Heme group color

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. 

Resonance-stabilized systems include car-boxylate groups, as in formate aliphatic hydrocarbons with conjugated double bonds, such as 1,3-butadiene and the systems known as aromatic ring systems. The best-known aromatic compound is benzene, which has six delocalized k electrons in its ring. Extended resonance systems with 10 or more 71 electrons absorb light within the visible spectrum and are therefore colored. This group includes the aliphatic carotenoids (see p.l32), for example, as well as the heme group, in which 18 k electrons occupy an extended molecular orbital (see p. 106). 

TThe iron-porphyrin (heme) group of hemoglobin, released from dying erythrocytes in the spleen, is degraded to yield free Fes+ and, ultimately, bilirubin. This pathway is arresting for its capacity to inject color into human biochemistry. 

Figure 1. (a) X-ray crystal structure of horse-heart ferricytochrome c.8 All protein atoms are shown in the C.-P.-K. form, while the heme group is shown in the stick form. All Arg and Lys residues are colored blue, while Glu and Asp are colored in red, to contrast the destribution of the most ionizable side chains, (b) The X-ray crystal structure of horse heart ferricytochrome c in complex with horse cytochrome c peroxidase (cep).9 The peroxidase is shown as a molecular surface model, with blue regions depicting positive and red representing negative electrostatic potential. Note the cluster of negative potential on ccp that surrounds the contact interface. 

Figure 5.17 Structure of myoglobin complexed with a heme group. The two residues chosen for mutation, Y103 and L72, are shown. Left Side view of myoglobin. Right Top view of myoglobin. Color scheme Carbon (white), oxygen (red), nitrogen (blue), iron (pink).

The cytochromes (from the Greek cell colors ) are a family of proteins containing prosthetic heme groups (see Chaps. 5 and 15). Mitochondria contain three classes of cytochromes a, b, and c, which have hemes of different structures. 

Cytochrome c. An electron carrier in the respiratory chain, cytochrome c consists of a small protein (114 amino acids in human) and a bound heme group. The iron atom in the heme group can accept an electron. It has a pink color with absorption at 415, 520, and 549nm that is derived from the heme group. 

Figure 11 Molecular model of the complex between Ru-65-c) t bs and Cc. The geometry of the complex is the same as that of the complex involving native cytochrome bs proposed by Salemme. The heme groups (red), and the ruthenimn complex (green) are highhghted. The atoms forming an electron-transfer pathway between the ruthenimn complex and the heme group of Ru-65-c) t bs are colored yellow. The lysine and arginine residues are blue, while aspartate and glutamate residues are red ...

Figure 15 X-ray crystal structure of the complex between yeast c) t bc and yCcT C)d c is colored grey, yCc is light blue, the heme groups are red, acidic residues on c)d c are red, basic residues on yCc are bine, and Phe230 is purple. The ruthenium complex on Cys39 is green...

Fig. 2.1 (a) Stereo view of the overall fold of CYP3A4 (PDB code IWOE) colored from blue at the N terminus through to red at the C-terminus. The heme group is shown as a ball-and-stick model in the center of the figure, (b) Stereo view of the active site of CYP3A4. The solvent-accessible... 

Fig. 4. Overall view of the RC structure. The Cj, chains of the protein are shown as grey ribbons. Refer to color plate 7 for the colors of the cofactors which are represented as blue (heme groups), red (special pair), green (accessory bacteriochlorophylls), lilac (carotenoid), yellow (bacteriopheophytins), orange (quinones) and cyan (non-heme iron). (See also Color Plate 7)...

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