Electron transport

Peter Mitchell theorized that the generation of ATP only occurred because Bacteria and mitochondria could pump protons across a membrane. These primary pumps lead to generation of a charge across the membrane known as the proton motive force. As the protons try to move back across the membrane the energy of their movement can be harnessed in a number of ways. The most important use of the PMF in several aerobic organisms is the generation of ATP normally using the enzyme f1f0 ATPase. This ATP-generating enzyme is found in the cytoplasmic membrane of Bacteria and the inner membrane of the mitochondria and in the absence of the PMF actually cleaves ATP into ADP plus phosphate. However, when PMF is applied, the ATPase works essentially in reverse, generating ATP from ADP and phosphate. It is thus known as a secondary pump. For all this to happen efficiently the organism or organelle must conform in several ways to the Mitchell hypotheses that are as follows:

? Protons are pumped across mitochondrial and bacterial membranes in like a way as to generate an electric potential across the membrane. A membrane-bound enzyme (ATPase) couples synthesis of ATP to the flow of protons down the electric potential gradient.

? Solutes can accumulate against a concentration gradient through the coupling of proton flow to the movement of the solute through a transmembrane protein. These cotransporters may act as symports, antiports or uniports.

? The flow of protons through the flagellar transmembrane proteins rotates components of the flagellum and allows a prokaryote to move.
There are five types of component molecule.

1.Enzymes that catalyze transfer of hydrogen atoms from reduced NAD+ to flavoproteins (NADH dehydrogenases).

2.Flavoproteins. Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The flavins are reduced through accepting a hydrogen atom from NADH and oxidized by losing an electron.

3.Electron carriers’ cytochromes.Cytochromes are porphyrin-containing proteins each of that can be reduced or oxidized by the loss of a single electron:

                                         cytochrome-Fe²⁺ --> cytochrome-Fe³⁺ + e-

4.Iron-sulfur proteins. These are carriers of electrons with a range of reduction potentials.

5.Quinones. These are lipid-soluble carriers that can diffuse through membranes carrying electrons from iron-sulfur proteins to cytochromes.Protons for the final reduction in the transfer chain are supplied through the disassociation of water providing the build up of hydroxyl ions on the inside of the membrane.

Protons flow back into the mitochondrial matrix or bacterial cell through the enzyme ATP synthase, driving ATP synthesis. This enzyme is in two parts one localized on the Bacterial cytoplasmic or mitochondrial matrix side and the other which spans the membrane to the outside of the bacterial cell or the intermembrane space of the mitochondrion.The rate of oxidative phosphorylation is set through the availability of ADP; electrons only flow down the chain when ATP is needed. When there are high levels of ATP and energy-richcompounds like NADH+ and FADH, accumulation of citric acid inhibits the citric acid.