Two-faced molecule offers NO explanation

	David Lawson
	Clare Stevenson
	Robert Eady
	Colin Andrew

A Janus molecule isolated from a soil bacterium could help explain the regulation of blood clotting and the male erection, according to a team at the John Innes Centre in Norwich (JIC). The hemoprotein cytochrome c' (cyt c') from the denitrifying soil microbe Alcaligenes xylosoxidans has a strong affinity for the neurotransmitter nitric oxide (NO), which it binds in a novel and unexpected fashion. This reaction is used to "soak up" and de-toxify NO which is produced in copious quantities as a natural by-product of the bacterium's metabolism.

The bacterial cytochrome c' has many properties in common with the NO-responsive enzyme guanylate cyclase, which plays a critical role in cellular signaling in animals. Crystallographers David Lawson and Clare Stevenson, and enzymologist Robert Eady, at JIC and chemist Colin Andrew at Oregon Institute have found an unexpected mechanism for the binding of NO to cyt c' which provides a new explanation for the selective activation of guanylate cyclase and related enzymes by NO. Previously, it was thought that gas molecules such as NO and CO were only able to bind to one face of the heme structure.

The team has now obtained three-dimensional X-ray structures of oxidized and reduced cyt c', and also of the reduced protein with NO or CO bound. As they had anticipated from spectroscopic and modeling studies NO disrupts the Fe-histidine bond to form a 5-coordinate adduct but unexpectedly the NO is bound to the proximal face of the heme.

Explaining why the hemoprotein guanylate cyclase was stimulated to a much greater extent (some 200 times) by the binding of NO, than the binding of CO (a fivefold increase in activity) was impossible until now. The team provides evidence that these different gases bind to different faces of the heme, and thus affect the surrounding protein structure to different extents. The work is reported in more detail in EMBO J [1].

"Our study contradicts the scientific dogma concerning the mode of action of a whole family of tremendously important biological molecules," explains Lawson. "The details of how these molecules function now need to be reinterpreted," he adds.

Reference:
1. EMBO J., 2000, 19(21), 5661 ^*

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