values is equally good for a 4Fe or a 3Fe center. The 13C ENDOR data are consistent with the binding of a carbon atom to either the Ni or the Fe component of the spin-coupled cluster. The 13C hyperfine couplings are similar to those determined earlier for the C0-bound form of the H cluster of the Clostridium pasteurianum hydrogenase, proposed to be the active site of hydrogen activation [Telser et al. (1987) J. Biol. Chem. 262, 6589-5694]. The 61 Ni ENDOR data are the first nickel ENDOR recorded for an enzyme. The EPR simulation using the ENDOR-derived hyperfine values for 61Ni is consistent with a single nickel site in the Ni-Fe-C complex. On the basis of our results and the M?ssbauer data [Lindahl et al. (1989) J. Biol. Chem. 265, 3880-3888], we propose the stoichiometry of the components of the Ni-Fe-C complex to be Ni1Fe3-4S greater than or equal to 4C1, with four acid-labile sulfides. 相似文献
8.
CO dehydrogenase from Clostridium thermoaceticum. EPR and electrochemical studies in CO2 and argon atmospheres 总被引:2,自引:0,他引:2
The EPR and redox properties of the metal complexes in CO dehydrogenase (CODH) from Clostridium thermoaceticum were studied. Controlled potential coulometric reductive titrations of CODH were performed under argon and CO2 atmospheres. In the titrations performed under argon, five to eight electrons/dimer were required for reduction, and four distinct EPR signals appeared. These included a signal with gave = 1.82 (Em approximately -220 mV), two signals with the same g values but different linewidths at gave = 1.94 (Em approximately -440 mV), and a signal at gave = 1.86 (Em approximately -530 mV). All of the S = 1/2 EPR signals had low spin concentrations; values between 0.2 and 0.3 spins/dimer were typically obtained for each signal. Features between g = 6 and 4, typical of S = 3/2 states, were also observed, and these may account, at least to some degree, for the low spin concentration values. Under CO2, and at negative potentials, CODH served as an electrocatalyst in the reduction of CO2 to CO. The apparent half-maximal activity for this reduction at pH 6.3 occurred at -430 mV, a potential near the thermodynamic value. An EPR signal, arising from a complex containing Ni, Fe, and the carbon from CO/CO2 developed along with this activity. The reduction of this complex is probably the last step to occur prior to the catalysis of CO2 reduction. 相似文献
9.
Many anaerobic bacteria fix CO2 via the Wood pathway of acetyl-CoA synthesis. Carbon monoxide dehydrogenase (CODH), also called acetyl-CoA synthase, accepts the methyl group from the methylated corrinoid/iron-sulfur protein (C/Fe-SP), binds a carbonyl group from CO, CO2, or the carboxyl of pyruvate, and binds coenzyme A. Then CODH catalyzes the synthesis of acetyl-CoA from these enzyme-bound groups. Here, we have characterized the methyl transfer steps involved in acetyl-CoA synthesis. We have studied the reactions leading to methylation of CODH by methyl iodide and shown an absolute requirement of the C/Fe-SP in this reaction. In addition, we have discovered and partly characterized two previously unknown exchange reactions catalyzed by CODH: between the methylated C/Fe-SP and methylated CODH and between methylated CODH and the methyl moiety of acetyl-CoA. We have performed these two exchange reactions, methylation of the C/Fe-SP, and methylation of CODH at controlled potentials. The rates of all these reactions except the exchange between methylated C/Fe-SP and methylated CODH are accelerated (from 1 to 2 orders of magnitude) when run at low potentials. Our results provide strong evidence for a nucleophilic redox-active metal center on CODH as the initial acceptor of the methyl group from the methylated C/Fe-SP. This metal center also is proposed to be involved in the cleavage of acetyl-CoA in the reverse reaction. 相似文献
10.
Levels of enzymes involved in the synthesis of acetate from CO2 in Clostridium thermoautotrophicum 总被引:11,自引:7,他引:4 下载免费PDF全文
The acetogenic bacterium Clostridium thermoautotrophicum, grown on methanol, glucose, or CO2-H2, contained high levels of corrinoids, formate dehydrogenase, tetrahydrofolate enzymes, carbon monoxide dehydrogenase, and hydrogenase. Cell-free extracts catalyzed pyruvate-dependent formation of acetate from methyltetrahydrofolate. These results suggest that C. thermoautotrophicum synthesizes acetate from CO2 via a formate-tetrahydrofolate-corrinoid pathway. 相似文献