Preliminary time-of-flight neutron diffraction study of human deoxyhemoglobin

Human hemoglobin (HbA) is an intricate system that has evolved to efficiently transport oxygen molecules (O₂) from lung to tissue. Its quaternary structure can fluctuate between two conformations, T (tense or deoxy) and R (relaxed or oxy), which have low and high affinity for O₂, respectively. The binding of O₂ to the heme sites of HbA is regulated by protons and by inorganic anions. In order to investigate the role of the protonation states of protein residues in O₂ binding, large crystals of deoxy HbA (∼20 mm³) were grown in D₂O under anaerobic conditions for neutron diffraction studies. A time-of-flight neutron data set was collected to 1.8 Å resolution on the Protein Crystallography Station (PCS) at the spallation source run by Los Alamos Neutron Science Center (LANSCE). The HbA tetramer (64.6 kDa; 574 residues excluding the four heme groups) occupies the largest asymmetric unit (space group P2₁) from which a high-resolution neutron data set has been collected to date.