This is an image of the three-dimensional structure myoglobin, which is an oxygen storage protein found in muscles. This image is using a ball-and-stick representation of the protein, often a ribbon representation is used instead to highlight both the secondary structure and the threading of the polypeptide through the folded tertiary structure. The oxygen is not bound by the protein directly, but is bound by an iron-heme molecule that is embedded in the myoglobin protein. Zooming in on the iron-heme molecule, you can observe that the iron is bound to the heme through four nitrogen atoms. Iron ions prefer to bind to four ligand, a fifth ligand site for the iron ion involves a nitrogen from the histidine side chain of His93, which is called the proximal histidine. A spacefill rendering of this histidine shows that it is tightly bound to the iron atom. The six ligand to the iron atom is the dioxygen molecule that is being bound by the myoglobin. There is another conserved histidine, which is called the distal histidine, and it is located next to the oxygen molecule. A spacefilling rendering of this histidine reveals that it is not tightly bound to the oxygen, rather, its presence helps the myoglobin select for oxygen over the hightly toxic carbon monoxide molecule, which prefers to bind to the iron atom straight on, instead of at an angle as shown for the oxygen. You can click here, to return to the beginning.

(Click-drag in the left, top and right margins to zoom, z-rotate and slice, respectively.)