In the body, the iron in the heme is coordinated to the fournitrogen atoms of the porphyrin and also to a nitrogen atom froma histidine residue (one of the amino-acid residues inhemoglobin) of the hemoglobin protein (see Figure 4). The sixthposition (coordination site) around the iron of the heme isoccupied by O2 when the hemoglobin protein isoxygenated.
Blood is an amazing and vitally important part of the body,because it contains many finely-tuned chemical systems that allowit to maintain the chemical environment needed for the body'smetabolism. One of the most important functions of blood isdelivering O2 to all parts of the body by thehemoglobin protein. O2 is carried in the hemoglobinprotein by the heme group. The heme group (a component of thehemoglobin protein) is a metal complex, with iron as the centralmetal atom, that can bind or release molecular oxygen. Both thehemoglobin protein and the heme group undergo conformationalchanges upon oxygenation and deoxygenation. When one heme groupbecomes oxygenated, the shape of hemoglobin changes in such a wayas to make it easier for the other three heme groups in theprotein to become oxygenated, as well. This feature helps theprotein to pick up oxygen more efficiently as the blood travelsthrough the lungs. Hemoglobin also enables the body to eliminateCO2, which is generated as a waste product, via gasexchange in the blood (CO2 exchanged for O2in the lungs, and O2 exchanged for CO2 inthe muscles). The species generated as waste by theoxygen-consuming cells actually help to promote the release of O2from hemoglobin when it is most needed by the cells. Hence,hemoglobin is a beautiful example of the finely tuned chemicalsystems that enable the blood to distribute necessary moleculesto cells throughout the body, and remove waste products fromthose cells.
Hemoglobin a Synthesis in the Developing Fetus — …
How do CO2 and H+ promote the release ofO2 from hemoglobin? These species help forminteractions between amino-acid residues at the interfaces of thefour subunits in hemoglobin. These interactions are called "salt bridges,"because they are between positively-charged and negatively-charged amino-acidresidues on different subunits of the same protein (Figure 8). When"salt bridges" form, the subunits are held in a position that"tugs on" the histidine that is attached to the heme iron. (See Figure5.) This favors the domed configuration, which is the deoxygenated form ofhemoglobin.