Hemoglobin proximal histidine

Although the same transition is expected for the 5-coordinate site in the Ni globins as for 6-coordinate models, ligand release is not observed, presumably, due to rapid geminate recombination in the protein. However, excitation of the 4-coordinate site of Ni hemoglobin does result in transient acquisition of a fifth axial ligand—most likely the proximal histidine. 

Four of the six coordination sites of the iron in hemoglobin are occupied by the nitrogen atoms of the pyrrol rings, and another is occupied by a histidine residue of the globin (the proximal histidine). The iron s sixth site is coordinated with oxygen in oxyhemoglobin and with H2O in deoxyhemoglobin. 

Figure 10 Conformational changes in human hemoglobin upon the O2 binding. The proximal histidine is pulled toward the porphyrin ring by oxygenation, and then the F-helix shifts position. Purple and blue units represent oxy (R-state) and deoxy (T-state) proteins and hemes, respectively...

In hemoglobin, the interactions between the subunits are known under the general term of allosteric properties and are of great physiological importance. They determine the cooperative binding of O2. In the deoxy form, the iron atom of each heme is in a high-spin (S =2) five-coordinate iron(II) state and lies about 0.5 A out of the heme plane in the direction of the proximal histidine [12]. The Fe-N(imida2ole) bond vector has 10° tilt off the heme normal. 

Of course, because of the generally poor agreement engendered by our inability to treat the disorder properly, the resultant standard deviations are larger than normal. Yet these are sufficiently small to enable us to reach our objective, an estimate of the movement of the proximal histidine group in cobalt-substituted hemoglobin upon oxygenation.15... 

Fe to shift into the ring center. The proximal histidine in the Fe coordination sphere (see Fig, lb) also moves and acts as a means of communicating between sub-units. Another feature of hemoglobin is designed to lessen its affinity for other ligands. The position of the distal histidine shown in Fig. lb forces coordination in a nonlinear geometry. This is favorable for 02 but not for strongly... 

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