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Titration of the Histidyl and -Amino Groups of Hemoglobin

From the ¹ From the Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27706

The titration data of Antonini, Wyman, Brunori, Fonticelli, Bucci, and Rossi-Fanelli (1), for human and horse oxy- and deoxyhemoglobins, have been analyzed in a way that yields the number of groups which titrate in the pH region normally assigned to imidazole and -amino groups.

The results suggest the possibility that the group which differs between the oxy- and deoxyproteins above pH 6.5, which is the group linked to the alkaline Bohr effect, may be an -amino group rather than an imidazole group, as has been assumed heretofore.

The number of unavailable imidazole groups, which arise from histidine residues obliged to be in the uncharged state in the native protein, is about 4 per heme in human hemoglobin and about 3 per heme in horse hemoglobin. This is considerably smaller than the number of unavailable imidazoles in sperm whale myoglobin, and is also less than the number of unavailable imidazoles estimated for horse hemoglobin in previous studies.

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