Cytochrome c is a highly conserved protein across the spectrum of species, found in plants, animals, and many unicellular organisms. This, along with its small size (molecular weight about 12,000
daltons), makes it useful in studies of
cladistics. Its primary structure consists of a chain of about 100
amino acids. Many higher order organisms possess a chain of 104 amino acids.
[3]
The cytochrome c molecule has been studied for the glimpse it gives into evolutionary biology. Both
chickens and
turkeys have identical sequence homology (amino acid for amino acid), whereas
ducks possess molecules differing by one amino acid. Similarly, both
humans and
chimpanzees have the identical molecule, while
rhesus monkeys share all but one of the amino acids:
[4] the 66th amino acid is
isoleucine in the former and
threonine in the latter.
[3] Pigs,
cows and
sheep also share identical cytochrome c molecules.
[3]