| STRUCTURE OF THE HEME PROSTHETIC GROUP
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| Heme is the O2-binding molecule common to Mb and Hb; it is a porphyrin molecule to which an iron atom (Fe2+) is coordinated (Fig. 4.1). The Fe-porphyrin prosthetic group is, with the exception of two propionate groups, hydrophobic, and planar. Heme becomes an integral component of the globin proteins during polypeptide synthesis; it is heme that gives globin proteins their characteristic purple-red color. Globins increase the aqueous solubility of the otherwise poorly soluble, hydrophobic heme prosthetic group. Once sequestered inside a hydrophobic pocket created by the folded globin polypeptide, heme encounters a protective environment that minimizes the oxidation (rusting) of Fe2+ to Fe3+ in the presence of O2. Such an environment is also essential for globins to bind and release O2. Should the iron atom become oxidized to Fe3+, generating the proteins metmyoglobin or methemoglobin, heme can no longer interact reversibly with O2. Ultimately, O2 storage and transport become compromised.
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| Figure 4.1 Structure of heme. Heme is a complex of porphyrin and iron. Left (top view): The carbon framework of protoporphyrin IX, a conjugated tetrapyrrole ring, is depicted in gray; O2 molecules are red. Iron (yellow sphere) prefers six ligands in an octahedral coordination geometry; pyrrole nitrogen atoms (blue spheres) provide four of these. Right (side view): In the globin structure, the planar heme is positioned between the proximal and distal histidines (His); only the former has an imidazole nitrogen (blue sphere) close enough to bond with iron. The α-helices that contain these histidines are shown in pink. In deoxygenated globins, the sixth position remains vacant, leaving a pentacoordinated iron. For globins in the oxygenated state, O2 occupies the sixth position. |
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