Characterization of the particulate methane monooxygenase metal centers in multiple redox states by X-ray absorption spectroscopy

Raquel L. Lieberman, Kalyan C. Kondapalli, Deepak B. Shrestha, Amanda S. Hakemian, Stephen M. Smith, Joshua Telser, Jane Kuzelka, Rajeev Gupta, A. S. Borovik, Stephen J. Lippard, Brian M. Hoffman, Amy C. Rosenzweig*, Timothy L. Stemmler

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

71 Scopus citations


The integral membrane enzyme particulate methane monooxygenase (pMMO) converts methane, the most inert hydrocarbon, to methanol under ambient conditions. The 2.8-Å resolution pMMO crystal structure revealed three metal sites: a mononuclear copper center, a dinuclear copper center, and a nonphysiological mononuclear zinc center. Although not found in the crystal structure, solution samples of pMMO also contain iron. We have used X-ray absorption spectroscopy to analyze the oxidation states and coordination environments of the pMMO metal centers in as-isolated (pMMOiso), chemically reduced (pMMOred), and chemically oxidized (pMMO ox) samples. X-ray absorption near-edge spectra (XANES) indicate that pMMOiso contains both CuI and CuII and that the pMMO Cu centers can undergo redox chemistry. Extended X-ray absorption fine structure (EXAFS) analysis reveals a Cu-Cu interaction in all redox forms of the enzyme. The Cu-Cu distance increases from 2.51 to 2.65 Å upon reduction, concomitant with an increase in the average Cu-O/N bond lengths. Appropriate Cu2 model complexes were used to refine and validate the EXAFS fitting protocols for pMMOiso. Analysis of Fe EXAFS data combined with electron paramagnetic resonance (EPR) spectra indicates that Fe, present as FeIII, is consistent with heme impurities. These findings are complementary to the crystallographic data and provide new insight into the oxidation states and possible electronic structures of the pMMO Cu ions.

Original languageEnglish (US)
Pages (from-to)8372-8381
Number of pages10
JournalInorganic chemistry
Issue number20
StatePublished - Oct 2 2006

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry


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