Oxidative phosphorylation is the sequence of reactions in mitochondria that convert energy from food into cellular energy by synthesizing ATP, the primary energy currency of cells. To drive the second step in oxidative phosphorylation, electrons must be passed to one of the electron carrier molecules of the electron transport system.
The ability to convert the energy from food molecules into cellular energy efficiently is crucial to cell survival. The central conversion system is oxidative phosphorylation, a sequence of reactions that take place in mitochondria (a type of organelle found in plant cells).
These reactions take high-energy electrons and use them to make adenosine diphosphate (ADP) and inorganic phosphate (Pi). The name “oxidative phosphorylation” derives from the fact that organic molecules are oxidized to provide the electrons that are used as an energy source to facilitate the phosphorylation of ADP.
Oxidative phosphorylationmay be divided into four general steps:
- obtaining high-energy electrons
- transferring energy fromthe electrons into cellular energy, accomplished via the electron transport system
- using the cellular energy from the electron transport system to establish a proton (H+) gradient across the inner mitochondrial membrane
- using the stored energy of the proton gradient to drive the synthesis of adenosine triphosphate (ATP)
Finding Electrons: Oxidation
Themain source of the electrons for the first step of oxidative phosphorylation is the chemical oxidation of organic molecules. Sources can include glycolysis, fatty acid oxidation, and the Krebs cycle (citric acid cycle).
Chemical oxidation results in a loss of electrons by the oxidized molecule; when the electrons are removed from a molecule they are picked up by an electron acceptor, or electron carrier. Electron carriers are molecules that can transport electrons between molecules in the cell, much as a package delivery service will carry a box between two addresses.
The most common electron carriers are nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD). Each one of these molecules can carry two electrons at a time. Once NAD+ or FAD molecules accept electrons, they are said to be chemically reduced and are denoted as NADH or FADH2.
To drive the second step in oxidation phosphorylation, the electrons carried by NADH and FADH2 must be passed to one of the electron carrier molecules of the electron transport system. The carriers that receive the electrons then pass them to other carriers in the system.
Every time one carrier gives a pair of electrons to another, a small amount of energy is released. This energy is used by the mitochondrion to pump H+ across the inner mitochondrial membrane. The electron pair continues to be transferred through a series of electron carriers until any extra energy they carry has been used for pumping H+.
The last carrier transfers what are now low-energy electrons to oxygen, which is the final electron acceptor in the series.When the electrons are accepted by the oxygen, it combines with two H+ to form a molecule of water.
The components of the electron transport system are embedded in the inner mitochondrial membrane, and they are arranged in four large complexes.
Each complex contains a component responsible for picking up the electrons and a protein portion that delivers the electrons to the next carrier in the chain. Each complex also contains additional proteins—in some cases as many as twenty—that attach the complex to the inner mitochondrial membrane.
In addition to these four complexes, there are two smaller electron carriers that can transport electrons between the larger complexes. One of these, cytochrome c, is a small protein. The other electron carrier is called ubiquinone, or coenzymeQ.
Every time an electron pair is delivered to a new carrier, it loses a certain amount of energy, and each of the carriers can only accept electrons that have a particular amount of energy. Therefore, an electron pair moves through the electron transport system in an exact order, going only to carriers that are able to accommodate electrons of a precise energy level.
The order in which electrons move through all the components of the electron transport system has been determined by Britton Chance and several other investigators. An electron pair entering the system would proceed as follows. (The four complexes are identified by Roman numerals I-IV.)
NADH → complex I → ubiquinone → complex III → cytochrome c → complex IV → oxygen
Complex II accepts electrons directly from FADH2, found in the matrix of the mitochondrion, then passes them to ubiquinone.
Establishing the Proton Gradient
The energy harvested during electron transport is used in the third step of the process to create an H+ gradient. Three of the complexes (I, III, and IV) contain an additional protein component that is able to use the harvested energy to move protons from the matrix across the inner membrane, into the space between the inner and outer membranes, which is called the inter membrane space.
The accumulation of protons in this inter membrane space results in proton concentration gradient across the inner membrane, from a high proton concentration in the inter membrane space to a low proton concentration in the matrix of the mitochondrion. The proton concentration gradient represents a stored form of energy, much like the capacitor in an electronic device.
Finally, in the fourth and final step of oxidative phosphorylation, the proton gradient is used to drive the synthesis of ATP. The energy from the proton gradient is used by an ATP-synthesizing enzyme, also found in the inner membrane of the mitochondrion called ATP synthase.
ATP synthase is a very large molecule. At very high magnification, the ATP synthase molecule looks like a lollipop sticking out from the inner membrane into the matrix of the mitochrondrion.
In 1961 Peter Mitchell proposed that the stepwise transfer of electrons by the electron transport system and the proton gradient worked together to synthesize ATP.
His proposal, called the chemiosmotic hypothesis, represented a radical departure from other ideas at the time. At first the chemiosmotic hypothesis found little support among scientists, but the chemiosmotic hypothesis has stood the test of time.
Although some details remain to be worked out, the experimental evidence accumulated by Mitchell, as well as by many other investigators since 1961, overwhelmingly supports this model. Mitchell received the Nobel Prize in Chemistry in 1978 for his proposal of the chemiosmotic hypothesis and the elegant research he performed in its support.
Work from the laboratory of scientist Efraim Racker has demonstrated that there are different functions for the two parts of the lollipop. The spherical part of the lollipop can be removed by mechanical shaking and has been found still to be able to synthesize ATP.
Racker called the sphere F1, or coupling factor 1. The “stick” portion of the lollipop is embedded in the inner mitochondrial membrane. This part of the enzyme acts as a tunnel for protons to travel back into the mitochondrion.
The stick portion of the enzyme can be inactivated by the antibiotic oligomycin, so it is called the Fo' or oligomycin-sensitive factor. Another name for the ATP synthase is thus F0F1ATPase.
The fine details of how the movement of protons through the channel in the stick drives ATP synthesis have not been completely worked out. The spherical portion of the molecule can make ATP without the proton gradient.
Once synthesized, however, the ATP remains tightly attached, so no additional ATP can be made by the enzyme. The large concentration of protons in the intermembrane space causes a net flux of protons back into thematrix through the tunnel provided by the ATP synthase.
Paul Boyer suggested that when protons move through the lollipop stick into the mitochondrion, the sphere changes its shape. This shape change, in turn, causes the enzyme to release newly synthesized ATP. The ATP can now be used by the cell, and the enzyme can now make more ATP.
Boyer’s proposal, which has gradually gained acceptance among researchers, is reasonable. Many biological enzyme reactions work by using similar shape changes to attach and release the substrates. A molecule of ATP is released for every three hydrogen ions that are returned to the matrix.