Mechanisms for Methane and Ammonia Oxidation by Particulate Methane Monooxygenase

Area of Science:

• Biochemistry
• Quantum Chemistry
• Enzyme Catalysis

Background:

• Particulate methane monooxygenase (pMMO) is a membrane-bound enzyme catalyzing methane and ammonia oxidation.
• Experimental characterization of the pMMO active site is challenging due to its membrane-bound nature.
• Recent advances in cryo-electron microscopy (cryo-EM), EPR, and X-ray spectroscopy have provided new insights into the active site structure.

Purpose of the Study:

• To perform the first quantum chemical study on a model of the pMMO active site (CuD).
• To elucidate the catalytic mechanisms of pMMO, focusing on the role of the copper active site.
• To investigate the electronic properties and reactivity of the active site during oxygen activation.

Main Methods:

• Quantum chemical calculations using a validated model of the pMMO active site (CuD).
• Investigation of reaction pathways involving oxygen and potential substrates.
• Analysis of proton-coupled electron transfer (PCET) steps and radical character.

Main Results:

• Identification of low-barrier catalytic mechanisms for pMMO.
• Discovery of two initial proton-coupled electron transfer steps to a bound O2 ligand.
• Observation of significant radical character in protonated oxygen species, facilitating substrate proton acceptance.

Conclusions:

• The study provides a quantum chemical basis for understanding pMMO's catalytic efficiency.
• The proposed mechanism highlights the importance of initial PCET steps and oxygen radical character.
• These findings offer crucial insights into the function of single-copper active sites in metalloenzymes.