Flavoprotein oxygenases

We have recently studied the -oxidation of cyanatryn further, however, and have come to the conclusion that it is catalysed by the cytochrome P450 system rather than by the flavoprotein -oxygenase (9). The conclusions were based on the following ... 

Of the various proposals for the mechanism of activation of molecular oxygen by flavoprotein oxygenases, we have chosen to believe that oxygen reacts with dihydroflavin to yield a 4a-hydroperoxyflavin (Equation 32). The inability to detect 4a-FlHOOH in the reaction of... 

N-oxidation can occur in a number of ways to give either hydroxylamines from primary and secondary amines [Eqs. (11) and (12)], hydroxamic acids from amides, or N-oxides from tertiary amines [Eq. (13)]. The enzyme systems involved are either cytochrome P450 or a flavoprotein oxygenase. Hydroxylamines may be further oxidized to a nitro compound via a nitroso intermediate [Eq. (11)]. Oximes can be formed by rearrangement of the nitroso intermediate or N-hydroxylation of an imine, that could in turn be derived by dehydration of a hydroxylamine [Eq. (11)]. N-Oxides may be formed from both tertiary arylamines and alkylamines and from nitrogen in heterocyclic aromatic systems, such as a pyridine ring. 

Kishore, G. M., and E. E. Snell Reactivity of an FAD-dependent Oxygenase with Free Flavins A New Mode of Uncoupling in Flavoprotein Oxygenases. Bio-chem. Biophys. Res. Commun. 87, 518 (1979). 

Dioxygenases that form ds-diols are composed of two or three components, forming ds-dihydrodiols or ds-diol carboxylic acids, respectively. The three-component dioxygenases are composed of a flavoprotein, a ferredoxin, and a terminal oxygenase. 

Cytochrome P-450 has been characterized in four stable states [Fe, Fe " RH, Fe RH, (O2—Fe ) RH (metastable)] during its oxygenase reaction cycle. In the complete native system a flavoprotein and a redoxin (putidaredoxin) act as electron donors but also as effectors that complement the cytochrome. In the more complex microsomal system the sequence and intermediates are less well defined the electron-transfer chain contains two flavoproteins and one cytochrome, whose interactions with cytochrome P-450 are still the subject of great controversy. 

I, 2-diol and NAD. This multiprotein complex contains an iron-sulfur flavoprotein, an iron-sulfur oxygenase, and ferredoxin. See also Phthalate Dioxygenase... 

Benzoate 1,2-dioxygenase [EC 1.14.12.10], also called benzoate hydroxylase, catalyzes the reaction of benzoate with dioxygen and NADH to generate catechol, carbon dioxide, and NAD+. This is a multiprotein system which contains a reductase which is an iron-sulfur flavoprotein (FAD) and an iron-sulfur oxygenase. 

It should be noted that non-heme oxygenases can also degrade aromatics such as biphenyls and naphthalene (Scheme 7.23). A naphthalene dioxygenase consists of a catalytic oxygenase component with a mononuclear iron site, an iron-sulfur flavoprotein reductase and an iron-sulfur ferredoxin transferring electrons from... 

Flavins are very versatile redox coenzymes. Flavopro-teins are dehydrogenases, oxidases, and oxygenases that catalyze a variety of reactions on an equal variety of substrate types. Since these classes of enzymes do not consist exclusively of flavoproteins, it is difficult to define catalytic specificity for flavins. Biological electron acceptors and donors in flavin-mediated reactions can be two-electron acceptors, such as NAD+ or NADP+, or a variety of one-electron acceptor systems, such as cytochromes (Fe2+/ Fe3+) and quinones, and molecular oxygen is an electron acceptor for flavoprotein oxidases as well as the source of oxygen for oxygenases. The only obviously common aspect of flavin-dependent reactions is that all are redox reactions. 

The other classes of flavoproteins in table 10.2 interact with molecular oxygen either as the electron-acceptor substrates in redox reactions catalyzed by oxidases or as the substrate sources of oxygen atoms for oxygenases. Molecular oxygen also serves as an electron acceptor and source of oxygen for metalloflavoproteins and dioxygenases, which are not listed in the table. These enzymes catalyze more complex reactions, involving catalytic redox components, such as metal ions and metal-sulfur clusters in addition to flavin coenzymes. 

Bacterial flavoprotein monooxygenase (cyclohexanone oxygenase) has been used for the oxidation and rearrangement of allyl selenides, but the products have not been analyzed for enantiomer composition10,11. 

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