Protonation states of histidine and other key residues in deoxy normal human adult hemoglobin by neutron protein crystallography.

ABSTRACT: The protonation states of the histidine residues key to the function of deoxy (T-state) human hemoglobin have been investigated using neutron protein crystallography. These residues can reversibly bind protons, thereby regulating the oxygen affinity of hemoglobin. By examining the OMIT F(o)-F(c) and 2F(o)-F(c) neutron scattering maps, the protonation states of 35 of the 38 His residues were directly determined. The remaining three residues were found to be disordered. Surprisingly, seven pairs of His residues from equivalent ? or ? chains, ?His20, ?His50, ?His58, ?His89, ?His63, ?His143 and ?His146, have different protonation states. The protonation of distal His residues in the ?(1)?(1) heterodimer and the protonation of ?His103 in both subunits demonstrates that these residues may participate in buffering hydrogen ions and may influence the oxygen binding. The observed protonation states of His residues are compared with their ?pK(a) between the deoxy and oxy states. Examination of inter-subunit interfaces provided evidence for interactions that are essential for the stability of the deoxy tertiary structure.

SUBMITTER: Kovalevsky A 

PROVIDER: S-EPMC2967419 | biostudies-literature | 2010 Nov

REPOSITORIES: biostudies-literature