The Electron Transport Chain

Oxidative Phosphorylation

Oxidative Phosphorylation is the indirect formation of ATP.

It involves a series of redox reactions in which oxygen is the final electron acceptor.

It is more complex than Substrate-Level Phosphorylation and therefore creates more ATP.

The reduction of the electron carrying molecules NAD+ and FAD to NADH and FADH2 are energy harvesting reactions that will transfer the majority of their free energy to the creation of ATP.

Electron Transport Chain

NADH and FADH2, transfer their electrons to a series of compounds (mostly proteins), which are associated with the inner mitochondrial membrane.

ETC Compounds

The compounds (in order): NADH dehydrogenase, ubiquinone (Q), the cytochrome b-c1 complex, cytochrome c, cytochrome oxidase complex.

ETC and Redox Reactions

The protein/compounds are arranged in order of increasing electronegativity (each successive compound wants the electrons more than the one before it).

Each compound is reduced by gaining two electrons from the one before it and oxidized by donating its two electrons to the one after it.

Free Energy from the ETC As the electrons are

passed they become more stable and generate free energy.

This free energy is used to pump protons (H+) into the intermembrane space from the matrix (active transport).

There are 3 proton pumps.

Oxygen

Oxygen is the final electron acceptor and it joins with two protons in the matrix to form water.

Steps in the ETC:

1. NADH, from pyruvate oxidation and the Krebs Cycle, gives up its two electrons (oxidized) to NADH dehydrogenase. (reduced) NAD+ is regenerated and

can pick up more H+ in glycolysis, pyruvate oxidation, or the Kreb's cycle (recycling of electron carriers).

2. The mobile carriers Q and cytochrome c shuttle electrons from one protein complex to the next until they reach the cytochrome oxidase complex.

(series of redox rxns)

3. Three of the protein complexes also act as proton pumps, using the free energy released to move protons from the matrix to the intermembrane space.

4. At the cytochrome oxidase complex, cytochrome oxidase, catalyzes the reaction between the electrons, protons and oxygen to form water.

(2H+ + ½ O2 + 2e- H

2O)

http://www.sp.uconn.edu/~terry/images/anim/ETS.html

What about FADH2?

FADH2 skips the first protein compound and starts at Q.

This means that NADH oxidation pumps three protons into the intermembrane space, while FADH2

oxidation pumps only two protons. Important to note that the NADH formed in glycolysis

in the cytoplasm cannot get to the matrix to take part in the ETC

It passes into the mitochondrial matrix through the glycerol-phosphate shuttle, where its electrons are passed to FADH2, therefore another FADH2 is essentially created in glycolysis.

Energy Transfer This process gives up 222 kJ/mol of free energy. The chemical potential energy of the electron’s position

is converted to an electrochemical potential energy of a proton gradient that forms across the inner mitochondrial membrane. High concentration in the intermembrane space, low

concentration in the matrix. The proton gradient will be used to produce ATP.

Three ATP are formed from NADH while two ATP are formed from FADH2..

There are many copies of the ETC along the cristae, therefore lots of ATP can be produced.

Chemiosmosis and Oxidative Phosphorylation

Due to the movement of electrons through the ETC and the proton pumps, there is now an electrochemical gradient across the inner membrane. (More protons outside in the IMS than in the matrix). difference in charge and a

difference in concentration. Protons need to move back in to

fix this difference BUT, the inner membrane is impermeable to protons.

ATP Synthase The protons are forced

through special proton channels that are coupled with ATP synthase (ATPase).

Therefore the electrochemical gradient produces a proton-motive force (PMF) that moves the protons through this ATPase complex.

Chemiosmosis and Oxidative Phosphorylation

Each time a proton comes through the ATPase complex, the free energy of the electrochemical gradient is reduced.

This energy is used to create ATP from ADP + P in the matrix.

1961 Peter Mitchell discovered this and coined the term chemiosmosis because the energy that drives ATP production comes from the osmosis of protons.

It took a long time for his theory to be accepted. He finally got his Nobel Prize in 1978.

ATP Synthase

http://www.sp.uconn.edu/~terry/images/anim/ATPmito.html

The importance of Oxygen

The continual production of ATP is dependent on the maintenance of a proton reservoir in the intermembrane space.

This depends on the continued movement of electrons and that depends on the availability of oxygen.

Therefore we need oxygen to prevent the ETC from being clogged up and we need food to provide the glucose that provides electrons for the ETC.