The Oxygen-Carrying Proteins

FIGURE 6.7 The three-dimensional structures of two proteins that contain substantial amounts of rx-helix in their structures. The helices are represented by the regularly coiled sections of the ribbon drawings. Myohemery-thrin is the oxygen-carrying protein in certain invertebrates, including Sipunculids, a phylum of marine worm. (Jane Richardson)... 

ERYTHROCYTES. Erythrocytes are biconcave diskshaped, blood cells (with pits or depressions in the center on both sides), the primary function of which is to transport hemoglobin, the oxygen-carrying protein. The biconcave shape of the erythrocyte provides a large surface volume ratio and thereby facilitates exchange of oxygen. The average diameter of erythrocytes is 7.5 pm, and thickness at the rim is 2.6 pm and in the center about 0.8 pm. The normal concentration of erythrocytes in blood is approximately 3.9-5.5 million cells per pL in women and 4.1-6 million cells per pL in men. The total life span of erythrocytes in blood is 120 days. 

Erythrocytes—Red blood cells the color comes from the oxygen-carrying protein hemoglobin inside the cell. 

An example of evolution via gene duplication is provided by the oxygen-carrying proteins of blood. 

Transport and storage. Many small molecules and ions are transported in the blood and within cells by being bound to carrier proteins. The best example is the oxygen-carrying protein hemoglobin. Iron is stored in various tissues by the protein ferritin. 

Hemoglobin, the oxygen-carrying protein of vertebrate blood, is similar in structure to myoglobin. However, hemoglobin is composed of four chains i.e., it has a quaternary structure. The four chains... 

An important group of naturally occurring amino acids complexes of iron are the hemerythrins —the oxygen-carrying proteins in a variety of marine worms. Most hemerythrins have molecular... 

N and O donors typically hexacoordinate, these can have extended carboxylate bridged stractures. The stracture and reactivity of such complexes have been examined as model systems in the search for explanations of the biological activation and transport of oxygen. The oxygen carrying proteins hemerythrins contain active centers " " in which two iron atoms are linked by both /x-oxo and carboxylate bridges provided by amino acid groups - these are discussed elsewhere in this Encyclopedia. 

The number of charges on a metal ion can be altered by addition or removal of an electron. Many transition metal ions found in humans can exist in a range of oxidation states. For example, copper has + 1 and + 2 (cuprous and cupric respectively), iron has + 2 and + 3 (ferrous and ferric respectively), but some of these states are not stable in aqueous solution at neutral pH without ligands being attached to the metal ion. Thus, the oxygen-carrying protein, haemoglobin, is able to maintain the... 

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