The monoiodotyrosine 74, formyltryptophan 59, mononitrotyrosine 67, and carboxymethylmethionine 80 derivatives of horse cytochrome c are defective in their ability to accept electrons from the succinate-cytochrome c reductase system, while their reactions with purified cytochrome c oxidase are essentially those of the native protein. The 4-nitrobenzo-2-oxa-1,3-diazole derivative of lysine 13 of horse cytochrome c and the bis-phenylglyoxal derivative of arginine 13 of Candida krusei cytochrome c have the opposite properties, in that they are readily reduced by the succinate-cytochrome c reductase (EC 1.3.99.1) system but are defective in their capability of transferring electrons to cytochrome c oxidase (EC 1.9.3.1). We conclude that electrons from mitochondrial cytochrome c reductase are transmitted to ferricytochrome c by a different pathway than electrons from ferrocytochrome c to cytochrome c oxidase. The present results are compatible with the concept that the mechanism of reduction involves an aromatic ring channel comprising residues 74, 59, 67, and 80, leading from the "left back" part of the protein to the heme iron. On the other hand, since residue 13 is immediately above the edge of the heme that is at the "front surface" of the molecule, we suggest that the electron leaves ferrocytochrome c to cytochrome c oxidase by way of the edge of pyrrole ring II or the adjacent surface-located sulfur of cysteinyl residue 17, which is thioether bonded to the heme. On this basis, the sites of electron entry and exit in cytochrome c would appear to be some 110 degrees of arc away from each other along the surface of the protein, explaining several previously observed phenomena.

Animals↗

PubMed1972

The primary structure of cytochrome c from the rust fungus Ustilago sphaerogena.

1. The complete amino acid sequence of cytochrome c from the basidiomycete Ustilago sphaerogena was determined from the amino acid compositions and sequences of either tryptic or chymotryptic peptides, and in homology with at least thirty other established sequences of cytochrome c. 2. The primary structure of the molecule bears all of the characteristics of a mammalian-type cytochrome c, showing the typical clustered distribution of hydrophobic and basic residues with a single polypeptide chain of 107 residues. 3. Like all other fungal cytochromes c, it possesses a free N-terminus, and one less residue at the C-terminus than vertebrate cytochromes c. The region of residues 70-80 is strictly conserved, as is histidine at position 18. Position 26 is occupied by an asparagine residue, in contrast to histidine which occurs at this location in most of the known sequences of mammalian-type cytochromes c. 4. In contrast to some other fungal and plant cytochromes c of known primary structures, the Ustilago cytochrome c molecule does not contain trimethyl-lysine. 5. The sequence of Ustilago cytochrome c differs from the sequences of human, horse, chicken, tuna, wheat, and baker's yeast proteins at loci 47, 43, 44, 44 and 38 respectively.

Amino Acid Sequence↗

PubMed1971