Internal monooxygenases

All the internal monooxygenases that have so far been purified and characterized contain flavin coenzymes. The external hydrogen donors include reduced NAD, reduced NADP, ascorbic acid and sulfhydryl compounds. Cofactors required for the external monooxygenases are flavin, pteridine, copper, nonheme iron and heme as cytochrome P-450. In some monooxygenase reactions, enzymes and/or electron carrier systems other than monooxygenase itself are involved in the transfer of an electron or hydrogen from the external hydrogen donor to the cofactor involved. 

Two classes of monooxygenases are known. Those requiring a cosubstrate (BH2 of Eq. 18-36) in addition to the substrate to be hydroxylated are known as external monooxygenases. In the other group, the internal monooxygenases, some portion of the substrate being hydroxylated also serves as the cosubstrate. Many internal monooxygenases contain flavin cofactors and are devoid of metal ions. 

An alternative mechanism for the oxidation of phenolic compounds is enzyme-catalyzed oxidation. Several classes of enzymes can catalyze this reaction. According to the Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB), these enzymes are part of the E C. 1 class of oxidoreductases (see the Internet web site http //www.chem.qmul.ac.uk/iubmb/enzyme/ECl). The three main classes of enzymes that catalyze the oxidation of phenolic compounds are the oxidoreductases that use oxygen as electron acceptor (E.C. 1.10.3), the peroxidases (E.C. 1.11.1), and monophenol monooxygenase (E.C. 

The name of internal monooxygenase was proposed by Hayaishi and Nozaki643 for this type of enzyme those that require a cofactor are called external monooxygenases. 

Chan, S. I., Nguyen, H.-H. T., Shiemke, A. K., and Lidstrom, M. E., 1992, Biochemical and biophysical studies toward characterization of the membrane-associated methane monooxygenase. 7th Intern. Symp. on Microbial Growth on Cl Compounds. J. C. Murrell, and D. P. Kelly. Andover UK, Intercept Ltd., 93nl07. 

Tyrosinase catalyzes two reactions, the hydroxylation of phenolic compounds in ortho-position (cresolase activity) and subsequently the oxidation of the diphenolic products (cat-echolase activity).Tyrosinase as well as another enzyme that catalyzes only the oxidation reaction, catecholoxidase (EC 1.10.3.1), belongs to the group of phenoloxidases. The monooxygenase nature of Ty was established by Mason and coworkers in a pioneering study using 0-labeled oxygen. The two-electron donor required in the hydroxylation reaction is the o-diphenol, which is generated internally from the monophenol substrate. 

Carbon monoxide inhibited the 6/3-. la-, and 16a-hydroxylation of testosterone by rat liver microsomes to different extents. A C0/02 ratio of 0.5 inhibited the la-, 6/i-, and 16a-hydroxylation reactions by 14%, 25%, and 36%, respectively, and the ratio of C0/02 needed for 50% inhibition of testosterone hydroxylation in the 16a-, 6/3-, and 7a-positions was 0.93, 1.54, and 2.36, respectively (36,48). Studies on the photochemical action spectrum revealed that CO inhibition of the three hydroxylation reactions was maximally reversed by monochromatic light at 450 nm, but there were differences in the shape of the photochemical reactivation spectra for the 6/3-, la-, and 16a-hydroxylation reactions (36,48). The data from our laboratory summarized above and at the First International Symposium on Microsomes and Drug Oxidation in 1968 pointed to multiple cytochromes P450 with different catalytic activities that were under separate regulatory control (36,45,46), and we indicated that the actual number of cytochromes that participate in the multiple hydroxylation reactions must await the solubilization and purification of the microsomal system (36). The use of different inducers of liver microsomal monooxygenases caused selective increases in the concentration of specific cytochromes P450 in fiver microsomes that greatly facilitated the isolation and purification of these hemoproteins. 

To understand the role of these bacteria in methane cycling, the methane oxidation system must be studied. In methanotrophs, methane is oxidized to methanol by an enzyme called the methane monooxygenase (MMO) (I), which uses methane, molecular oxygen, and reducing equivalents to produce methanol and water. All known methanotrophs contain a membrane-bound MMO, called the particulate methane monooxygenase (pMMO). The presence of this enzyme system is correlated with the complex internal membrane system found in all known methanotrophs. 

However, a few species of methanotrophs also have the ability to produce a second cytoplasmic enzyme, called the soluble methane monooxygenase (sMMO) (1, 2) (Table I). When the sMMO is present, the complex internal membrane systems are absent. The sMMO has been intensively studied in the past few years, and much is known about this enzyme (5, 6) (Table II). It was purified and characterized from three strains, and in all three cases it consists of three components containing a total of five polypeptides. The genes for these polypeptides, were cloned and sequenced from two strains and show... 

The latter enzymes had earlier been called mixed function oxidases by Mason in order to characterize their double functions as oxygenases and oxidases. For the reduction of one oxygen atom of molecular oxygen to water two electrons from an external donor have to be supplied. Sometimes the substrate itself can provide these reducing equivalents, so that for these enzymes the term internal monooxygenases may be used ... 

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