Spin equilibria in human methemoglobin: effects of bezafibrate and inositol hexaphosphate as measured by susceptometry and visible spectroscopy.
Spin equilibria in human methemoglobin: effects of bezafibrate and inositol hexaphosphate as measured by susceptometry and visible spectroscopy.
复制标题
人高铁血红蛋白的自旋平衡：通过电纳法和可见光谱测量苯扎贝特和肌醇六磷酸的影响。
DOI：
10.1021/bi00438a055
复制
发表时间：
1989
期刊：
影响因子：
2.9
通讯作者：
DiIorio,EE
中科院分区：
文献类型：
--
作者：
Noble,RW;DeYoung,A;Vitale,S;Cerdonio,M;DiIorio,EE
Revised Manuscript Received March 9, 1989 abstract: The effects of inositol hexaphosphate (IHP) and a second allosteric effector, bezafibrate, on the spin-state equilibria of the mixed-spin derivatives of ferric human hemoglobin A are examined. Changes in spin-state equilibrium are monitored by measuring absorption spectra in the visible region (460-700 nm) as well as by direct measurements of magnetic susceptibility by means of a superconducting fluxmeter. The addition of IHP at pH 6.5 results in a measurable shift in thespin equilibria of these derivatives toward higher spin. However, the addition of bezafibrate in the presence of IHP results in still larger shifts toward the high-spin form. The changes in the free energies of the spin-state equilibria resulting from the combination of these twoeffectors are similar in magnitude to that which results from the R-state to T-state transition in carp hemoglobin. e origin or mechanism of the ligand-linked changes in the ligand affinity of hemoglobin remains a topic of exploration and controversy. Perutz (1972) postulated a linkage between the ligand-induced movement of the proximal histidine residue with respect to the heme plane and the conformational equilibrium of the protein. Within the framework of the two-state model of Monod, Wyman, and Changeux (1965), the movement of this histidine toward theheme plane is re-sisted in the low-affinity T state while occurringwith greater facility in the high-affinity R state of the protein. This of course results in a thermodynamic linked function (Wyman, 1964) between ligand binding and protein conformation. Tests of this hypothesis have given mixed results. Rousseau et al.(1984) examined the resonance Raman spectra of the heme regions of ligand-saturated R and T states of hemoglobin and found no evidence for differences in bond energies to account for their differences in ligand affinity. Onthe other hand, Nagai and Kitagawa (1980) demonstrated a clear difference in the stretching frequencies of the iron-proximal histidine bond in deoxygenated high-and low-affinity states of hemoglobin. Studies on a wide variety of hemoglobins have demonstrated a correlation between this iron-histidine stretching frequency and ligand affinity (Friedman, 1985). However, the actual changes in bond energy indicated by these frequency shifts are small compared to the associated changes in the free energy of ligand binding (Ondrias et al., 1982). An alternative basis for an affinity change could be a steric resistance to the positioningof the ligand at the heme iron on the distal side of the heme. However, in examining thebinding
