Crystal structures of the soluble methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath) demonstrating geometrical variability at thedinuclear iron active site

The oxidation of methane to methanol is performed at carboxylate-bridged di nuclear iron centers in the soluble methane monooxygenase hydroxylase (MMOH ). Previous structural studies of MMOH, and the related R2 subunit of ribon ucleotide reductase, have demonstrated the occurrence of carboxylate shifts involving glutamate residues that ligate the catalytic iron atoms. These s hifts are thought to have important mechanistic implications. Recent kineti c and theoretical studies have also emphasized the importance of hydrogen b onding and pH effects at the active site. We report here crystal structures of MMOH from Methylococcus capsulatus (Bath) in the diiron(II), diiron(III ), and mixed-valent Fe(II)Fe(III) oxidation states, and at pH values of 6.2 , 7.0, and 8.5. These structures were investigated in an effort to delineat e the range of possible motions at the MMOH active site and to identify hyd rogen-bonding interactions that may be important in understanding catalysis by the enzyme. Our results present the first view of the diiron center in the mixed-valent state, and they indicate an increased lability for ferrous ions in the enzyme. Alternate conformations of Asn214 near the active site according to redox state and a distortion in one of the alpha -helices adj acent to the metal center in the diiron(II) state have also been identified . These changes alter the surface of the protein in the vicinity of the cat alytic core and may have implications for small-molecule accessibility to t he active site and for protein component interactions in the methane monoox ygenase system. Collectively, these results help to explain previous spectr oscopic observations and provide new insight into catalysis by the enzyme.