Myoglobin, heme derivatives

Myoglobin Heme derivatives Catalyst/O storage/ electron transfer [13, 21, 22]... 

Cytochromes were first named and classified on the basis of their absorption spectra (Figure 21.9), which depend upon the structure and environment of their heme groups. The b cytochromes contain iron—protoporphyrin IX (Figure 21.10), the same heme found in hemoglobin and myoglobin. The c cytochromes contain heme c, derived from iron-protoporphyrin IX by the covalent attachment of cysteine residues from the associated protein. UQ-cyt c... 

A similar study was performed on ruthenium-modified myoglobins, in which AG variations were obtained by changing the nature of the ruthenium complex covalently bound to the protein, and by substituting a porphyrin to the heme [137]. It is gratifying to observe that, in spite of the rather heterogeneous character of this series, the study leads to an estimation of 1.9 to 2.4 eV for A which is consistent with the value 2.3 eV derived in section 3.2.1 from temperature dependent experiments. Satisfactory agreement between the results given by the two methods is also observed in the case of ruthenium-modified cytochrome c [138]. 

The cases of myoglobin and hemoglobin are not rare. Many enzymes are dependent for their function on the presence of a nonprotein group. For example, cytochrome c also contains a prosthetic group similar, but not identical, to heme, as do a number of other proteins. These are known generically as heme proteins. There is a family of enzymes that contain a flavin group, the flavoproteins. Another family contains pyridoxal phosphate, a derivative of vitamin Be. There are a number of other examples. 

In Ref. [279] the technique of protein modification was used to study the dependence of the rate of photoinduced electron tunneling on the distance between TZnP and Ru(III) sites in modified myoglobins. The modified proteins were prepared by substitution of zinc mesoporphyrin IX diacid for the heme in four various pentaammineruthenium (III) derivatives of sperm whale myoglobin (NH3)5Ru(His-48)Mb, (NH3)5Ru(His-12)Mb, (NH3)5Ru(His-116)Mb and (NH3)5Ru(His-81)Mb. Metal-to-metal distance between ZnP and (NH3)5Ru(His) ranges in this seria from 16.1-18.8 A for His-48 to 27.8-30,5 for His-12. The rate constant of electron tunneling decreases in this series in accordance with Eq. (1) with ve = 7.8 x 10s s 1 and ae = 2.2 A at T = 298 K. 

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