Abstract
13 CO. Geminate rebinding of CO to the denatured Hb was accelerated more than 1000 times, suggesting that the native structure of the Hb is required to suppress efficient geminate rebinding of CO, as is the case in Mb. The geminate yield and rate for CO rebinding are almost the same in both the denatured Hb and Mb. Similarity in the equilibrium spectrum and rebinding dynamics of CO indicates that the state of the denatured Hb is very similar to that of the denatured Mb. In the denatured Hb, quaternary contact of the protein is likely severed, with the denatured protein existing as an independent subunit much like Mb. Mb, an oxygen storage protein, is a monomeric protein that contains a single heme group as the active ligand-binding site of the protein. Hb, an oxygen transport protein, is a tetrameric protein, each unit of which contains a heme group, with the global tertiary structure of each unit similar to that of Mb. Hb exhibits cooperative binding of ligands essential to efficient transport of O2. 2 The cooperativity of Hb is accompanied by a substantial change in its quaternary structure. Whereas the peptide sequence of Mb is different from that of Hb, amino acid residues near the heme in both proteins are highly conserved. The residues surrounding the bound ligand are invariant in both proteins, except for isoleucine 107 in Mb that is replaced with leucine 107 in Hb. The rebinding of CO to Mb and Hb after photolysis are most often used to study the relationships between protein structure, function, and dynamics. Though the rebinding dynamics of the ligands to both proteins are similar in general, they are different in detail. Whereas cooperative binding of the ligand to Hb makes its behavior difficult to compare directly to the binding of Mb, the difference is often attributed to arise from the quaternary structure of Hb. The dynamics and structural characterization of denatured proteins can be useful in understanding protein folding as well as various biological phenomena such as regulation of the cellular activity and onset of neurodegenerative disease. 3
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