PMID: 27377386

Authors:

Dajnowicz S, Seaver S, Hanson BL, Fisher SZ, Langan P, Kovalevsky AY, Mueser TC

Title:

Visualizing the Bohr effect in hemoglobin: neutron structure of equine cyanomethemoglobin in the R state and comparison with human deoxyhemoglobin in the T state.

Journal:

Acta Crystallogr D Struct Biol. 2016 Jul 1;72(Pt 7):892-903. doi:, 10.1107/S2059798316009049. Epub 2016 Jun 28.

Abstract:

Neutron crystallography provides direct visual evidence of the atomic positions of deuterium-exchanged H atoms, enabling the accurate determination of the protonation/deuteration state of hydrated biomolecules. Comparison of two neutron structures of hemoglobins, human deoxyhemoglobin (T state) and equine cyanomethemoglobin (R state), offers a direct observation of histidine residues that are likely to contribute to the Bohr effect. Previous studies have shown that the T-state N-terminal and C-terminal salt bridges appear to have a partial instead of a primary overall contribution. Four conserved histidine residues [alphaHis72(EF1), alphaHis103(G10), alphaHis89(FG1), alphaHis112(G19) and betaHis97(FG4)] can become protonated/deuterated from the R to the T state, while two histidine residues [alphaHis20(B1) and betaHis117(G19)] can lose a proton/deuteron. alphaHis103(G10), located in the alpha1:beta1 dimer interface, appears to be a Bohr group that undergoes structural changes: in the R state it is singly protonated/deuterated and hydrogen-bonded through a water network to betaAsn108(G10) and in the T state it is doubly protonated/deuterated with the network uncoupled. The very long-term H/D exchange of the amide protons identifies regions that are accessible to exchange as well as regions that are impermeable to exchange. The liganded relaxed state (R state) has comparable levels of exchange (17.1% non-exchanged) compared with the deoxy tense state (T state; 11.8% non-exchanged). Interestingly, the regions of non-exchanged protons shift from the tetramer interfaces in the T-state interface (alpha1:beta2 and alpha2:beta1) to the cores of the individual monomers and to the dimer interfaces (alpha1:beta1 and alpha2:beta2) in the R state. The comparison of regions of stability in the two states allows a visualization of the conservation of fold energy necessary for ligand binding and release.