Chemiosmosis and G3P shuttle

Mitochondrial_electron_transport_chain—Etc4.svg

Chemiosmosis:

The generation of ATP by the movement of H+ across a membrane during cellular respiration. ATP synthase allows protons to pass through the membrane and uses the kinetic energy to phosphorylate ADP, making ATP.

  • The membrane must be impermeable to ions so that a proton concentration gradient can be maintained.
  • Specific transporters allow movement of ions across the membrane.
  • Electron transport through the ETC creates a proton concentration gradient in which the cystolic side of the inner mitochondria membrane has a higher conc of protons.
  • ATP synthase catalyzes ADP phosphorylation driven by the movement of protons across the inner membrane from the cytosol to the matrix.
  •  NAD and FAD are carriers pass electrons to the ETC in the inner mitochondrial membrane, which in turn pass them to other proteins in the ETC. The energy available in the electrons is used to pump protons from the matrix across the inner mitochondrial membrane, storing energy in the form of a transmembrane electrochemical gradient.
  • The protons move back across the inner membrane through the enzyme ATP synthase.
  • The flow of protons back into the matrix of the mitochondrion via ATP synthase provides enough energy for ADP to combine with inorganic phosphate to form ATP.
  • The electrons and protons at the last pump in the ETC are taken up by oxygen to form water.

 

Evidence to support postulates: 

Postulates explain the effect of uncouplers which bind to the H+ and carries them across the membrane to the matrix, thereby affecting the proton conc gradient. The second postulate demonstrates that the medium becomes acidic when mitochondria under anaerobic conditions are given small amounts of oxygen. Acidifying the media can form ATP from ADP and Pi without the need for electron transfer. ATPase activity in damaged mitochondria acts in reverse using the proton gradient to synthesize ATP i.e an ATP synthase.

G3P Shuttle:

This is an alternative mechanism that leads to the reduction of Ubiquinone.

Glycerol-3-Phosphate is converted to Dihydroxyacetone phosphate (irreversible). While this is done, mitochondrial G3P Dehydrogenase reduces E-FAD to E-FADH2 which when oxidized back to E-FAD causes Q to be reduced to QH2 which enters into oxidative phosphorylation.  When Dihyoxyacetone is reconverted to G3P (reversible), NADH + H+ is used and oxidised to NAD+.

g3p

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