cytochromes

If iron-sulfur clusters are one kind of wire in the cell's electrical circuits, cytochromes are another, and they too are colored — the name literally means cell pigments. They are the heme proteins whose job is not to carry oxygen, like hemoglobin, but to carry electrons. They are everywhere energy is harvested: the chain of cytochromes in your mitochondria is exactly what your cells use to extract energy from food using oxygen, and a closely related set runs photosynthesis in plants.

A cytochrome uses the same iron-in-a-porphyrin heme unit as hemoglobin, but it is wired for a completely different task. Instead of leaving a binding site open for oxygen, both axial positions above and below the iron are usually filled by amino-acid groups from the protein, so nothing can dock there. The iron then has just one trick: to flip between iron(III) (oxidized) and iron(II) (reduced), accepting an electron from the carrier before it and handing it to the carrier after it. By stringing many cytochromes together, each tuned to a slightly different reduction potential, the cell builds a downhill staircase of electron carriers, releasing the energy of that electron flow in small, usable steps rather than one destructive burst. Different families are named by the light they absorb — cytochrome a, b, and c — and one famous member, cytochrome c oxidase, is where the journey ends as electrons are finally combined with oxygen to make water.

Cytochromes matter because they are the heart of the energy economy of almost all aerobic life, and the small differences in their reduction potentials, set by the surrounding protein, are what let the cell capture energy efficiently. An honest distinction worth holding onto: the iron in a cytochrome is doing redox chemistry, switching oxidation state to pass electrons, whereas the iron in hemoglobin must NOT change oxidation state — it stays iron(II) while it reversibly binds O2. Same heme, opposite design rules. And like iron-sulfur clusters, cytochromes are a frequent target of poisons such as cyanide, which jams cytochrome c oxidase and halts respiration.