It is known that the cooperativity manifested in the binding of ligands to hemoglobin (Hb) is mediated by conformational changes of the protein. However the dynamic pathways relating the ligand bindings to the protein conformational changes remain unclear. This problem has been studied by photolyzing Hb and myoglobin (Mb) complexes, and examining the results metastable states by various spectroscopies. It is believed that in these metastable states, the deoxy heme is constrained in a metastable configuration due to an unrelaxed protein tertiary conformation and consequently the heme-globin interactions are manifestsed in the form of a structural distortion. But the interpretation of the spectroscopic data is difficult and the structural dynamics of these metastable states are poorly understood. We believe that the technique of time-resolved extended x-ray absorption fine structure (EXAFS) developed in our laboratory provides a unique probe for studying the structural aspect of the heme-globin interactions. The EXAFS of Hb or Mb can be reliably used for determining the distances of atoms within 3.2 angstroms from the iron atom. A set of time-resolved EXAFS of a photolyzed sample is a series of EXAFS, each measured at a chosen time after photolysis. With an on-line transient optical absorption monitor, the fraction of the hemes which have rebound ligand can be determined. Subtracting the contribution of the rebound hemes from the transient EXAFS, one obtains the EXAFS of the photoexcited deligated states. The sensitivity of time-resolved difference EXAFS is sufficient for studying the structural evolution of these deligated states. We propose to study the photolyzed carbomonoxy (CO) complexes of Mb, Hb and Hb sumunits with 100 s-resolved EXAFS and simultaneous optical absorption measurements. By varying the temperature we can study the deligated states with or without CO in the heme pocket. Such studies will elucidate the dynamics of energy transfer between heme and protein. Our long-term goal is to understand the dynamic pathways relating the ligand bindings to the protein conformational changes. Besides its obvious relevance to the functions of lung and heart, the mechanism of cooperative ligand bindings to Hb is also the basis for understanding the allosteric effect in enzymes.
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