cytochrome
Summary
A cytochrome is a heme-containing protein whose iron cycles between Fe³⁺ and Fe²⁺ to pass electrons one at a time down the electron transport chain. Most testable: cytochrome c carries electrons from Complex III to Complex IV, which reduces O₂ to H₂O.
Detail
Meaning: A cytochrome is a protein with a heme group whose iron switches between Fe³⁺ (oxidized) and Fe²⁺ (reduced), so it carries one electron at a time. In the electron transport chain, electrons pass down carriers of rising reduction potential. Complex III (cytochrome bc₁) hands electrons from ubiquinone to cytochrome c, a mobile carrier in the intermembrane space. Complex IV (cytochrome c oxidase) passes them to O₂, forming H₂O. Energy released drives proton pumping.
Equation: ΔG°′ = −nFΔE°′, where n is moles of electrons transferred (mol), F is Faraday's constant (96,485 C/mol), ΔE°′ is the standard reduction potential difference (V), and ΔG°′ is the free energy change (J/mol). Electrons flow toward higher E°′.
On the MCAT: Passages ask for the order of carriers or the effect of an inhibitor. Cyanide and carbon monoxide block Complex IV, so upstream carriers stay reduced, O₂ use falls, and ATP synthesis falls.
Common confusion: Cytochromes carry electrons only, one at a time, whereas NADH and FADH₂ carry two. Iron in hemoglobin stays Fe²⁺ when it binds O₂, but cytochrome iron cycles between Fe³⁺ and Fe²⁺.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Biochemistry (Voet and Voet)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.