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Thursday, 30 September 2004

Surprising Ways of Binding Throw Clearer Light on Global Nitrogen Cycle

Elitza I. Tocheva and Michael E.P. Murphy
The University of British Columbia, Vancouver, BC, Canada

Nitric oxide (NO) is a small but powerful biologically active molecule that can protect or destroy cells. The bacterial enzyme that creates NO can, however, also turn nitrogen fertilizers into ozone-depleting NO and nitrous oxide, a greenhouse gas. Researchers at the University of British Columbia now have a better understanding of how that enzyme, nitrite reductase (NiR), works. Elitza Tocheva and Michael Murphy used SSRL's macromolecular crystallography facilities to study difficult-to-prepare crystal complexes of NiR bound to NO or to nitrite (which NiR turns into NO).

The results were surprising. NO and nitrite attach to a copper atom at an active site in the enzyme. Both atoms in NO, a nitrogen and an oxygen, bind directly to the copper, rather than the usual case in NO-metal binding where just the nitrogen attaches to the metal. The novel NO-copper geometry may be relevant to other kinds of copper-containing enzymes that play roles in neurodegenerative diseases. Nitrite also bound to copper in NiR in an unexpected way, with the nitrogen moved away from the plane of the nitrite and Cu atoms. This makes the nitrogen atom tilt towards the active site. Determining the structures also solved a puzzle: now scientists can imagine how nitrite can be reduced to NO without having to do an unprecedented backflip at the enzyme's active site. This insight into the structure of NiR could lead to ways to inhibit its activity (to prevent release of environmentally dangerous gases) or ways to use the enzyme to remove nitrogen from specific environments.