Book of Abstracts: Albany 2003
June 17-21 2003
Equilibrium O2 Distribution in the Zn2+-Protoporphyrin IX Deoxymyoglobin Mimic: Application to Oxygen Migration Pathway Analysis
Proton spin relaxation induced by the triplet ground state of O2 in the zinc-containing diamagnetic analog of sperm whale deoxymyoglobin has been measured as a function of oxygen concentration. As no covalent binding of oxygen to the metal occurs in the zinc species, the relaxation effects of O2 on the protein 1H resonances arise exclusively via much weaker noncovalent interactions. The relaxation effects at the amide proton sites are found to be highly localized and are derived almost exclusively from O2 binding at the four previously identified xenon binding sites. Relative binding constants of 1.0, 0.08, 0.07 and 0.23 were determined for the Xe 1, Xe 2, Xe 3 and Xe 4 sites, respectively, with an absolute binding constant of 20 M-1 estimated for Xe 1. In combination with earlier measurements of the kinetics of the heme binding of oxygen, these equilibria measurements enable a more detailed analysis of models characterizing O2 entry and egress. A correlation is established between the fraction of O2 which enters the Fe2+-binding site via rotation of the distal histidine sidechain (so-called "histidine gate") and the experimentally observable O2 (or CO) lifetime in the Xe 1 site. A physiological role for these secondary oxygen binding sites is proposed in enhancing the O2 association reaction by facilitating competition with water binding in the distal heme pocket.