Heterogeneous motions within human apohemoglobin

Conservation of the secondary and tertiary protein organization of human apohemoglobin was observed at temperatures ranging from 7 to 25 degrees C using CD spectra in the far-UV (200-250 nm) and near-UV (250-300 nm) regions. The dynamics of apohemoglobin were probed using fluorescence quenching experiments on the Trp residues and an extrinsic dye (ANS or bis-ANS) located in the heme cavities. The long decay time of the dye emission (> 10 ns) reveals the dynamics of the protein matrix averaged over the whole molecule, The short decay time of the Trp residue emission (congruent to 3 ns) probes the dynamics of their close vicinities. When the temperature rises from 10 to 20 degrees C, the average intraproteic motions throughout the whole apohemoglobin matrix are greatly accelerated, whereas the hydrophobic protein regions around the alpha 14, beta 15 and beta 37 Trp residues appear much less animated. These dynamic differences between the behavior of the softer matrix and the packed rigid regions containing the tryptophans could be one of the requisites for apohemoglobin stability. We suspect that the highly rigid tryptophan domains in human apohemoglobin are likely to be knots.