13. This image shows 1 electrostatic bond present
between amino acids in the beta chain of deoxyhemoglobin. The heme
group is still shown in red. The bond is between the carboxyterminal
His146 and Asp94. His146 of the beta chain is protonated in the deoxygenated
state, but not the oxygenated state. Its pK is raised due to the
proximity of the negatively charged Asp94 side chain.
14. One of the hydrogen bonds between polypeptide chains is shown in the following figure. In this image, 2 bonds are shown between the beta and alpha polypeptide chains. The top bond is of interest it is present in the deoxygenated state of hemoglobin, but is broken during the transition to the oxygenated state. This bond is a hydrogen bond is shown between Asp99 (beta chain) and Tyr42 (alpha).
No bond is present between the Asn102 (beta) and Asp94
(alpha) in the deoxygenated state, the 4.82A distance is too far apart
for a hydrogen bond. However these residues do interact in the oxygenated
protein. It would be acceptable to highlight these residues in an
image for your paper, but the bond is not present and should not be shown
for the deoxygenated state.
15. The final image highlights residues that would be appropriate for a discussion of the sickle cell hemoglobin. The two alpha chains are colored yellow and orange while the beta subunits are cyan and blue. Glu6 of the beta chains is colored green; this residue is mutated to a valine in sickle cell anemia. In sickle cell hemoglobin the Val6 interacts with Phe85 (magenta) and Leu88 (purple) of another hemoglobin molecule in the deoxygenated state. The image shows that amino acid residue 6 of the beta chain is not in close proximity to Phe85 or Leu88 in a single hemoglobin protein.
References:
Perutz, M.F., H. Muirhead, J.M. Cox, and L.C.G. Goaman. Three-dimensional Fourier Synthesis of Horse Oxyhaemoglobin at 2.8 A Resolution: The Atomic Model. Nature 219: 131-139 (1968).
Fermi, G., M.F. Perutz, and B. Shaanan. The Crystal Structure of Human Deoxyhaemoglobin at 1.74 A Resolution. J. Mol. Biol. 175: 159-174 (1984).
Stryer, L. Biochemistry, Fourth Edition, W.H. Freeman and Company, New York (1995).
Voet, D. and J. Voet. Biochemistry, Third Edition,
John Wiley & Sons, Hoboken, NJ (2004).