Researchers ID structure of key intermediate in enzyme converting methane to methanol; potential for synthetic fuels
A team from the University of Minnesota and Michigan State University has identified the structure of the key intermediate “Q” in the enzyme methane monooxygenase (MMO). MMO catalyzes the O2-dependent conversion of methane to methanol in methanotrophic bacteria, thereby preventing the emission into the atmosphere of approximately one billion tons of this potent greenhouse gas annually.
Q is one of the most powerful oxidizing intermediates occurring in nature. Exploiting this extreme oxidizing potential is of great interest for bioremediation and the development of synthetic approaches to methane-based alternative fuels and chemical industry feedstocks, the authors noted in their paper, published in the journal Nature. The insight gained into the formation and reactivity of Q from the structure reported is an important step towards harnessing this potential, the authors suggested.
The study, led by John Lipscomb at Minnesota and Denis Proshlyakov at Michigan State, confirms that Q has a diamond-shaped core consisting of two highly oxidized iron atoms connected by twin, single-oxygen atom bridges.
Lipscomb and other scientists had hypothesized this molecular makeup before, but this is the first scientific proof of the structure. Historically, observing the core has been difficult at best because Q’s lifespan within MMO’s catalyst cycle is just a few seconds. Methane is also one of the hardest bonds to break.
To determine the core, researchers used newly developed time-resolved resonance Raman vibrational spectroscopy to measure expansion and contraction motions within the diamond core. By making the measurements in a continuously flowing stream of MMO, the researchers could accumulate Q’s weak visible spectrum for hours despite its short lifetime. The long continuous flow required 40 grams of the enzyme, which required almost one year to purify.
Scientists have been studying Q for 20 years. Now we know its molecular structure, and this will hopefully become a stepping stone for researching possible uses for methane in bioremediation, transportable biofuels, and chemical products.—John Lipscomb
This discovery also allowed researchers to identify the point in the MMO cycle at which atmospheric O2 split into oxygen atoms, one of which is transferred into methane to make methanol.
A tremendous amount of energy is released when the methane bond is broken, making it an attractive fuel. However, it is difficult to transport, especially outside of the US. Converting methane to liquid methanol makes it easy to transport and a good alternative to petroleum for energy. Methanol is also often used in the development of many drugs, chemical processes, and synthetic products.
Other enzymes, including RNR (the enzyme that provides the building blocks of DNA), are likely to contain similar diamond core structures. Lipscomb hopes understanding one diamond core could provide the tools required for studying other similar structures.
Lipscomb plans to continue studying Q and MMO, particularly the other intermediates in the reaction cycle. He hopes researchers and physicians will take the results and apply them to other projects studying enzymes, biofuels, drug discovery, and related sciences.
This work was supported by the NIH grants GM40466 and GM100943 (to J.D.L.) and grant GM096132 (to D.A.P.).
Rahul Banerjee, Yegor Proshlyakov, John D. Lipscomb & Denis A. Proshlyakov (2015) “Structure of the key species in the enzymatic oxidation of methane to methanol” Nature doi: 10.1038/nature14160