Oxygen in hemoglobin

Heme A porphyrin ring that has a Fe+ ion coordinately bound in the center of the molecule. Heme binds oxygen in hemoglobin and myoglobin and serves as an electron carrier in the cytochromes. 

Figure 9-2. Binding of molecular oxygen in hemoglobin/myoglobin.

The transport of oxygen in hemoglobin solutions is similarly a more complex process than diffusion with the reaction A 4 B = AB, as it has most often been modelled. The extent of oxyhemoglobin formation varies sigmoidally, rather than hyperbolically with the equilibrium partial pressure of oxygen, a consequence of coopera-tive oxygen binding at the four sites per protein molecule (29). 

The metals play several different roles in living systems. As cations (.Na, K, Ca , and Mg" ), they serve to maintain the balance between intracellular and extracellular fluids, nerve transmissions, and other activities. They are also needed for protein functions. For example, the Fe " ion binds oxygen in hemoglobin molecules and Cu, Zn , and Mg ions are essential for enzyme activity. In addition, calcium in the form of Ca5(P04)3(0H) and Ca3(P04)2 is an essential component of teeth and bones. 

Iron is indispensable in the human body (see Mineral nutrients). The average adult body contains 3 grams of iron. About 65% is found in hemoglobin, which carries oxygen from the lungs to the various parts of the body. Iron is also needed for the proper functioning of cells, muscles, and other tissues (4). 

The allosteric effect is seen in hemoglobin which can exist in two quaternary stmctural states oxygenated (R) or deoxygenated (T). The binding of one O2 or some other effector to one of the subunits stabilizes the R form as compared to the T form. Binding of a second and third O2 stabilizes it even further. 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Porphyrins, derivatives of porphin, are best known for their role as oxygen earriers in hemoglobin. 

Hemoglobin (Hb) binds to both oxygen and carbon monoxide. When tiie carbon monoxide replaces the oxygen in an organism, the following reaction occurs ... 

Oxygenation of Hemoglobin Triggers Conformational Changes in the Apoprotein... 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

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. 

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