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Papers In Press, published online ahead of print September 9, 2005
Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461
Corresponding Author: jfriedma{at}aecom.yu.edu
After photodissociation, ligand rebinding to myoglobin (Mb) exhibits complex kinetic patterns associated with multiple first order geminate recombination processes occurring within the protein and a simpler bimolecular phase representing second order ligand rebinding from the solvent. A smooth transition from cryogenic-like to solution phase properties can be obtained using a combination of sol-gel encapsulation, addition of glycerol as a bathing medium, and temperature tuning (-1565° C). This approach was applied to a series of double mutants, MbCO (H64L/V68X, X=A, V, L, N and F), which were designed to examine the contributions of the position 68(E11) side chain to the appearance and disappearance of internal rebinding phases in the absence of steric and polar interactions with the distal histidine. Based on the effects of viscosity, temperature, and the stereochemistry of the E11 side chain, the three major phases, B->A, C->A and D->A, can be assigned respectively to ligand rebinding from: i) the distal heme pocket, ii) the Xe cavities prior to large amplitude side chain conformational relaxation, and iii) the Xe cavities after significant conformational relaxation of the position 68(E11) side chain. The relative amplitudes of the BA and CA phases depend markedly on the size and shape of the E11 side chain, which regulates sterically both ligand return to the heme iron atom and ligand migration to the Xe cavities. The internal Xe cavities provide a transient docking site that allows side chain relaxations and the entry of water into the vacated distal pocket, which in turn slows ligand recombination markedly.
J. Biol. Chem, 10.1074/jbc.M506333200
Submitted on June 10, 2005
Revised on August 29, 2005
Accepted on September 9, 2005
The position 68(E11) side chain in myoglobin regulates ligand capture, bond formation with heme iron, and internal movement into the Xe cavities
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