Enzyme redox catalysis

Redox catalysis Zn, Fe, Cu, Mn, Mo, Co, V Se, Cd, Nl Enzymes (see Table 11.4 for more Information) Reactions with oxygen (Fe, Cu) Oxygen evolution (Mn) Nitrogen fixation (Fe, Mo) Inhibition of llpid peroxidation (Se) Carbonic anhydrase (Cd) Reduction of nucleotides (Co) Reactions with H2 (Nl) Bromoperoxidase activity (V)... 

Electron tunneling may also be of significance for redox catalysis, including enzyme catalysis. In particular it may turn out to be a tool for carrying out catalytic reactions via multi-electron paths. For instance, according to the data of ref. 11, the two-electron reduction of molecular oxygen to hyd-... 

Furthermore, it should be noted that the same metal can often play different roles in different enzymes. For example, nickel(II) displays electrophilic catalysis in urease and redox catalysis in hydrogenase. 

In dehalogenating enzymes of anaerobic microorganisms, corrinoid cofactors have a newly discovered further role in the redox catalysis of the energy conserving dehalogenation of chloro(hydro)carbon compounds ( dehalorespiration ), and the specific redox properties of the protein-bound unusual corrinoids are of particular current interest. 

The large size of the pores of MCM-41 has also allowed the entrapment of enzymes, such as cytochrome c, papain and trypsin [193]. Enzyme entrapment has been extensively performed with sol-gel materials. The types of applications of redox catalysis using enzyme-mesoporous materials is expected to parallel the sol-gel materials, which is discussed in the last section of this chapter. 

As implied above, there is nothing dramatically special about photocatalysis. It is simply another type of catalysis alongside, as it were, redox catalysis, acid-base catalysis, enzyme catalysis, thermal catalysis and others. Consequently, it is worth reemphasising that any description of photocatalysis must correspond to the general definition of catalysis. This said, it could be argued that the broad label photocatalysis simply describes catalysis of a photochemical reaction. 

Oxyfenyl heme centers (Fe ==0) are now believed to be reactive intermediates in all heme enzymes that undergo redox catalysis. Fe =0 species have been observed or predicted in heme peroxidases (i), catalases (2), oxygenases (3), and cytochrome c oxidase 4). Myoglobin (Mb), which normally functions as a reversible 02-binding protein (5) and does not undergo redox catalysis, will, however, react with H2O2 to generate an Fe =0 center (6, 7). 

Many enzymes use redox centers to store and transfer electrons during catalysis. These redox centers can be composed of metals such as iron or cobalt, or organic cofactors such as quinones, amino acid radicals, or flavins. In order to fully appreciate the catalytic mechanisms of these enzymes, it is often necessary to determine the free energy required to reduce or oxidize their protein redox centers. This is called the redox potential. The measurement of enzyme redox potentials can be performed by either direct or indirect electrochemical methods. The type of electrochemistry suitable for a particular protein system is simply dictated by the accessibility of its redox center to the electrode surface. Because most reactions catalyzed by enzymes occur within hydrophobic pockets of the protein, the redox sites are often far from the surface of the protein. Unless an electron transfer path exists from the protein surface to the redox center, it is not feasible to use direct electrochemistry to measure the redox potential. Since only a few enzymes (most notably certain heme-containing enzymes) have such electron transferring paths and... 

Optimization of reaction complex topology implies the positioning of charged enzyme groups such that there is an optimum interaction with (activated) reactants. Significant protonation (in protolytic enz5unes) or electron transfer (in redox catalysis) can already occiu in the adsorbed state. This is due to the unique electrostatic properties of the enzyme, which is shielded from the solvent by its hydrophobic peptide framework. 

The EC mechanism is central to the concept of homogeneous redox catalysis, which is used to promote a redox reaction between an electrogenerated mediator and soluble reactant, under conditions where heterogeneous electron transfer to the reactant is restricted on kinetic grounds. Many redox processes between soluble mediators and oxidoreductase enzymes have also been shown to reduce to the simple EC mechanism under limiting conditions. ... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

Industrial applications inclnde the production of petrochemicals, fine chemicals and pharmacenticals (particnlarly throngh asymmetric catalysis), hydrometallurgy, and waste-treatment processes. Many life processes are based on metallo-enzyme systems that catalyse redox and acid-base reactions. 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

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