Monooxygenase iron-containing

There is a great variety of iron-containing monooxygenases however, the heme-containing monooxygenases known so far are almost based on two classes of hemoproteins, the cytochromes P-450 and the closely related NO synthases [22],... 

Some of the first catalytic model systems for the simulation of the function of methane monooxygenase comprise monomeric as well as dimeric iron-containing model complexes bearing hydro-tris(pyrazolyl)borate ligands [6]. These complexes, e.g. 3, catalyze the oxidation of aromatic and aliphatic carbon-hydrogen bonds in the presence of oxygen (1 atm), acetic acid and zinc powder at room temperature (Scheme 2). 

Non-heme iron-containing monooxygenases are able to catalyze the oxidation of hydrocarbons to the corresponding hydrocarbon alcohols and have been found in methanotrophic bacteria [58]. 

Structure of the Iron Center Formation of the Iron Center and Tyrosyl Radical Spectroscopy of the Diferric Iron Center Spectroscopy of the Tyrosyl Radical Redox Properties of the Iron Center Mixed-Valent Form of the Iron Center Diferrous Form of the Iron Center Inhibitors to Iron-Containing Ribonucleotide Reductase Methane Monooxygenase A. Spectroscopy of the MMOH Cluster X-Ray Structure of MMOH... 

In addition to these three, there are also monooxygenase enzymes containing single nonheme iron or copper ions, or nonheme iron plus an organic cofactor such as a reduced pterin at their active sites.Just as with the dioxygenase enzymes, we do not know how similar the mechanisms of the different metal-containing monooxygenase enzymes are to one another. The enzyme for which we have the most information is cytochrome P-450, and we will therefore focus our discussion on that system. Speculations about the mechanisms for the other systems are discussed at the end of this section. 

Non-heme iron-containing monooxygenases, such as methane monooxygenase and o-hydroxylase, analogously to cytochrome P450, can transfer the oxygen atom from molecular oxygen to the C-H bonds of alkanes [24],... 

Another type of iron-containing monpoxygenase was flrst described by Bernhardt et al. ) and contains a two iron-two-acid-labile-sulfur cluster. It was isolated from bacteria and catalyzes the 0-demethylation of 4-methoxybenzoate The corresponding electron transport chain involves NADH, a flavoprotein and a second iron-sulfur protein It seems that many more bacterial monooxygenases belong to this type rather than to the heme-sulfur-containing category. 

These oxidation reactions require oxygen (O2) and tetrahydrobiopterin as a cofactor. Thus, as shown in Scheme 13.39, 7,8-dihydroneopterin 3 -triphosphate (generated from guanosine triphosphate [GTP] as seen in Scheme 12.118) is converted to 6-pyruvoyl-5,6,7,8-tetrahydropterin by an elimination reaction and two keto-enol isomerizations. The process is catalyzed by the enzyme 6-pyruvoyltetra-hydropterin synthase (EC 4.2.3.12). Then, via an intermediate, written as an equilibrium between a-hydroxyketones (named dihydrosepiapterin) linked by a common enol, reduction to tetrahydrobiopterin is effected (in two separate steps) by 2 equivalents of NADPH used by the enzyme sepiapterin reductase (EC 1.1.1.153). Tetrahydrobiopterin is the cofactor involved in the National Institutes of Health (NIH) shift (cf. Chapter 6) pathway used by the iron-containing enzyme phenylalanine 4-monooxygenase (EC 1.14.16.1) to convert phenylalanine (Phe, F) to tyrosine (Tyr, Y) and is converted to (6i )-6-(L-erythro-l,2-dihydroxypropyl)-5,6,7,8-tetrahydro-4a-hydroxypterin in the process. 

Scheme 1339. A representation of the conversion of phenylalanine (Phe, F) to tyrosine (Tyr, Y) using tetrahydrobiopterin as the cofactor (involved in the NIH shift [cf. Chapter 6] pathway) used by the iron-containing enzyme phenylalanine 4-monooxygenase (EC 1.14.16.1). EC numbers and some graphic materials provided in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmnl.ac.uk/iubmb/enzyme/.

In Scheme 13.40 and as noted above, the action of the iron-containing enzyme tyrosine 3-monooxygenase (EC 1.14.16.2) is shown to effect the conversion of tyrosine (Tyr, Y) and oxygen (O2) to 3,4-dihydroxyphenylalanine (L-dopa), while the cofactor tetrahydrobiopterin undergoes oxidation to 4a-hydroxytetrahydrobiop-terin. Then, the general aromatic-L-amino acid decarboxylase (EC 4.1.1.28), an enzyme that uses pyridoxal as a cofactor, effects the decarboxylation of the bisphe-noUc add to the corresponding amine, dopamine [3,4-dihydroxyphenethylamine, 2-(3,4-dihydroxyphenyl)ethanamine]. 

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