Credit: J. BLAIR/CORBIS

Methanol could become an alternative source of energy, possibly replacing the less environmentally friendly combustion of coal and petroleum. Methanol can be produced by oxidizing methane, the main component of natural gas. But there are no catalysts that can efficiently perform this chemical reaction on an industrial scale at low temperature.

Several natural enzymes can carry out the reaction, however. Elsewhere in this issue (Nature 434, 177–182; 2005), Raquel L. Lieberman and Amy C. Rosenzweig report the X-ray crystal structure of one of them — particulate methane monooxygenase (pMMO) from the bacterium Methylococcus capsulatus. The structure helps to settle years of controversy by revealing that the protein is a cylindrical trimer, with three metal centres in each subunit. Two of the centres contain copper; one centre is mononuclear (containing a single copper ion), whereas the other is dinuclear (with two copper ions in close proximity). The other metal centre contains a zinc atom, which Lieberman and Rosenzweig believe came from the buffer used to crystallize the protein; they think that the natural protein contains either copper or iron at this centre.

It is still unclear which of the metal centres is the catalytic site — it could be any of them, say the authors. However, clues to the answer might come from structural similarities between a subunit of pMMO and cytochrome c oxidase subunit II, the terminal enzyme in the respiratory chain, which contains a dinuclear copper centre involved in electron transfer. The dinuclear copper centre of pMMO could likewise be responsible for shuttling electrons to the catalytic site, which may be the nearby mononuclear copper centre.

The structure also shows which amino-acid side chains are associated with each metal ion. But much remains to be done. For example, the resolution of the structure is not high enough to determine whether the metal centres contain additional ligands, such as water, that could be functionally important. Nonetheless, the new work is a step along the way to finding out how pMMO works, and towards the long-term goal of developing small-molecule catalysts that might replicate methane-to-methanol conversion in an industrial setting.