Chemiosmosis

The chemiosmotic coupling, as is the substrate chain, a route for the synthesis of ATP from ADP Pi in living cells. For this, the osmotic energy of a proton or Natriumgradienten is used. This coupling is referred to as chemiosmotic hypothesis, also known as Mitchell hypothesis. It was first erected in 1961 by Peter D. Mitchell. 1978 Mitchell was awarded for the discovery of the chemiosmotic coupling with the Nobel Prize for Chemistry.

Basic principle

By the activity of proton pump different pH values ​​can be established on both sides of a biomembrane. But it can also sodium ions are translocated. This process requires energy, since the transport is against a concentration gradient and the membrane is energized by ( ΔΨ ). The transmembrane protein ATP synthase utilizes the return transport of protons or sodium ions along this concentration gradient for the synthesis of cellular energy carrier ATP from ADP.

The energy for the construction of the gradient is provided either by the difference between the redox potentials for the oxidation of high-energy substances ( oxidative phosphorylation ) or light energy ( photophosphorylation ).

Oxidative phosphorylation

In the respiratory chain of mitochondria several protein complexes can act as a proton transporter: These are in addition to NADH dehydrogenase (complex I), the cytochrome -c reductase ( complex III ) (via the Q- cycle), and cytochrome c oxidase ( complex IV ). The energy comes from the high redox potential of NADH or FADH2 to oxygen. A gain of the gradient is performed by consumption of protons in the formation of water from oxygen and protons through the cytochrome c oxidase.

Photophosphorylation

In the oxygenic photosynthesis of green plants and cyanobacteria the cytochrome b6f complex acts ( Cyt b6f ) of the redox chain as a proton pump.

  • The energy comes from the first redox potential of the electrons that are transported through the redox chain acyclic in ATP production from the Photosystem II (PS II) to the photosystem I (PS I). The energy of the electrons comes from the excitation by the light in photosystem II
  • In cyclic phosphorylation electrons are excited in photosystem I by light and transmitted via ferredoxin back to the cytochrome b6.

The proton gradient is enhanced by that takes place on the side of thylakoid interior the cleavage of water to oxygen and protons, and thus the proton concentration is further increased. On the side of the stroma NADPH is formed by consumption of protons.

Also in the anoxygenic photosynthesis a proton gradient is built up by light energy through a photosystem. Finally (eg Halobacterium salinarum ) use the light energy through bacteriorhodopsin directly to build a proton gradient also Haloarchaea.

Swell

  • Biological process
  • Biochemical reaction
  • Pathway
Gene Ontology Substrate-level phosphorylation Adenosine triphosphate Adenosine diphosphate ATP-Synthase Luminous energy Periplasmic space Cytoplasm Intermembrane space Q cycle Digital Object Identifier David Keilin
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