Mechanism of hydroxylation catalyzed by the MMO complex

Methane and other substrate hydroxylation by dioxygen occurs with the participation of all three components of the enzyme MMOH, MMOR and MMOB (Feig and Lippard, 1994 Wallar and Lipscomb, 1996 and references therein). The redox potential of the transition [Fe(III)-Fe(III)] - [Fe(II)-Fe(II)] is E0 = 0.048 V in the MMOH resting state and changes into -0.084, +0.097 and +-0.100 V after the addition of MMOB, MMOR and (MMOB +- MMOR), respectively (Waller and Lipscomb, 1996). 

The latter compound attracts special interest because it forms more rapidly in the absence of substrates (k = 1.2 s 1) than it autodecays (k = 0.05 s 1) and, therefore, can be directly investigated by physicochemical methods. The Mossbauer spectrum of compound Q from M. trichosporium indicates that the diiron center consists of two high-spin antiferromagnetically-coupled iron atoms, each in the Fe(IV) state bridged by oxygen atom. Compound Q reacts very quickly with methane and other substrates with the formation of compound T. The latter releases a product and is transformed to diferric MMOH. 

The mixed-valent [Fe(II)Fe(III)] state of MMOH from has the ability to accommodate simultaneously several molecules (methanol, water and DMSO) as recently demonsrated by ENDOR spectroscopy (Willems et al., 1998). The structure of the binuclear iron-methanol complex and the detailed mechanism of the complex dissociation were investigated with the use of density function methods (Bash et al., 2001a.b). 

Among other discussed concepts concerning the MMO substrate hydroxylation in the compound Q active site, the following suggested mechanisms should be mentioned. 

Formation of a protein radical, for example Cys 151, RS, which promotes the synchronous insertion of oxygen atoms across the substrate C-H bond (Waller and Limscomb, 1996, Shilov, 1997). The absence of rearranged products of the radical clock substrates for MMOH isolated from M. capsulatus raises the possibility in principle, of such a mechanism. 

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