Cytochrome type proteins

In addition to the effect of mutations at Phe-82 on the stability of the cytochrome c active site, the intense, negative Soret Cotton effect in the circular dichroism spectrum of ferricytochrome c is profoundly affected by the presence of non-aromatic amino acid residues at this position [115]. Recent examination of six position-82 iso-l-ferricytochrome c mutants establishes that while Tyr-82 exhibits a Soret CD spectrum closely similar to that of the wild-type protein, the intensity of the negative Soret Cotton affect varies with the identity of the residue at this position in the order Phe > Tyr > Gly > Ser = Ala > Leu > He, though the Ser, Ala, He, and Leu variants have effectively no negative Soret Cotton effect [108]. 

Fig. 2a-c. Stereodiagram of the yeast iso-1-cytochrome c surface, (a) Surface of the wild-type protein (b) surface of the Ser-82 mutant (c) surface of the Gly-82 mutant. (Modified from Refs. [123, 124])... 

To evaluate the functional role(s) of this residue, six mutations, Lys, His, Glu, Asn, Leu, and Ala, were introduced at this site, and the electrochemical and NMR properties of the resulting proteins examined [134]. Contrary to expectation, removal of Arg-38 did not result in a change in the dependence of the cytochrome c reduction potential on pH. Instead, as the electron-withdrawing ability of the residue substituted at position-38 decreased, the reduction potential also decreased, with the greatest decrease (50 mV) observed for the Ala mutant. The variation of reduction potential with pH, however, remained essentially the same as that previously observed for the wild-type protein. 

The two differently bound methionine sulphur in the b and c-type haem proteins encouraged measurements of the respective S 2p binding energy value. In myoglobin, a representative of the b-type proteins, the methionine residue is exclusively located in the polypeptide side chain with no direct metal sulphur bonding. By way of contrast cytochrome c has one methionine coordinated to iron. 

The CYPs, the carbon monoxide-binding pigments of microsomes, are heme proteins of the b cytochrome type. Originally described as a single protein, there are now known to be more than 2000 CYPs widely distributed throughout animals, plants, and microorganisms. A system of nomenclature utilizing the prefix CYP has been devised... 

Redox proteins that include quinone cofactor units play important roles in biological ET processes. Some of the quinoproteins include the quinone cofactor in a non-covalently linked configuration, such as the pyrroloquinoline quinone, PQQ, dependent enzymes, whereas other quinoproteins include the quinone cofactor covalently-linked to the protein, for example topaquinone (2,4,6-trihydroxyphenylalanine quinone, TPQ) dependent enzymes. A number of quinoproteins include in addition to the quinone cofactor an ET cofactor unit in another protein subunit. These cofactors may be metal ions or a cytochrome-type heme cofactor such as D-fructose dehydrogenase that is a heme containing PQQ-dependent enzyme. ... 

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