title: Proton transfer at helium temperatures during dioxygen activation by heme monooxygenases.
abstract: In the first measurement of enzymatic proton transfer at liquid helium temperatures, we examine protonation of the peroxo-ferriheme state of heme oxygenase (HO) produced by in situ radiolytic cryoreduction of oxy-HO in H2O and D2O solvents at ca. 4 K and above, and compare these findings with analogous measurements for oxy-P450cam and for oxy-Mb. Proton transfer in HO occurs at helium temperatures in both solvents; it occurs in P450cam at approximately 50 K and higher; in Mb it does not occur until T > 170 K. For Mb, this transfer at 180 K is biphasic, and the majority phase shows a solvent kinetic isotope effect of 3.8. We discuss these results in the context of the picture of environmentally coupled tunneling, which links proton transfer to two classes of protein motions: environmental reorganization (lambda in Marcus-like equations) and protein fluctuations ("active dynamics"; gating) which modulate the distance of proton transfer.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Roman
authors.initials: R
authors.lastname: Davydov
authors.forename: Sergey
authors.initials: S
authors.lastname: Chemerisov
authors.forename: David E
authors.initials: DE
authors.lastname: Werst
authors.forename: Tijana
authors.initials: T
authors.lastname: Rajh
authors.forename: Toshitaka
authors.initials: T
authors.lastname: Matsui
authors.forename: Masao
authors.initials: M
authors.lastname: Ikeda-Saito
authors.forename: Brian M
authors.initials: BM
authors.lastname: Hoffman
mesh_terms: D019475:Camphor 5-Monooxygenase / Q000737:chemistry / Q000378:metabolism; D003080:Cold Temperature; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry / Q000378:metabolism*; D009211:Myoglobin / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D010108:Oxyhemoglobins / Q000737:chemistry / Q000378:metabolism
chemical_list: D009211:Myoglobin; D010108:Oxyhemoglobins; C017595:oxymyoglobin; D006419:Heme Oxygenase (Decyclizing); D019475:Camphor 5-Monooxygenase; D010100:Oxygen