Nonheme iron proteins, mechanism

The rapid-quench method [78] was used in Ref. 83 to analyze the mechanism of a bacterial phenylalanine hydroxylase, a mononuclear nonheme iron protein that uses tetrahydropterin as the source of the two electrons needed to activate O2 for the hydroxylation of phenylalanine to tyrosine. Mossbauer spectra of samples prepared by freeze-quenching the reaction of the complex enzyme— Fe(ll)-phenylalanine-6-methyltetrahydropterin with O2 revealed the accumulation of an intermediate at short reaction times (20-100 ms). The Mossbauer parameters of the intermediate (3 = 0.28mms, A q= l.26mms ) suggested it to be a high-spin Fe(IV) complex similar to those that have previously been detected in the reactions of other mononuclear Fe(ll) hydroxylases. 

An additional component is the iron-sulfur protein (FeS nonheme iron) (Figure 12-6). It is associated with the flavoproteins (metallofiavoproteins) and with cytochrome b. The sulfur and iron are thought to take part in the oxidoreduction mechanism between flavin and Q, which involves only a single e change, the iron atom undergoing oxidoreduction between Fe " and Fe k... 

LOXs are proteins containing a single atom of nonheme iron in catalytic center, with the ferric enzyme in an active form. The free radical-mediated mechanism of LOX-catalyzed process may be presented as follows (see also Figure 26.1) ... 

Evidence for an alternative oxidative stress protection mechanism in sulfate-reducing bacteria has begun to emerge. Table 10.1 provides data on the proteins implicated in this alternative system. All but one of these proteins contain distinctive types of nonheme iron active sites. This chapter describes recent results on three of these novel proteins DcrH, Rbo, and Rbr, all from Desulfovibrio vulgaris HUdenborough. 

The ferriheme protein metmyoglobin (metMb) at the physiological pH 7.4 was reported to bind the NO molecule reversibly yielding the nitrosyl adduct [metMb(NO)] the kinetics of the association and dissociation processes were investigated and a limiting dissociation mechanism was proposed (58,68). 2-His-l-Glu nonheme iron center engineered into myoglobin was reported capable to bind Fe(II) and reduce NO to N2O (69). 

Flavodi-iron proteins containing a distinctive nonheme diiron/ flavin mononucleotide active site, catalyze reductive scavenging of dioxygen and NO in air sensitive microorganisms. Anaerobic addition of NO up to one NO per diferrous rmit results in formation of a diiron mononitrosyl complex, whereas further addition of NO results in two reaction pathways, one of which produces N2O and the diferric site and the other which produces a stable diiron-dinitrosyl complex. The production of N2O upon addition of NO to the mononitrosyl deflavo-protein was interpreted in terms of the li5rponitrite mechanism (71). 

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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