Scheme 1. Binding of Gaseous Ligands to Ferrous Cb H-NOX^a.

^aEach heme species is labeled in red. The rate constants labeled in italic are estimated by computer modeling. Both association and dissociation of O₂ to ferrous Cb H-NOX (A) are assumed to be fast, and the rate constants are not determined, although K_D(O₂) of Cb H-NOX was measured by stopped-flow titration method. K_D(NO) is the affinity for 6c NO-heme-His complex. The putative hydrogen bonding from the hydroxyl group of Y139 to O₂-heme is represented by a dashed line. Intermediate D, bracketed in parentheses, is a quaternary complex as proposed before³² and is not observed in experiment, for its extreme transient nature.