Fig. 6.
Heme-helix strain model for NO-induced activation of H-NOX proteins. In the basal, FeII-unligated (gold) state, the heme (orange square) is distorted from planarity from a coordination network in the proximal involving proline 116 (green pentagon) and the heme-ligating histidine 103 (blue pentagon). Upon binding of NO (blue and red circles) the iron–histidine bond is severed, allowing for histidine 103 dissociation and movement of proline 116 away from the heme. These movements result in a more planar heme, structural changes in the signaling helix (αF; pink cylinder) to stabilize H103 in the outward conformation, and rotation of the distal subdomain with respect to the proximal subdomain. The resulting active conformation communicates NO binding to the signaling effector domain (shown as a gray rectangle) through contacts bridging the distal and proximal subdomains.