The High and Low Spin Iron Sites and the T/to R/Transition in Carp Hemoglobin

Abstract

The molecular mechanism of the modulation of the protein activity and in particular the control of oxygen binding in hemoglobin (Hb) at the level of atomic structure has been object of large interest. Most of the studies have been addressed to the location of the cooperativity free energy required to switch between the two quaternary structure: the low affinity unligated (deoxy) tense Td structure and the high affinity ligated relaxed R/ structure. The free energy can be located: a) at the heme site(1) b) at the subunit interface(2) and/or c) distributed over other bonds of the protein (3-5). X-ray absorption spectroscopy provides Via the analysis of EXAFS (x-ray absorption fine structure) and XANES (x-ray absorption near edge structure) information on the local structure of active sites in solution. The early studies of high and low affinity forms of deoxygenated adult hemoglobin (HbA) and Hb Kempsey by EXAFS(6,7) and the more recent studies of low and high affinity carp hemoglobin (carp Hb) by XANES(8,9) have shown that the average distance between iron and its first neighbours does not change more than 0.01 Å comparing the two conformations in the unligated (or ligated) forms. These studies show that most of the cooperative energy associated with the change of the quaternary structure is not located at the active site but subtle important structural changes at the active sites, induced by the quaternary conformation, are expected from the indication of changes of magnetic susceptibility.KeywordsQuaternary StructureSpin TransitionXANES SpectrumIron SiteCooperative EnergyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.