Oxidative phosphorylation

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The Electron Transport Chain. Not all structures represent current knowledge of electron transport chains -- see talk page for more details.

Oxidative phosphorylation is the terminal process of cellular respiration in eukaryotes. During oxidative phosphorylation electrons are transferred from NADH or FADH₂ — created in glycolysis, fatty acid metabolism and the Krebs cycle — to molecular oxygen, via a series of protein complexes located in the inner mitochondrial membrane. Protons are pumped from the mitochondrial matrix into the intermembrane space as a result of this flow of electrons. This generates a pH gradient and a transmembrane electrical potential across the membrane. This is a form of potential energy which is referred to as a proton-motive force. The protons flow back into the mitochondrial matrix via a large protein complex, ATP synthase. This is how most of the ATP is created in a eukaryotic cell. This is the reaction that ATP synthase catalyses:

ADP^3- + H⁺ + P_i ↔ ATP^4- + H₂O

The synthase functions almost as a mechanical motor, with each NADH molecule contributing enough proton motive force to generate 2.5 ATP. Each FADH₂ molecule is worth 1.5 ATP. All together, the 10 NADH and 2 FADH₂ molecules contributed through oxidation of glucose (glycolysis, conversion of pyruvate to acetyl-CoA, and the Krebs cycle) account for 23 of the 30 total ATP energy carrier molecules. It is worth noting that these ATP values are maximum values and in reality protons leak across the membrane causing somewhat lower values.

Photophosphorylation, which occurs when plants synthesize glucose during photosynthesis, also uses ATP synthase and a proton gradient to generate ATP. The process occurs across the thylakoid membrane when chlorophyll is energized by light and donates an excited electron to an electron transport chain.

Inhibitors

There are a few well-known toxins that affect the process of oxidative phosphorylation and can lead to breakdown of the chain:

1. Cyanide interrupts the electron transport chain in the inner membrane of the mitochondrion because it binds more strongly than oxygen to the Fe³⁺ (ferric iron ion) in cytochrome a₃, preventing this cytochrome from combining electrons with oxygen.
2. Oligomycin functions by inhibiting the ATP synthase protein and preventing it from generating ATP from the proton gradient.
3. CCCP (m-chloro-carbonylcyanide-phenylhydrazine) destroys the proton gradient by allowing the protons to flow out of the membrane. Without the gradient, the ATP synthase cannot function and ATP synthesis breaks down.
4. A detergent, or substance that destroys cellular membranes by breaking apart their lipid bilayers, will destroy the membrane used in the process and prevent a proton gradient.
5. Rotenone prevents the transfer of electrons from Fe-S centers in Complex I (most notably) to ubiquinone. The electrons entering into Complex I are those derived from NADH, and provide the bulk of the reducing potential to the electron transport chain.

For each of these toxins, a backup will cause everything before it to break down as well. For example, if oligomycin is added, protons cannot pass through. As a result, the H⁺ pumps are unable to pass protons through because the gradient becomes too strong for them to overcome. NADH and FADH₂ are then not oxidized and the citric acid cycle ceases to operate because there are no NAD⁺ and FAD coenzymes to be reduced.

Reactive oxygen species

Molecular oxygen is ideal as a terminal electron acceptor due to its high affinity for electrons. However, the reduction of molecular oxygen yields potentially harmful intermediates. The transfer of four electrons results in the production of water, which is not harmful. The transfer of one or two electrons produces superoxide anion and peroxide as shown below:

${\begin{matrix}\quad &{e^{-}}&\quad &{e^{-}}\\{{\mbox{O}}_{2}}&\longrightarrow &{\mbox{O}}_{2}^{\underline {\bullet }}&\longrightarrow &{\mbox{O}}_{2}^{2-}\\\quad &\quad &{\mbox{Superoxide}}&\quad &{\mbox{Peroxide}}\\\quad &\quad &{\mbox{anion}}&\quad &\quad \end{matrix}}$

These compounds and, particularly, their reaction products such as hydrogen peroxide are very harmful to many cellular components.

Due to the efficiency of the cytochrome c oxidase complex very few partly reduced intermediates are released. Inevitable small amounts of superoxide anion and peroxide are released though.Species which can be produced from these such as Hydrogen peroxide (H₂O₂) and the hydroxyl radical (•OH) are collectively referred to as reactive oxygen species or ROS. There are many enzymes whose task is to convert ROS into less reactive species. Chief among theses enzymes is superoxide dismutase. This enzyme converts superoxide radicals into molecular oxygen and hydrogen peroxide as shown below:

${\begin{matrix}\quad &\quad &\quad &{\mbox{Superoxide}}&\quad &\quad &\quad \\\quad &\quad &\quad &{\mbox{dismutase}}&\quad &\quad &\quad \\{\mbox{2O}}_{2}^{\underline {\bullet }}&+&{\mbox{2H}}^{+}&\longleftarrow \!\longrightarrow &{\mbox{O}}_{2}&+&2{\mbox{H}}_{2}{\mbox{O}}_{2}\\\end{matrix}}$

There are two varieties of the enzyme within eukaryotes. The hydrogen peroxide released from the cytochrome c oxidase complex and by superoxide dismutase is scavenged by the enzyme catalase. Catalase converts hydrogen peroxide into molecular oxygen and water as shown below:

${\begin{matrix}\quad &{\mbox{Catalase}}&\quad &\quad &\quad \\2{\mbox{H}}_{2}{\mbox{O}}_{2}&\longleftarrow \!\longrightarrow &{\mbox{O}}_{2}&+&2{\mbox{H}}_{2}{\mbox{O}}\\\end{matrix}}$

Superoxide dismutase and catalase are both remarkably efficient and carry out their reactions at or near the diffusion-limited rate.

Antioxidant vitamins C and E also serve to convert ROS to less harmful compounds. Vitamin E cannot exist in the mitochondrial matrix since it will not mix with water; it is hydrophobic. It instead exists in the inner membrane of the mitochondrion and protects the membrane from being harmed by ROS. Vitamin C is hydrophilic and carries out its reactions in the mitochondrial matrix.

External links

• Interactive Molecular model of succinate dehydrogenase
• Interactive Molecular model of Coenzyme Q - cytochrome c reductase
• Interactive Molecular model of cytochrome c oxidase

References

1. Leninger, D., Cox, M. Principles of Biochemistry 3rd Ed., Worth Publications, New York, NT., 2001.