Controlled Oxidation of Hydrocarbons by the Membrane-Bound Methane Monooxygenase: The Case for a Tricopper Cluster Export

@article{citeulike:3332177, abstract = {Abstract: The growing need for inexpensive methods to convert methane to methanol has sparked considerable interest in methods that catalyze this process. The integral membrane protein particulate methane monooxygenase (pMMO) mediates the facile conversion of methane to methanol in methanotrophic bacteria. Most evidence indicates that pMMO is a multicopper enzyme, and these copper ions support redox, dioxygen, and oxo-transfer chemistry. However, the exact identity of the copper species that mediates the oxo-transfer chemistry remains an area of intense debate. This highly complex enzyme is notoriously difficult to purify because of its instability outside the lipid bilayer and tendency to lose its essential metal cofactors. For this reason, pMMO has resisted both initial identification and subsequent isolation and purification for biochemical and biophysical characterization. In this Account, we describe evidence that pMMO is a multicopper protein. Its unique trinuclear copper cluster mediates dioxygen chemistry and O-atom transfer during alkane hydroxylation. Although a recent crystal structure did not show this tricopper cluster, we provide compelling evidence for such a cluster through redox potentiometry and EPR experiments on the holo enzyme in pMMO-enriched membranes. We also identify a site in the structure of pMMO that could accommodate this cluster. A hydrophobic pocket capable of harboring pentane, the enzymes largest known substrate, lies adjacent to this site. In addition, we have designed and synthesized model tricopper clusters to provide further chemical evidence that a tricopper cluster mediates the enzymes oxo-transfer chemistry. These biomimetic models exhibit similar spectroscopic properties and chemical reactivity to the putative tricopper cluster in pMMO. Based on computational analysis using density functional theory (DFT), triangular tricopper clusters are capable of harnessing a singlet oxene upon activation by dioxygen. An oxygen atom is then inserted via a concerted process into the CH bond of an alkane in the transition state during hydroxylation. The turnover frequency and kinetic isotope effect predicted by DFT show excellent agreement with experimental data.}, address = {Institute of Chemistry, Academia Sinica, Nankang, Taipei 11509, Taiwan, ! A. A. Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125}, author = {Chan, Sunney I. and Yu, Steve S.}, citeulike-article-id = {3332177}, citeulike-linkout-0 = {http://dx.doi.org/10.1021/ar700277n}, citeulike-linkout-1 = {http://pubs.acs.org/doi/abs/10.1021/ar700277n}, day = {19}, doi = {10.1021/ar700277n}, journal = {Acc. Chem. Res.}, keywords = {alkane, methanotrophy, pmmo}, month = {August}, number = {8}, pages = {969--979}, posted-at = {2008-09-24 15:56:42}, priority = {2}, title = {Controlled Oxidation of Hydrocarbons by the Membrane-Bound Methane Monooxygenase: The Case for a Tricopper Cluster}, url = {http://dx.doi.org/10.1021/ar700277n}, volume = {41}, year = {2008} }

TY - JOUR ID - citeulike:3332177 L3 - citeulike-article-id:3332177 N2 - Abstract: The growing need for inexpensive methods to convert methane to methanol has sparked considerable interest in methods that catalyze this process. The integral membrane protein particulate methane monooxygenase (pMMO) mediates the facile conversion of methane to methanol in methanotrophic bacteria. Most evidence indicates that pMMO is a multicopper enzyme, and these copper ions support redox, dioxygen, and oxo-transfer chemistry. However, the exact identity of the copper species that mediates the oxo-transfer chemistry remains an area of intense debate. This highly complex enzyme is notoriously difficult to purify because of its instability outside the lipid bilayer and tendency to lose its essential metal cofactors. For this reason, pMMO has resisted both initial identification and subsequent isolation and purification for biochemical and biophysical characterization. In this Account, we describe evidence that pMMO is a multicopper protein. Its unique trinuclear copper cluster mediates dioxygen chemistry and O-atom transfer during alkane hydroxylation. Although a recent crystal structure did not show this tricopper cluster, we provide compelling evidence for such a cluster through redox potentiometry and EPR experiments on the holo enzyme in pMMO-enriched membranes. We also identify a site in the structure of pMMO that could accommodate this cluster. A hydrophobic pocket capable of harboring pentane, the enzymes largest known substrate, lies adjacent to this site. In addition, we have designed and synthesized model tricopper clusters to provide further chemical evidence that a tricopper cluster mediates the enzymes oxo-transfer chemistry. These biomimetic models exhibit similar spectroscopic properties and chemical reactivity to the putative tricopper cluster in pMMO. Based on computational analysis using density functional theory (DFT), triangular tricopper clusters are capable of harnessing a singlet oxene upon activation by dioxygen. An oxygen atom is then inserted via a concerted process into the CH bond of an alkane in the transition state during hydroxylation. The turnover frequency and kinetic isotope effect predicted by DFT show excellent agreement with experimental data. IS - 8 JF - Acc. Chem. Res. AD - Institute of Chemistry, Academia Sinica, Nankang, Taipei 11509, Taiwan, ! A. A. Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125 EP - 979 TI - Controlled Oxidation of Hydrocarbons by the Membrane-Bound Methane Monooxygenase: The Case for a Tricopper Cluster VL - 41 SP - 969 KW - alkane KW - methanotrophy KW - pmmo AU - Chan, Sunney AU - Yu, Steve PY - 2008/08/19/ UR - http://dx.doi.org/10.1021/ar700277n ER -