The structural and functional analysis of the hemoglobin D component from chicken.

Oxygen binding by chicken blood shows enhanced cooperativity at high levels of oxygen saturation. This implies that deoxy hemoglobin tetramers self-associate. The crystal structure of an R-state form of chicken hemoglobin D has been solved to 2.3-A resolution using molecular replacement phases derived from human oxyhemoglobin. The model consists of an alpha2 beta2 tetramer in the asymmetric unit and has been refined to a R-factor of 0.222 (R-free = 0.257) for 29,702 reflections between 10.0- and 2.3-A resolution. Chicken Hb D differs most from human oxyhemoglobin in the AB and GH corners of the alpha subunits and the EF corner of the beta subunits. Reanalysis of published oxygen binding data for chicken Hbs shows that both chicken Hb A and Hb D possess enhanced cooperativity in vitro when inositol hexaphosphate is present. The electrostatic surface potential for a calculated model of chicken deoxy-Hb D tetramers shows a pronounced hydrophobic patch that involves parts of the D and E helices of the beta subunits. This hydrophobic patch is a promising candidate for a tetramer-tetramer interface that could regulate oxygen binding via the distal histidine.