Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 127, Issue 16, Pages 5854-5861Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja042365f
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Funding
- NIDDK NIH HHS [R01 DK035090-23, DK035090, R01 DK035090] Funding Source: Medline
- NIGMS NIH HHS [R29 GM052002, GM52002, R01 GM052002] Funding Source: Medline
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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 imiclazole, 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 imiclazole 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 with the analogous rate in myoglobin (Mb) leads to a determination of the difference in their distal free energy barriers: Delta G(D) approximate to 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 (similar to 3 kJ/mol), then lead to a distal pocket enthalpic barrier of H-D approximate to 9 +/- 2 kJ/mol. These results agree well with the predictions of a simple model and with previous independent room-temperature measurements (Tian et al. Phys. Rev. Lett. 1992, 68, 408) of the enthalpic MbCO rebinding barrier (18 +/- 2 kJ/mol).
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