Histidine, in myoglobin

Olson JS, Mathews AJ, Rohlfs RJ, Springer BA, Egeberg KD, Sligar SG, Tame J, Renaud JP, Nagai K (1988) The role of the distal histidine in myoglobin and hemoglobin. Nature 336 265-266... 

Figure Bl.2.11. Biologically active centre in myoglobin or one of the subunits of haemoglobin. The bound CO molecule as well as the proximal and distal histidines are shown m addition to the protohaeme unit. From Rousseau D L and Friedman J M 1988 Biological Applications of Raman Spectroscopy vol 3, ed T G Spiro (New York Wiley). Reprinted by pennission of John Wiley and Sons Inc.

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. 

Fig. 1. Relative positions of the surface histidines (12, 48, 81, and 116) and the heme with its axial histidine in ruthenated sperm whale myoglobin. The edge-edge ET distances are 12.7 (His48), 19.1 (His81), 20.1 (Hisll6), and 22.1 A (Hisl2) [12]...

There is no counterpart to the distal histidine of myoglobin in hemoglobin. 

As in myoglobin, hemoglobin (Fig. 7-23), and cytochrome c (see Fig 16-8), one axial coordination position on the iron of most heme proteins (customarily called the proximal position) is occupied by an imidazole group of a histidine side chain. However, in cytochrome P450 and chloroperoxidase a thiolate (-S ) group from a cysteinyl side chain, and in catalase a phenolate anion from a tyrosyl side chain, occupies the proximal position. The sixth or distal coordination position is occupied by the sulfur atom of methionine in cytochrome c and most other cytochromes with low-spin iron but cytochromes b5 and c3 have histidine. The high-spin heme proteins, such as cytochromes c, ... 

Silver nitrate has been found to react with cysteine, as in hemoglobin,412 or more often with histidine, as in myoglobin, trypsin and carboxypeptidase A.414 In most cases the reaction was similar to that for Hg2+. 

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