Biochemistry Lecture No. 18: Electron Transport
Friday October 19 , 2012
-Most electron carriers are embedded in the four large integral membrane protein complexes of the
electron transport chain. Electron carriers differ in their affinity for electrons (from strongest reducing
agent to strongest oxidant). Electrons pass from a carrier with low affinity (strong reductants like NADH)
to a carrier with higher affinity (strong oxidants like O )2 The electron transport chain has five types of
electron carriers (in approximate order of increasing affinity for electrons): Flavins, iron-sulfur centres,
ubiquinone, cytochromes, and copper centres.
-Flavins consists of a flavin (isoalloxazine) ring that can accept two electrons (along with two protons).
Examples of flavins include FADH and2riboflavin (vitamin B ). 2
-Each iron-sulphur centre can accept one electron (per centre), in which it goes from a Fe to a Fe .
Two forms of iron-sulphur centres are present in nature: Fe S (sq2 2e planar centre that accepts
2electrons) and Fe S4 4attice structure centre that accepts 4 electrons).
-Ubiquinone is a lipid molecule that freely diffuses in the membrane (also called Coenzyme Q or simply
Q). This electron carrier accepts two electrons, one at a time. The intermediate between oxidized Q (no
electrons added) and reduced QH (2 el2ctrons added) is known as ubisemiquinone QH (1 electron
added) and can act as a free radical if not immediately reduced.
-Cytochromes are proteins that contain heme groups, with an iron ion at the centre and can accept one
electron (Fe to Fe ). A notable example of this electron carrier is cytochrome c, which is a peripheral
membrane protein that accepts electrons from Complex III and gives electrons to Complex IV.
-Copper centres consist of copper ions, usually coordinated by histidine side chains, and can accept one
electron (Cu to Cu ). +
Electron Transport Complexes:
-The electron transport chains is comprised of four large complexes (I, II, III and IV) each made up of
many polypeptide chains, embedded in the inner mitochondrial membrane and contain electron carriers
as cofactors. -The flow of electrons between complexes is as follows:
NADH I Q III cytochrome c IV O 2
FADH 2 II Q III cytochrome c IV O 2
NADH Dehydrogenase (Complex I):
-Also known as Complex I, NADH dehydrogenase accepts two electrons from the mitochondrial matrix. It
pumps 4 protons across the IM membrane (from the matrix to the intermembrane space) per electron