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MODEL STUDIES ON NONHEME MONOOXYGENASES - CHEMICAL MODELS FOR NONHEME IRON AND COPPER MONOOXYGENASES MAS AMI ITO, KIYOSHI FUJISAWA, NOBUMASA KITAJIMA, AND YOSHIHIKO MORO-OKA Research Laboratory of Resources Utilization Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
8.1. Introduction Occurrence of a wide variety of nonheme iron monooxygenases is known as described in Chapter 7. The best characterized example among this class of proteins is methane monooxygenase [1-3] of which the active site consists of a paired iron centers [4]. Because of its remarkable catalytic activity, effective for aerobic oxidation of methane to methanol, the structure/catalytic mechanism of methane monooxygenase (MMO) has attracted the attention of many chemists, and the X-ray structural analysis of the resting state [5] of its hydroxylase component (MMOH) as well as the spectroscopic identification of some reaction intermediates have been recently accomplished [6-8]. The active site structure of MMOH determined by X-ray crystallography is illustrated in Fig. I. Two iron(II) ions are triply bridged with two carboxylate groups and a hydroxide. One of the carboxylate ligands originates from the acetate contained in the buffer solution used for the crystallization. A dioxygen carrying protein for some marine worms, hemerythrin, is known to contain a similar dinuclear iron site [9-12]. The precise coordination structure of the active site and the dioxygen binding fashion in this protein have been the subject of numerous investigations, which have been reviewed extensively [9-12]. It is noteworthy that hemerythrin binds dioxygen as schematically illustrated below in eq. (1), resulting in the formation of a stable asymmetric hydroperoxo adduct, whereas usually iron hydroperoxo species are supposed to be extremely unstable and reactive. 345
T. Funabiki (ed.), Oxygenases and Model Systems © Kluwer Academic Publishers 1997
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M.ITO. K.FUJISA WA. N.KITAJ/MA. and Y.MORO-OKA
Figure I. The active site structure of MMOH [5).
+ O2
(1)
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 [l3-15] 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-O-(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 oxo iron species as
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the reactive intermediate in the catalytic reaction. Considering the cur
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