Carbon monoxide binding properties of domain-swapped dimeric myoglobin

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Myoglobin (Mb) is a monomeric oxygen storage hemoprotein, and has been shown to form a domain-swapped dimer. In this study, monomeric and dimeric carbon monoxide (CO)-bound Mb (MbCO) exhibited similar absorption spectra. The CO stretching frequencies of MbCO were observed at 1,932 and 1,944 cm-1 for both monomeric and dimeric MbCO. The resonance Raman (RR) bands for the stretching between the heme iron and axial ligands were observed at the same frequencies for the monomer and dimer of deoxygenated Mb (deoxyMb) and MbCO, respectively (ν Fe-His, 220 cm-1; ν Fe-C, 507 cm-1), showing that the Fe-His bond strength of deoxyMb and the Fe-CO bond strength of MbCO did not change by the dimerization. Time-resolved RR measurements showed that the dynamics of the structural changes at the heme active site after CO photo-dissociation of MbCO was similar between monomeric and dimeric Mb [monomer, (5.2 ± 1.8) × 106 s-1; dimer, (6.2 ± 1.1) × 106 s-1 at room temperature]. These results show that the heme coordination structure, the protein environment around the bound CO, and the protein relaxation character are similar between monomeric and dimeric MbCO. Although the active site structure was similar between the monomer and dimer, the CO binding rate constant of dimeric Mb [(1.01 ± 0.03) × 106 M-1 s-1 at 20 °C] was about twice larger than that of the monomer [(0.52 ± 0.02) × 106 M-1 s-1 at 20 °C], presumably due to the expansion of the channel between the Xe3 cavity and the solvent by the dimerization.

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