Enzymic Oxidative Coupling

F. Holz, Automatic, Continuous-Flow, Photometric Determination of Soil Enzyme Activities by the Use of (Enzyme) Oxidative Coupling Reactions. Part 1. Determination of Catalase Activity [in German]. Landwirtsch. Forsch., 39 (1986) 139. 

Oshima R, Yamauchi Y, Watanabe C, Kumanotani J 1985 Enzymic oxidative coupling of urushiol in sap of the lac tree, Rhus vernicifera. J Org Chem 50 2613-2621... 

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

Cytochrome c oxidase is an enzyme that couples the one-electron oxidation of cytochrome c to the four-electron reduction of 02 and is thus a crucial component of respiration. Cytochrome c contains the redox-active heme c, while cytochrome c oxidase contains a dinuclear Cua redox site in subunit II and three redox-active sites in subunit I heme a, heme a3, and Cur. It is believed that heme a is an electron-transfer site, while heme a3 and Cur function together at the 02 reduction site. 

The basic study was performed on copper complexes with N,N,N, N1-tetramethylethane-1,2-diamine (TMED), which were known to be very effective oxidative coupling catalysts (7,12). From our first kinetic studies it appeared that binuclear copper complexes are the active species as in some copper-containing enzymes. By applying the very strongly chelating TMED we were able to isolate crystals of the catalyst and to determine its structure by X-ray diffraction (13). Figure 1 shows this structure for the TMED complex of basic copper chloride Cu(0H)Cl prepared from CuCl by oxidation in moist pyridine. 

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

The fungal enzyme from R. praticola was able to catalyze the oxidative coupling of pentachlorophenol (PCP) and syringic acid, a representative of phenol carboxylic acids from lignin occurring in HS structures. 

Cytochrome c and ubiquinol oxidases are part of an enzyme superfamily coupling oxidation of ferrocytochrome c (in eukaryotes) and ubiquinol (in prokaryotes) to the 4 e /4 reduction of molecular oxygen to H2O. After this introduction, we will concentrate on the cytochrome c oxidase enzyme. The two enzymes, cytochrome c oxidase (CcO) and ubiquinol oxidase, are usually defined by two criteria (1) The largest protein subunit (subunit I) possesses a high degree of primary sequence similarity across many species (2) members possess a unique bimetallic center composed of a high-spin Fe(II)/(III) heme in close proximity to a copper ion. Cytochrome c oxidase (CcO) is the terminal... 

In recent years, numerous applications of such peroxidase-catalyzed oxidative coupling of phenols and aromatic amines have been reported (Table 7). These peroxidase-catalyzed biotransformations lead to modified natural products with high biological activities [110-118]. Several examples have also been described for the oxidative coupling of phenols with peroxidases and other oxidative enzymes from a variety of fungal and plant sources as whole cell systems... 

It has been demonstrated that Mn is the preferred substrate for MnP (13-17). The enzyme oxidizes Mn to Mn and the Mn produced, complexed with a suitable carboxylic acid ligand (12-16), diffuses from the enzyme and in turn oxidizes the organic substrates (6,8,13-17). Thus the Mn ion participates in the reaction as a diffusible redox couple (Fig. 1) rather than as an enzyme-binding activator. In support of this concept, we have demonstrated that chemically prepared Mn complexed with a carboxylic acid ligand such as malonate or lactate mimics the reactivity of the enzyme (6,8,14,15). 

In cases where an oxidizing enzyme is coupled with a second, non-redox enzyme, obviously, only the oxidizing component needs to be considered in terms of electron transfer cofactor regeneration. 

From L-tyrosine, or alternatively from L-phenylalanine, there is one further alkaloid biosynthesis pathway. This is the galanthamine pathway (Figure 38). Galanthamine synthesizes with tyramine, norbelladine, lycorine, crinine, N-demethylnarwedine and Al-demethylgalanthamine. Schiff base and reduction reaction, oxidative coupling and enzyme NADPH and SAM activity occur in this pathway. Schiff base is a reaction for the ehmination of water in formation with the C—N bonds process. 

From norbelladine, through the activity of the SAM, the 4 -0-methylnorbelladine synthesizes, and again is transformed to lycorine, crinine and, by oxidative coupling, to A-demethylarwedine, which is the object of enzyme NADPH activity. Galanthamine is synthesized by transformation trough the activity of the SAM from A-demethylgalanthamine. 

Oxidative coupling of (5)-(-)-laudanosoline (5) with horseradish peroxidase in the presence of hydrogen peroxide, studied by Brossi et al. (27), afforded dibenzopyrrocoline (—)-6 in 81% yield, and conversion to (5)-(—)-0-meth-ylcryptaustoline (14) by methylation provided additional proof for the absolute configuration of this and related alkaloids. Enzyme specificity in the C— coupling reaction was demonstrated with similar oxidation of (/ )-(—)-laudanosoline methiodide, which afforded an aporphine converted by O-meth-ylation to (R)-(-)-glaucine. 

Finally, mention must be made of possible conjugation reactions in which a covalent bond is formed between a contaminant molecule and a second contami nant molecule or soil organic matter. Oxidative coupling reactions of phenolics and aromatic amines are catalyzed by extracellular enzymes, clays, and oxides (Wang et al., 1986 Liu et al., 1987 Fluang, 1990). The bioavailability of the synthetic organic within the product is reduced or possibly eliminated (Dec et al., 1990 Allard et al., 1994). 

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