Methane-oxidising bugs sidestep copper uptake.

All living organisms need copper to serve as a cofactor in crucial redox enzymes (eg cytochrome oxidase, superoxide dismutase). Larger doses of it are toxic to most cells, and nature has evolved complex mechanisms to handle and detoxify copper ions (see Chem. Brit., Oct. 2001, p24). Some bacteria, however, need all the copper they can get. Researchers in the US have now elucidated the structure of a complex molecule that bacteria secrete into the medium solely for the purpose of catching copper ions.

Hyung Kim, during studies with David Graham at the University of Kansas at Lawrence, US, studied the growth of methane-oxidising bacteria including Methylosinus trichosporium and Methylococcus capsulatus under copper-limited conditions. Graham had found that the bacteria enriched the medium with a novel organic compound, which he called methanobactin, when copper ions were scarce. When copper was provided, the bacteria rapidly absorbed the methanobactin along with the copper. The bacteria 'undergo a spectacular change in morphology in response to copper; filling the cell with membranes to increase the surface area for copper-requiring particulate methane monooxygenase activity (pMMO),' explains Nigel Robinson, University of Newcastle, UK.

Kim solved the crystal structure of methanobactin, revealing a novel ligand system built around a peptide backbone but containing both standard amino acids and other kinds of molecular building blocks (eg an ester, imidazole, pyrrolidine). The copper (I) ion is coordinated in a distorted tetrahedral arrangement by two imidazole nitrogens and two thionyl ligands, and the overall shape of the molecule echoes the coordination tetrahedron. Kim's team has identified several smaller molecules that had previously been linked to copper uptake as methanobactin fragments.

The findings, which Robinson calls 'an important addition to our understanding of metal-ion homeostasis,' suggest that methanobactin serves copper uptake in a similar way to the iron uptake function of the molecules known as siderophores.

Michael Gross