Respiratory pigments found widely in aerobic
organisms which transfer energy within cells.
They are proteins containing a chelate
complex of an iron prosthetic group and
a porphyrin system, and so include hemoglobin.
Cytochromes undergo a series of oxidation
and reduction reactions, transferring
electrons from a substrate to each other
and finally to an electron-acceptor. In cellular respiration
the substrates are the hydrogen-acceptors in the citric
acid cycle, the electron acceptor is oxygen,
and the energy produced is stored in ATP. See
also photosynthesis. Cytochromes
are, in fact, the principal route by which atmospheric oxygen enters into