title: CO rebinding to protoheme: investigations of the proximal and distal contributions to the geminate rebinding barrier.
abstract: The rebinding kinetics of CO to protoheme (FePPIX) in the presence and absence of a proximal imidazole ligand reveals the magnitude of the rebinding barrier associated with proximal histidine ligation. The ligation states of the heme under different solvent conditions are also investigated using both equilibrium and transient spectroscopy. In the absence of imidazole, a weak ligand (probably water) is bound on the proximal side of the FePPIX-CO adduct. When the heme is encapsulated in micelles of cetyltrimethylammonium bromide (CTAB), photolysis of FePPIX-CO induces a complicated set of proximal ligation changes. In contrast, the use of glycerol-water solutions leads to a simple two-state geminate kinetic response with rapid (10-100 ps) CO recombination and a geminate amplitude that can be controlled by adjusting the solvent viscosity. By comparing the rate of CO rebinding to protoheme in glycerol solution with and without a bound proximal imidazole ligand, we find the enthalpic contribution to the proximal rebinding barrier, H(p), to be 11 +/- 2 kJ/mol. Further comparison of the CO rebinding rate of the imidazole bound protoheme with the analogous rate in myoglobin (Mb) leads to a determination of the difference in their distal free energy barriers: DeltaG(D) approximately 12 +/- 1 kJ/mol. Estimates of the entropic contributions, due to the ligand accessible volumes in the distal pocket and the xenon-4 cavity of myoglobin ( approximately 3 kJ/mol), then lead to a distal pocket enthalpic barrier of H(D) approximately 9 +/- 2 kJ/mol. These results agree well with the predictions of a simple model and with previous independent room-temperature measurements of the enthalpic MbCO rebinding barrier (18 +/- 2 kJ/mol).
authors.affiliation: Department of Physics and Center for Interdisciplinary Research on Complex System, Northeastern University, Boston 02115, USA.
authors.forename: Xiong
authors.initials: X
authors.lastname: Ye
authors.forename: Anchi
authors.initials: A
authors.lastname: Yu
authors.forename: Georgi Y
authors.initials: GY
authors.lastname: Georgiev
authors.forename: Florin
authors.initials: F
authors.lastname: Gruia
authors.forename: Dan
authors.initials: D
authors.lastname: Ionascu
authors.forename: Wenxiang
authors.initials: W
authors.lastname: Cao
authors.forename: J Timothy
authors.initials: JT
authors.lastname: Sage
authors.forename: Paul M
authors.initials: PM
authors.lastname: Champion
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry* / Q000378:metabolism*; D019277:Entropy; D005990:Glycerol / Q000737:chemistry; D006418:Heme / Q000737:chemistry* / Q000378:metabolism*; D007700:Kinetics; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman
chemical_list: D006418:Heme; D002248:Carbon Monoxide; D005990:Glycerol