Electron Transport Overview
A typical cell contains approximately 2000 mitochondria, each of which is about the size of a bacterium. The mitochondrion's extensively invaginated inner membrane contains a complex system of enzymes and cofactors that produce ATP, the cellular energy currency used by virtually all forms of life. Where does the energy come from and how is it channeled specifically into ATP synthesis? The answers to these questions form one of the most fascinating stories in biochemistry.
Introduction
The source for most of the energy utilized in ATP synthesis is the oxidation of NADH, which releases 218 kJ of free energy per mole under standard conditions. To maximize the recovery of this free energy in usable form, the oxidation of NADH occurs over several steps that are mediated by a series of electron carriers in the inner mitochondrial membrane and which serve to ferry electrons from NADH to O2. Some of these redox centers are highly mobile, and others are components of large integral membrane protein complexes. The free energy released in this series of redox reactions is stored in an electrochemical gradient of protons across the inner membrane. This gradient is subsequently harnessed by ATP synthase, which is also embedded in the inner membrane.
In this overview, we will review the roles of the three central complexes and the two mobile carriers in the mitochondrial electron transport chain. You can skip to a particular topic by using the menu at the top right. The energy graph labels can be changed with the menu at the top left. You can pause the presentation at any time with the pause/play button. The presentation will automatically pause after each complex. Click on Play to begin.
Complex I
NADH is the source of most electrons in the electron transport chain and binds to Complex I on the matrix side of the membrane. Complex I contains flavin mononucleotide or FMN, a redox-active prosthetic group that resembles FAD but lacks its AMP moiety. NADH transfers 2 electrons to FMN to yield the fully reduced FMNH2. Note that the activation barrier on the energy graph has been lowered, representing schematically the catalytic role of Complex I.
FMNH2 passes its electrons, one at time, to the mobile carrier molecule, ubiquinone, or coenzyme Q, abbreviated here as CoQ. This is possible because FMN has a stable radical oxidation state known as FMNH•. The electrons each pass through a series of 6 or 7 iron-sulfur clusters along their way to coenzyme Q. During the electron transfer, in a way not fully understood, Complex I also pumps 4 protons from the matrix to the intermembrane space. Once the electron transfer is complete, CoQH2, the fully reduced form of coenzyme Q which is nonpolar, diffuses into the membrane.
Before we move on to the next step of the electron transport process, you should note that Complex II, a transmembrane complex that is not shown here, also functions to transfer a pair of electrons to coenzyme Q. However, in the case of Complex II, the source of the electrons is the FADH2 produced in the citric acid cycle. Since the standard reduction potential of FADH2 is only slightly less than that of coenzyme Q, Complex II does not pump any protons out of the matrix.
Complex III
The two electrons from Complex I or Complex II are shuttled to Complex III by CoQH2. It binds initially to a site close to the cytosolic side of Complex III, which is also called cytochrome bc1. Since the next mobile electron carrier, the peripheral membrane protein cytochrome c, can only shuttle one electron at a time, one of the electrons from reduced coenzyme Q will take a somewhat circuitous route known as the Q cycle.
Reduced coenzyme Q transfers one of its electrons through an iron-sulfur cluster and a cytochrome c1 center to a cytochrome c that is temporarily bound to Complex III in the intermembrane space. In this process, the proton released by CoQH2 finds its way to the intermembrane space.
The second electron transfer also releases a proton into the intermembrane space, though the electron instead passes sequentially through a pair of b-type cytochromes buried deep inside the complex. The resulting fully oxidized coenzyme Q now diffuses to a binding site on Complex III near the matrix side of the membrane, where the sequestered electron is returned, producing the radical form of coenzyme Q, abbreviated CoQH•.
So far, only one electron has been passed to cytochrome c. A second cytochrome c receives its electron from a different reduced coenzyme Q molecule borrowed from the pool available in the inner mitochondrial membrane. As before, one electron goes to cytochrome c, while the other is passed on to the radical coenzyme Q, fully reducing it.
The reduced coenzyme Q borrowed in the previous step is thus regenerated, the net result being one reduced coenzyme Q becoming fully oxidized. The oxidized coenzyme Q can now return to Complex I and shuttle another pair of electrons.
Complex IV
The oxidized cytochrome c molecules shuttle their electrons to Complex IV, which is also known as cytochrome c oxidase. Complex IV passes the electrons on to diatomic oxygen, thereby reducing it to two water molecules. This reaction requires 4 electrons and hence two cycles of the previous steps are needed to carry out this terminal oxidation step. The first two electrons are passed through a binuclear copper center near the cytosolic surface to a pair of a-type cytochromes deep inside the complex. The iron atom in the second of these cytochromes is associated with an essential copper ion to form a binuclear complex to which diatomic oxygen binds. As each electron is transferred into Complex IV, one proton is transported from the matrix to the intermembrane space.
The first 2 electrons to enter Complex IV reduce its bound oxygen molecule to the peroxy state. The next electron, along with 2 protons from the matrix, fully reduces one of the oxygen atoms to water. With the acquisition of the fourth electron, the water transfers one of its hydrogens to the other oxygen atom creating two hydroxide ions, which become waters when they acquire two more protons from the matrix. These two water molecules diffuse back into the matrix.
Summary
For each molecule of oxygen reduced, 20 protons are transferred from the matrix to the intermembrane space. In addition, 4 matrix protons have been used to build 2 waters from the oxygen. Four electrons are transported, requiring 2 NADH molecules as input. The free energy released by the redox steps is stored in the proton gradient produced. These 20 protons will be transferred back to the matrix by ATP synthase in a manner that drives the synthesis of approximately 6 ATP molecules, a process that is known as oxidative phosphorylation. The oxidation of 2 FADH2 molecules initiated by Complex II also yields the four electrons necessary to reduce oxygen to water but pumps only 12 protons out of the matrix and hence yields only around 4 ATPs.