title: Mechanism of ligand binding to Ni(II)-Fe(II) hybrid hemoglobins.
abstract: The geminate and bimolecular binding of CO, O2 and NO to [alpha-Ni(II)]2-[beta-Fe(II)]2 and [alpha-Fe(II)]2-[beta-Ni(II)2] hybrid hemoglobins has been studied. Biomolecular reactions: At pH 6.6 and 20 degrees both hybrids bind CO at 0.15 x 10(6) M-1 s-1. Reactions with oxygen: At pH 6.6 the on rates are 4.8 and 7.5 x 10(6) M-1 S-1 for alpha- and beta-hybrids, respectively; the off rate is approximately 2 x 10(3) S-1 for both. At pH 8 the alpha-Fe shows cooperativity whereas the beta-hybrid does not. Nanosecond geminate reactions: Faster bimolecular rates correlate with larger geminate amplitudes; thus alpha-Fe hybrids have larger amplitudes, and O2 geminate amplitudes are larger than those with CO. At pH 8.50% of O2 recombines with the alpha-hybrid. With NO, nanosecond geminate recombination is observable only with the beta-hybrid. Picosecond reactions: alpha-Hybrids show picosecond recombination of O2. With NO, alpha-hybrids recombine at 30 ns-1, beta-hybrids at 0.3 ns-1. The NO picosecond rates correlate with the molecular dynamics which shows ligands leaving the beta-Fe atom early and regularly, but remaining near the alpha-Fe atom. The results may be explained by assuming an interaction between the alpha-subunits giving rise to a high-affinity faster-reacting form, whereas the beta-subunits only become fast-reacting when an R-T conformation change analogous to that of hemoglobin A takes place. A third allosteric state is postulated to explain the results.
authors.affiliation: Department of Physics, Jichi Medical School, Tochigi-Ken, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Shibayama
authors.forename: T
authors.initials: T
authors.lastname: Yonetani
authors.forename: R M
authors.initials: RM
authors.lastname: Regan
authors.forename: Q H
authors.initials: QH
authors.lastname: Gibson
mesh_terms: D000494:Allosteric Regulation; D001665:Binding Sites; D002248:Carbon Monoxide / Q000378:metabolism; D003196:Computer Graphics; D005296:Ferrous Compounds / Q000378:metabolism*; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D009532:Nickel / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism
chemical_list: D005296:Ferrous Compounds; D006454:Hemoglobins; D008024:Ligands; D009569:Nitric Oxide; D006418:Heme; D009532:Nickel; D002248:Carbon Monoxide; D010100:Oxygen