Ligand-dependent allosteric transformation of hemoglobins from the blood clam, Anadara Broughtonii

Abstract

1. 1. Oligomeric Hb I and II of Anadara broughtonii, which are unusual with respect to having no Bohr effect, were shown to have a R-T transformation on ligand-binding on the basis of the following experimental results. (a) Iodoacetamide reacted preferentially with the oxyforms of the hemoglobins. (b) CD spectra at the far-ultraviolet regions significantly changed on ligand-binding. (c) 1-Anilinonaphthalene-8-sulfonate bound to the hemoglobins with a preference for deoxyforms. From these results and previous findings [1], it is concluded that the absence of the Bohr effect in these hemoglobins is due to the lack of the Bohr proton ionizing groups in the molecules. 2. 2. Hb I and II treated with p-chloromercuribenzoate, designated as PMB-I and PMB-II, showed greatly increased oxygen affinity and decreased cooperativity. CD spectra at the far-ultraviolet of the PMB-Hb in the oxygen liganded state gave similar patterns to those of native oxygenated Hb. However no changes in the spectra were observed on deoxygenation. These findings suggest that the PMB-I and PMB-II retain their native oxy conformation even in the deoxy states. The PMB-modification might prevent the initial ligand-induced conformational change within the protomers.