Monooxygenase nonheme iron-containing

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. 

The answer is d. (Murray, pp 123—148. Scriver, pp 2367—2424. Sack, pp 159—175. Wilson, pp 287-317.) Some monooxygenases found in liver endoplasmic reticulum require cytochrome P450. This cytochrome acts to transfer electrons between NADPH, O2, and the substrate. It can be an electron acceptor from a flavoprotein. In the mitochondrial electron transport chain, flavoproteins donate electrons to coenzyme Q, which then transfers them to other cytochromes. Flavoproteins that are oxidases often react directly with molecular oxygen to form hydrogen peroxide. Flavoproteins can be NADH dehydrogenases that oxidize NADH and transfer the electrons to coenzyme Q. The electron transfer centers of flavoproteins in the electron transport chain contain nonheme iron and sulfur. 

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. 

There is a possibility that a couple of nonheme monooxygenases other than methane monooxygenase possess paired iron centers, but most of the iron proteins are suggested to contain a monomeric iron site. These include tyrosine hydroxylase [13-15] phenylalanine hydroxylase [16, 17], and isopenicillin N synthase [18,19]. Unfortunately, the active site structures of this class of enzymes have not been elucidated to date. Neither have the reaction mechanisms of these understood (recently, the crystal structure of isopenicillin N synthase has been reported) [20]. The function of this enzyme is not hydroxylation reaction but catalyzes the cyclization of L-5-(a-aminoadipoyl)-L-cysteinyl-D-valine to afford isopenicillin, while the catalytic reaction of this enzyme is assumed to include the reductive activation of dioxygen which affords water and high valent 0x0 iron species as... 

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