User talk:Simpsons contributor/Oxidative phosphorylation

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[edit] The electron-transport chain

During the Krebs cycle NADH and FADH2 are produced. During oxidative phosphorylation these are used to reduce molecular oxygen to water. This occurs in a number of electron-transfer reactions through four protein complexes. The electrons begin at complex I and travel through to complex IV, where the reduction of molecular oxygen takes place.

[edit] Complex I

Complex I, also known as NADH-Q oxidoreductase, is the first protein in the electron-transport chain.

[edit] Complex II

Complex II, also known as succinate-Q reductase, is the second protein in the electron-transport chain.

[edit] Complex III

Complex III, also known as Q-cytochrome c oxidoreductase or simply cytochrome reductase, is the third protein in the electron-transport chain.

[edit] Complex IV

Complex IV, also known as cytochrome c oxidase, is the fourth and final protein in the electron transport chain.

[edit] ATP Synthase

[edit] 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{dismatase} & \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} & \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 sine 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.

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