Ni-CODHs

Four crystal structures of Ni CODHs have been determined from the following organisms C. hydrogenoformans [87,88], Rhodospirillum (R.) rubrum [89] and the bifunctional CODH/ACS from M. thermoacetica [90,91]. In each case CODH has a very similar homodimeric quaternary structure with a diameter of about 100 A in the largest dimension and a total of five FeS clusters (Fig. 3A). An initially unexpected [4Fe - 4S] center, now called the D-cluster, is coordinated by the two subimits very close to the molecular surface. Each subunit also binds an additional [4Fe-4S] center, called the B-cluster, as well as the catalytic Ni-containing C-cluster. There is an electron transfer pathway between the physiological redox partner, the exposed D-cluster, then the B-cluster of one subunit and finally the C-cluster of the other subunit. The electron flow direction will depend on whether the enzyme reduces CO2 or oxidizes CO (Eq. 2). 

Fig. 5 Active site of MoCu-CODH (a) and Ni-CODH (b). The bidentate sulfur ligands of the MoCu-CODH denoted by So are part of a molybdopterin cytosine dinucleotide cofactor. Sj denotes an inorganic sulfide ligand...

Ni-CODH enzymes, found in anaerobic bacteria which have an [Ni4Fe5S] cluster as the active site. 

There is also the related class of Ni-CODH/ACS (acetyl CoA synthetase), where the CODH enzyme is part of a larger protein in which CO2 reduction is coupled to acetyl CoA synthesis. 

CODH and ACS shown to occur at discrete [Ni-X-Fe4S4] clusters, called Cluster C and Cluster A, respectively. ... 

The two subunits of CODH/ACS have been dissociated to offer a clearer picture of the ACS active site 135). The holoenzyme contains 2 Ni, 12 Fe, and 14 S 120) that are organized into 3 discrete clusters, whereas the isolated a subunit contains only 1 Ni and 4 Fe and has spectroscopic properties similar to those of Cluster A in the native enzyme 186). Based on EXAFS spectroscopy of the a. subunit, the Ni site in Cluster A has been proposed to be coordinated to 2 sulfur ligands at 2.19 A and 2 nitrogen or oxygen ligands at 1.89 A in a distorted square plane 186). 

Another possible mechanism also involves a methyl radical (mechanism 3). In this case, electron transfer from one of the reduced clusters on CODH/ACS to methyl-Co(III) would form a methyl-Co(II) species that can disproportionate to form Co(I) and methyl-Ni(II). 

The subunits of CODH/ACS have been isolated (see earlier discussion). The isolated a subunit contains one Ni and four Fe and has spectroscopic properties (186) similar to those of Cluster A, the active site of acetyl-CoA synthesis (212). Unfortunately, it has no ACS activity. Therefore, ACS activity may reside in the a subunit or it may require both the a and the fi subunits. If Clusters B and/or C of the B subunit are involved in acetyl-CoA synthesis, one possible role could be in electron transfer. Although acetyl-CoA synthesis and the CO/ exchange reactions do not involve net electron transfer, both of these reactions are stimulated by ferredoxin, indicating that internal electron transfer within CODH/ACS may be required during the reaction (121). Further studies with the isolated subunits and the reconstitu-... 

The biologically uncommon Ni center associated with FeS clusters is a powerful and unique catalytic unity. In this chapter we have reviewed the structural and mechanistic aspects of three NiFeS centers the active site of hydrogenase and Clusters A and C of CODH/ACS. In the former, the association of a Ni center with the most unusual FeCOCN2 unit is a fascinating one. Model chemists, spectroscopists, and crystallographers have joined efforts to try and elucidate the reaction mechanism. Although a consensus is being slowly reached, the exact roles of the different active site components have not yet been fully established. Ni appears to be the catalytic center proper, whereas the unusual Fe center may be specially suited to bind a by-... 

Although the crystal structure of CODH or CODH/ACS has not yet been solved, a great deal of work has been done on these enzymes and plausible catalytic mechanisms have been proposed. Concerted action between the Ni ion and one of the Fe centers of a 4Fe-4S cluster are thought to elicit the formation of CO2 from CO. But perhaps the most extraordinary reaction is the one catalyzed by Cluster A the insertion of CO to a Ni-CHs complex. Through the two reactions catalyzed by CODH/ACS, the highly toxic, CO is not only removed, but is used as a source of carbon and electrons. 

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

Mononuclear Ni complexes have been investigated as functional models for individual steps of the reactions mediated by the CODH/acetyl coenzyme A synthase.2018-2020 These are mentioned in the respective sections on mononuclear Ni complexes. The dinuclear type (770) complexes are... 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

The reactions of carbon dichalcogenides other than C02 with Ni° have also been studied quite intensively.2460,2461 Inter alia, Ni polymeric complexes with metallic properties have been prepared using CS2 as the starting material.2462 Some further impetus was given by the observation that CS2 can mimic the binding of CO to the CODH enzyme active site, although CS2 itself does not undergo oxidation/reduction and does not appear to bind to center C (the site of CO oxidation and C02 reduction).2463... 

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