Heme-based sensors in biological systems

The past several years have been witness to a staggering rate of advancemen t in the understanding of how organisms respond to changes in the availabil ity of diatomic molecules that are toxic and/or crucial to survival. Heme-b ased sensors presently constitute the majority of the proteins known to sen se NO, O-2 and CO and to initiate the chemistry required to adapt to change s in their availabilities. Knowledge of the three characterized members of this class, soluble guanylate cyclase, FixL and CooA, has grown substantial ly during the past year. The major advances have resulted from a broad rang e of approaches to elucidation of both function and mechanism. They include growth in the understanding of the interplay between the heme and protein in soluble guanylate cyclase, as well as alternate means for its stimulatio n. Insight into the O-2-induced structural changes in FixL has been supplie d by the single crystal structure of the heme domain of Bradyrhizobium japo nicum. Finally, the ligation environment and ligand interchange that facili tates CO sensing by CooA has been established by spectroscopic and mutagene sis techniques.