title: How do heme-protein sensors exclude oxygen? Lessons learned from cytochrome c', Nostoc puntiforme heme nitric oxide/oxygen-binding domain, and soluble guanylyl cyclase.
abstract: CONCLUSIONS
  Ligand selectivity for dioxygen (O(2)), carbon monoxide (CO), and nitric oxide (NO) is critical for signal transduction and is tailored specifically for each heme-protein sensor. Key NO sensors, such as soluble guanylyl cyclase (sGC), specifically recognized low levels of NO and achieve a total O(2) exclusion. Several mechanisms have been proposed to explain the O(2) insensitivity, including lack of a hydrogen bond donor and negative electrostatic fields to selectively destabilize bound O(2), distal steric hindrance of all bound ligands to the heme iron, and restriction of in-plane movements of the iron atom.
  
  
  BACKGROUND
  Crystallographic structures of the gas sensors, Thermoanaerobacter tengcongensis heme-nitric oxide/oxygen-binding domain (Tt H-NOX(1)) or Nostoc puntiforme (Ns) H-NOX, and measurements of O(2) binding to site-specific mutants of Tt H-NOX and the truncated β subunit of sGC suggest the need for a H-bonding donor to facilitate O(2) binding.
  
  
  RESULTS
  However, the O(2) insensitivity of full length sGC with a site-specific replacement of isoleucine by a tyrosine on residue 145 and the very slow autooxidation of Ns H-NOX and cytochrome c' suggest that more complex mechanisms have evolved to exclude O(2) but retain high affinity NO binding. A combined graphical analysis of ligand binding data for libraries of heme sensors, globins, and model heme shows that the NO sensors dramatically inhibit the formation of six-coordinated NO, CO, and O(2) complexes by direct distal steric hindrance (cyt c'), proximal constraints of in-plane iron movement (sGC), or combinations of both following a sliding scale rule. High affinity NO binding in H-NOX proteins is achieved by multiple NO binding steps that produce a high affinity five-coordinate NO complex, a mechanism that also prevents NO dioxygenation.
  
  
  CONCLUSIONS
  Knowledge advanced by further extensive test of this "sliding scale rule" hypothesis should be valuable in guiding novel designs for heme based sensors.
authors.affiliation: Division of Hematology, University of Texas Health Science Center at Houston, Houston, Texas 77225, USA. ah-lim.tsai@uth.tmc.edu
authors.forename: Ah-Lim
authors.initials: AL
authors.lastname: Tsai
authors.forename: Emil
authors.initials: E
authors.lastname: Martin
authors.forename: Vladimir
authors.initials: V
authors.lastname: Berka
authors.forename: John S
authors.initials: JS
authors.lastname: Olson
mesh_terms: D045305:Cytochromes c' / Q000737:chemistry / Q000378:metabolism*; D006162:Guanylate Cyclase / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000737:chemistry / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000378:metabolism*; D000071756:Soluble Guanylyl Cyclase
chemical_list: D045305:Cytochromes c'; D006420:Hemeproteins; D018160:Receptors, Cytoplasmic and Nuclear; D006418:Heme; D006162:Guanylate Cyclase; D000071756:Soluble Guanylyl Cyclase; D010100:Oxygen