Histidine hemoglobins

X = N (Histidine) Hemoglobin, Myoglobin, Peroxidase X = O (Tyrosine) Catalase... 

The imidazole ring is an essential component of many biological systems in proteins as part of the side chain of the amino acid histidine (hemoglobin and myoglobin), in nucleic acids as part of the purine ring of adenine and... 

Fig. 2. Reaction of diphosphoglycerate (2,3-DPG) and deoxyhemoglobin. The molecule fits into the central cavity of hemoglobin and forms salt bridges with valine NA(I)p, histidines NA2(2)p, H2I(I43)p, and lysine EF6(82)p. A, E, and E correspond to specific hemoglobin hehces and NA is the sequence...

The preparation of histidine by the hydrolysis of hemoglobin and precipitation with mercuric chloride in alkaline solution was... 

In hemoglobin M, histidine F8 (His F8) has been replaced by tyrosine. The iron of HbM forms a tight ionic complex with the phenolate anion of tyrosine that stabilizes the Fc3 form. In a-chain hemoglobin M variants, the R-T equilibrium favors the T state. Oxygen affinity is reduced, and the Bohr effect is absent. P Ghain hemoglobin M variants exhibit R-T switching, and the Bohr effect is therefore present. 

Hemoglobin also functions in CO2 and proton transport from tissues to lungs. Release of O2 from oxyHb at the tissues is accompanied by uptake of protons due to lowering of the of histidine residues. 

Each heme unit in myoglobin and hemoglobin contains one ion bound to four nitrogen donor atoms in a square planar arrangement. This leaves the metal with two axial coordination sites to bind other ligands. One of these sites is bound to a histidine side chain that holds the heme in the pocket of the protein. The other axial position is where reversible binding of molecular oxygen takes place. 

Figure 4.12 Distal histidine hydrogen bonding structure for hemoglobin (left) and a heme model (right). (Reprinted with permission from Figure 12 of Momenteau, M. Reed, C. A. Chem. Rev., 1994, 94, 659-698. Copyright 1994, American Chemical Society.)...

Fig. 45. Effect of oxygenation of Hemoglobin M -B(B2A6,w-0h)on one of the histidine i4N-EN-DOR lines (Washed whole cells). (Adapted from Ref. 243)...

Buffers resist changes in pH. Substances can act as buffers at their pK values. In proteins, the amino acids with ionizable R-groups can act as buffers, altiiough the only amino acid that is useful in maintaining physiologic pH (7.2-7.4) is histidine with an R-group pK near 7. Hemoglobin can act as an intracellular buffer in red blood cells because it contains histidyi residues. 

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