Ligation and quaternary structure induced changes in the heme pocket of hemoglobin: a transient resonance Raman study

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Abstract
The extent to which ligation and quarternary structure modify the heme-heme pocket configuration is determined by generating and analyzing transient resonance Raman spectra from various photolyzed and partially photolyzed Hb. From small frequency shifts in Raman band I (.apprx. 1355 cm-1) it was determined that ligation induces a configurational change about the heme. The extent to which ligation modifies the heme pocket is influenced by the quarternary structure. With respect to the structural parameter responsible for variations in the .pi.* orbital electron density of the porphyrin, the degree of alteration of the heme pocket configuration relative to deoxy-Hb(T) follows the sequence: liganded Hb(R) > liganded Hb(R) + IHP [inositol hexaphosphate] > liganded Hb(T) [.alpha. chain > .beta. chain] > deoxy-Hb(R). This progression on configurations also forms a sequence with respect to the retentiveness of the heme pocket as reflected in the ligand dynamics associated with geminate recombination. The heme-heme pocket of the R-state Hb, relative to those of the T-state species, apparently favors ligand retention in a dynamic, as well as thermodynamic, sense. The analysis of these and other related data implicates a ligation and quarternary structure modulated electronic and/or electrostatic interaction between the .pi. system of the porphyrin and the surrounding heme pocket as the basis for this variation in ligand dynamics as well as for the energetics of cooperativity.