Enzymes with Mononuclear Active Sites

Because of the increasing availability of protein structures, our understanding of nonheme iron enzymes with mononuclear active sites has also developed significantly since the first edition of this book. In addition to the previously reported... 

A wide range of soluble redox enzymes contain one or more intrinsic [2Fe-2S]2+ +, [3Fe-4S]+ , or [4Fe S]2+ + clusters that function in electron transport chains to transfer electrons to or from nonheme Fe, Moco/Wco, corrinoid, flavin, thiamine pyrophosphate (TPP), Fe S cluster containing, or NiFe active sites. Many have been structurally and spectroscopically characterized and only a few of the most recent examples of each type are summarized here. Dioxygenases that function in the dihydroxylation of aromatics such as benzene, toluene, benzoate, naphthalene, and phthalate contain a Rieske-type [2Fe-2S] + + cluster that serves as the immediate electron donor to the monomeric nonheme Fe active site see Iron Proteins with Mononuclear Active Sites). The xanthine oxidase family of molybdoenzymes see Molybdenum MPT-containing Enzymes) contain two [2Fe-2S] + + clusters that mediate electron transfer between the Moco active site and the Other soluble molybdoen-... 

Superoxide Reductase (see Iron Proteins with Mononuclear Active Sites). Detoxification of reactive oxygen species in anaerobic microorganisms has recently been shown to center around SOR, a novel mononuclear iron enzyme that reduces superoxide to hydrogen peroxide (see equation 4), rather than dismuting superoxide to oxygen and hydrogen peroxide as is the case for the superoxide dismutases found in aerobic organisms. 

Cobalt B Enzymes Coenzymes Cytochrome Oxidase Iron Heme Proteins Electron Transport Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Models of Protein Active Sites Metallocenter Biosynthesis Assembly. Metalloregulation Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors Nitrogenase Catalysis Assembly Photosynthesis Tungsten Proteins Vanadium in Biology Zinc DNA-binding Proteins. 

Cobalt Inorganic Coordination Chemistry Copper Inorganic Coordination Chemistry Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 2 Sites Cytochrome Oxidase Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases Iron Proteins with Mononuclear Active Sites Metal Ion Toxicity Metal-related Diseases of Genetic Origin Metallocenter Biosynthesis Assembly Metalloregulation Peptide Metal Interactions Zinc Enzymes. 

N-Heterocycles as ligands in dioxygen activation by enzymes with mononuclear nonheme iron-containing active sites 96CRV2607. 

Que, L. and R. Y. N. Ho (1996). Dioxygen activation by enzymes with mononuclear non-heme iron active sites. Chem. Rev. 96(7) 2607-2624. 

Mononuclear Active Sites). This reaction has important commercial applications, and nitrile hydratase has been nsed indnstrially for enzymatic production of acrylamide from acrylonitrile. The enzyme contains a mononuclear Fe(II) (or Co(II)) active site, the metal ion being bound in a highly unusual coordination involving two deprotonated peptide amides together with three cysteine side chains (Figure 4). 

Another important contribution of pulse radiolysis is in the evaluation of redox processes in native SODs and development of SOD mimics. SOD is an endogenous antioxidant enzyme which catalyzes the conversion of Oj radicals to H2O2. Different types of SODs are present in cells such as Mn-SOD in mitochondria and Cu, Zn-SOD in the cytosol and in extracellular surfaces. Reactions of O " radicals with the active site of native SODs from bacterial and animal sources have been examined. In one recent study involving superoxide reductase (SOR) from Desulfoarculus baarsii, the precise step responsible for the catalytic action was examined. Its active site contains an unusual mononuclear ferrous center. Since protonation processes are essential for the catalytic action, the pH dependence of the redox properties of the active site, both in the absence and in the presence of O radicals, was studied using pulse radiolysis. The results confirmed that the reaction of SOR with O2" radicals involves two reaction intermediates, an iron(III)-peroxo species and an iron(III)-hydroperoxo species. The protonation takes place in the second step, and therefore responsible for its catalytic activity. 

Figure 17.3 Anatomy of a redox enzyme representation of the X-ray crystallographic structure of Trametes versicolor laccase III (PDB file IKYA) [Bertrand et al., 2002]. The protein is represented in green lines and the Cu atoms are shown as gold spheres. Sugar moieties attached to the surface of the protein are shown in red. A molecule of 2,5-xyhdine that co-crystallized with the protein (shown in stick form in elemental colors) is thought to occupy the broad-specificity hydrophobic binding pocket where organic substrates ate oxidized by the enzyme. Electrons from substrate oxidation are passed to the mononuclear blue Cu center and then to the trinuclear Cu active site where O2 is reduced to H2O. (See color insert.)...

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 zinc acetate complex of tris(3-/-butyl-5-methylpyrazol-l-yl)borate was prepared as a structural model for carbonic anhydrase and comparison with the enzyme active site structures confirmed that the complexes are excellent structural models.239 A mononuclear zinc hydroxide complex can also be formed with the tris(pyrazolyl) borate ligand system as a structural model for carbonic anhydrase.240... 

One last class of mononuclear non-haem iron enzyme that we have not yet considered, consists of the microbial superoxide dismutases with Fe(III) at their active site. The crystal structure of the E. coli enzyme shows a coordination geometry reminiscent of protocatechuate 3,4-dioxygenase, with four endogenous protein ligands, three His and one Asp residue, and one bound water molecule (Carlioz et ah, 1988). 

Amino acid is one of the most important biological ligands. Researches on the coordination of metal-amino acid complexes will help us better understand the complicated behavior of the active site in a metal enzyme. Up to now many Ln-amino acid complexes [50] and 1 1 or 1 2 transition metal-amino acid complexes [51] with the structural motifs of mononuclear entity or chain have been synthesized. Recently, a series of polynuclear lanthanide clusters with amino acid as a ligand were reported (most of them display a Ln404-cubane structural motif) [52]. It is also well known that amino acids are useful ligands for the construction of polynuclear copper clusters [53-56], Several studies on polynuclear transition metal clusters with amino acids as ligands, such as [C03] [57,58], [Co2Pt2] [59], [Zn6] [60], and [Fe ] [61] were also reported. 

The first zinc enzyme to be discovered was carbonic anhydrase in 1940, followed by car-boxypeptidase A some 14 years later. They both represent the archetype of mono-zinc enzymes, with a central catalytically active Zn2+ atom bound to three protein ligands, and the fourth site occupied by a water molecule. Yet, despite the overall similarity of catalytic zinc sites with regard to their common tetrahedral [(XYZ)Zn2+-OH2] structure, these mononuclear zinc enzymes catalyse a wide variety of reactions, as pointed out above. The mechanism of action of the majority of zinc enzymes centres around the zinc-bound water molecule,... 

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