CO Rebinding to Protoheme: Investigations of the Proximal and Distal Contributions to the Geminate Rebinding Barrier
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- 作者：Xiong Ye ; Anchi Yu ; Georgi Y. Georgiev ; Florin Gruia ; Dan Ionascu ; Wenxiang Cao ; J. Timothy Sage ; and Paul M. Champion
- 刊名：Journal of the American Chemical Society
- 出版年：2005
- 出版时间：April 27, 2005
- 年：2005
- 卷：127
- 期：16
- 页码：5854 - 5861
- 全文大小：208K
- 年卷期：v.127,no.16(April 27, 2005)
- ISSN：1520-5126
文摘
The rebinding kinetics of CO to protoheme (FePPIX) in the presence and absence of a proximalimidazole 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 equilibriumand transient spectroscopy. In the absence of imidazole, a weak ligand (probably water) is bound on theproximal 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 withrapid (10-100 ps) CO recombination and a geminate amplitude that can be controlled by adjusting thesolvent viscosity. By comparing the rate of CO rebinding to protoheme in glycerol solution with and withouta bound proximal imidazole ligand, we find the enthalpic contribution to the proximal rebinding barrier, Hp,to be 11 ± 2 kJ/mol. Further comparison of the CO rebinding rate of the imidazole bound protoheme withthe analogous rate in myoglobin (Mb) leads to a determination of the difference in their distal free energybarriers: 
GD 
12 ± 1 kJ/mol. Estimates of the entropic contributions, due to the ligand accessible volumesin the distal pocket and the xenon-4 cavity of myoglobin (~3 kJ/mol), then lead to a distal pocket enthalpicbarrier of HD 9 ± 2 kJ/mol. These results agree well with the predictions of a simple model and withprevious independent room-temperature measurements (Tian et al. Phys. Rev. Lett. 1992, 68, 408) of theenthalpic MbCO rebinding barrier (18 ± 2 kJ/mol).
GD 12 ± 1 kJ/mol. Estimates of the entropic contributions, due to the ligand accessible volumesin the distal pocket and the xenon-4 cavity of myoglobin (~3 kJ/mol), then lead to a distal pocket enthalpicbarrier of HD 9 ± 2 kJ/mol. These results agree well with the predictions of a simple model and withprevious independent room-temperature measurements (Tian et al. Phys. Rev. Lett. 1992, 68, 408) of theenthalpic MbCO rebinding barrier (18 ± 2 kJ/mol).