title: Carbon monoxide poisoning is prevented by the energy costs of conformational changes in gas-binding haemproteins.
abstract: Carbon monoxide (CO) is a product of haem metabolism and organisms must evolve strategies to prevent endogenous CO poisoning of haemoproteins. We show that energy costs associated with conformational changes play a key role in preventing irreversible CO binding. AxCYTcp is a member of a family of haem proteins that form stable 5c-NO and 6c-CO complexes but do not form O(2) complexes. Structure of the AxCYTcp-CO complex at 1.25 Å resolution shows that CO binds in two conformations moderated by the extent of displacement of the distal residue Leu16 toward the haem 7-propionate. The presence of two CO conformations is confirmed by cryogenic resonance Raman data. The preferred linear Fe-C-O arrangement (170 ± 8°) is accompanied by a flip of the propionate from the distal to proximal face of the haem. In the second conformation, the Fe-C-O unit is bent (158 ± 8°) with no flip of propionate. The energetic cost of the CO-induced Leu-propionate movements is reflected in a 600 mV (57.9 kJ mol(-1)) decrease in haem potential, a value in good agreement with density functional theory calculations. Substitution of Leu by Ala or Gly (structures determined at 1.03 and 1.04 Å resolutions) resulted in a haem site that binds CO in the linear mode only and where no significant change in redox potential is observed. Remarkably, these variants were isolated as ferrous 6c-CO complexes, attributable to the observed eight orders of magnitude increase in affinity for CO, including an approximately 10,000-fold decrease in the rate of dissociation. These new findings have wide implications for preventing CO poisoning of gas-binding haem proteins.
authors: Molecular Biophysics Group, Institute of Integrative Biology, Faculty of Health and Life Sciences, University of Liverpool, Liverpool L69 7ZB, United Kingdom.
authors: Svetlana V
authors: SV
authors: Antonyuk
authors: Neil
authors: N
authors: Rustage
authors: Christine A
authors: CA
authors: Petersen
authors: Jamie L
authors: JL
authors: Arnst
authors: Derren J
authors: DJ
authors: Heyes
authors: Raman
authors: R
authors: Sharma
authors: Neil G
authors: NG
authors: Berry
authors: Nigel S
authors: NS
authors: Scrutton
authors: Robert R
authors: RR
authors: Eady
authors: Colin R
authors: CR
authors: Andrew
authors: S Samar
authors: SS
authors: Hasnain
mesh_terms: D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics; D002248:Carbon Monoxide / Q000737:chemistry* / Q000378:metabolism; D002249:Carbon Monoxide Poisoning / Q000378:metabolism / Q000517:prevention & control; D003460:Crystallization; D018360:Crystallography, X-Ray; D045305:Cytochromes c' / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D009154:Mutation; D010084:Oxidation-Reduction; D011485:Protein Binding; D011487:Protein Conformation; D013059:Spectrum Analysis, Raman
chemical_list: D001426:Bacterial Proteins; D045305:Cytochromes c'; D005296:Ferrous Compounds; D006418:Heme; D002248:Carbon Monoxide