Nitrile-hydratase 2+ complex

Heinrich L, Li Y, Vaissermann J, Chottard J-C (2001) A bis(carboxamido-N)diisocyanidobis (sulfenato-S)cobalt(HI) complex, model for the post-translational oxygenation of nitrile hydratase thiolato ligands. Eur J Inorg Chem 1407-1409... 

Figure 24 A five-coordinate Co complex of an N3S2 ligand, a proposed model for the active site of nitrile hydratase...

Many simple complexes have been prepared as models of active sites of biomolecules. For example, a reactive five-coordinate thiolate Co complex (Figure 24) was prepared to model the active site of nitrile hydratase, a Co or Fe metalloenzyme that promotes the conversion of nitriles to amides. The synthesized model complex is facile in its uptake and release of azide and thiocyanate, indicating that an appropriate nonleaving group environment enhances ligand displacement sufficiently for catalytic paths in non-redox active Co metalloenzymes. Other examples have appeared earlier in this report. 

While Fe(SCys)4, [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters all function as one-electron donors or acceptors, the more complex double-cubane [8Fe-7S] cluster that is found only in nitrogenases (see Nitrogenase Catalysis Assembly) has the potential to mediate two-electron transfer processes.Three methods have been employed to functionalize Fe-S centers for substrate binding and activation. The first involves having an accessible Fe coordination site as in the mononuclear Fe centers of nitrile hydratase and SOR, and the [4Fe-4S] clusters at the active sites of hydratases/dehydratases and radical-5-adenosylmethionine (SAM) enzymes.Indeed the recent recognition of the importance of the superfamily of radical-SAM enzymes in initiating radical reactions, via cluster-mediated reductive cleavage of SAM to yield a... 

Functional models of nitrile hydratase should reproduce the key reactions occuring at the active site. In addition to the catalytic hydrolysis of nitriles to amides, these also include photo-induced cleavage of coordinated nitric oxide and thiolate oxygenation under aerobic conditions. The nitrosyl complex 3 displays significant structural and electronic features in common with NHasCdark These spectroscopic similarities, however, do not translate to a functional model of photo-regulated NO dissociation. 

Most industrial enzymatic processes refer to reactions conducted by hydrolases in aqueous medium for the degradation of complex molecules (often polymers) into simpler molecules in conventional processes with limited added value (Neidelman 1991). Reasons underlying are clear since hydrolases are robust, usually extracellular and have no coenzyme requirements, which makes them ideal process biocatalysts. Enzyme immobilization widened the scope of application allowing less stable, intracellular and non-hydrolytic enzymes to be developed as process biocatalysts (Poulsen 1984 D Souza 1999), as illustrated by the paradigmatic case of glucose isomerase for the production of HFS (Carasik and Carroll 1983) and the production of acrylamide from acrylonitrile by nitrile hydratase (Yamada and Kobayashi 1996). 

Table 1 collects selected metric parameters and IR stretching frequencies (wno) fot a representative set of six-coordinated (6C) complexes of iron, ruthenium, and osmium, and few iron 5C-compounds. A number of Fe(III) heme proteins are involved in the regulation of the NO biosynthesis by the enzyme NO synthase, in NO transport (as vasodilator) in nitrophorins and in NO inhibition processes of cytochrome P450 and related enzymes (45). Heme enzymes are also intermediates in cyt cd nitrite reductases (NIRs), (46) and in NO reduction by a fungal cyt P450 NO reductase (47). The nonheme Fe(III) nitrile hydratase enzyme (NHase) is relevant to the microbial assimilation of organic nitriles, using... 

For organisms which express both pathways for nitrile hydrolysis, the stereochemical pathways can be very complex. The latter is illustrated by the microbial resolution of cx-aryl-substituted propionitriles using a Rhodococcus butanica strain (Scheme 2.109) [697]. Formation of the natural L-acid and the o-amide indicates the presence of an L-specific amidase and a nonspecific nitrile hydratase. However, the occurrence of the (5)-nitrile in case of Ibuprofen (R = i-Bu, e.e. 73%) proves the enantioselectivity of the nitrile hydratase [694]. In a related approach, Brevibacter-ium imperiale was used for the resolution of structurally related a-aryloxypropionic nitriles [698]. 

Lee CM, Hsieh CH, Dutta A, Lee GH, Liaw WF. 2003. Oxygen binding to sulfur in nitrosylated iron—thiolate complexes relevance to the Fe-containing nitrile hydratases. JAm Chem Soc 125(38) 11492-11493. 

Several synthetic Co(III) and Fe(III) complexes have been generated as models for the active-site metal center in nitrile hydratases [8, 10]. However, few have been examined in terms of water coordination and acidity. Cobalt complexes supported by Ns-type donor ligands, with two carboxamido nitrogen donors, exhibit a pfCa near 7 (Fig. 8.14a and b) [59, 60]. Introduction of two thiolate sulfur donors into the Co(III) coordination sphere increases the pK of the bound water to 8.3 (Fig. 8.14c) [61]. Interestingly, oxidation of one of the sulfur donors to a S-bound sulfmate (Fig. 8.14d) reduces the pK by around 1 unit [62]. As the active-site metal center in nitrile hydratases contain oxidatively modified cysteine residues coordinated to the metal center [8], it has been suggested that the oxidized sulfur donors play a role in modulating the acidity of the metal-bound water molecule. 

A model complex for Fe(III)-containing nitrile hydratase Et4N[Fe(PyPS)(H20)], Fig. 8.15) has been shown to exhibit a pfC value around 2 units lower than that found for the Co(III) analog (Fig. 8.14c) [63]. Notably, this value is also well below that found for [Fe(3,4-TDTA)(H20)] (pfQ = 8.2), a compound in which TDTA is a tetra anionic ligand with a N2O4 donor set [64]. 

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