Electron transport chain

Electron transport chain (ETC)

• It is the membrane associated to electron carrier system which has two basic functions;

1. To mediate the transfer of electrons from primary donor to terminal acceptor(may be O₂ in case of aerobic respiration)
2. To converse some of the energy released during electron transfer (in the form of proton motive force) for ATP synthesis. It is also called as the respiratory chain which is the sequence of the oxidation-reduction reaction. These reduce reactions are carried out by electron carrier and electron carrier enzymes.
3. As energy flows through their chain, much of the free energy is conserved in the form of ATP and the process is oxidative phosphorylation.

• Site of occurrence

• Mitochondria, chloroplast (eukaryotes)
• Cell membrane (prokaryotes)
• During photosynthetic, ATP is produced by the process of photophosphorylation.

1. Oxidation-reduction reactions:

It is involved in energy conversion in the cell.

• Chemically oxidation is defined as removal of electrons from substance and reduction is the addition of electrons to its substance.
• Biochemically,oxidation and reduction frequently involve the transfer of both electrons and protons.

Oxidizing agent (oxidant)

• Absorbs e^- and becomes reduced

Eg; Fe⁺³+e^-→Fe²⁺

• Reducing agent (reductant)

Donates e^- and become oxidized

Eg; 2H++2e-→H₂

For an oxidation, a subsequently reduction most also occur i.e a pair of substances (redox couple) is involved in which one is reduced and other is oxidized.Each redox couple is called as oxidation-reduction system.

Standard oxidation-reduction potential of an oxidation-reduction system or Electromotive potential

• One O-R system may tend to absorb electrons from another O-R system.
• The tendency to absorb electrons is expressed by standard oxidation-reduction potential.Which is measured electrically under standardize condition
• Its unit is express in volt.
• More positive the potential,the greater the oxidizing ability of the system.
• As electrons from electron donor at the top of the electron tower move through the electron transport chain, the electrons are caught by the electron acceptor at various level. The difference in reduction potential between redox pair is expressed. and use to derive ATP synthesis by oxidative phosphorylation.

• Components of ETC

• Electron carrier enzyme
• Protein electron carrier
• Non-protein electron carrier

Oxidative phosphorylation begins with the entry of electrons into respiratory chain. Most of these electrons arise from the action of dehydrogenase that collects electrons from catabolic pathways and funnels them into universal electron acceptors like nicotinamide nucleotide (NAD+) (NADP) flavin nucleotide (FMN) (FAD)^.

1. Nicotinamide nucleotide linked dehydrogenases: Most electron entering the respiratory chain arise from the action of dehydrogenase that uses co-enzymes (NAD⁺or NADP) as a group they are designated's NAD-linked dehydrogenases.
2. Flavoprotein:

• These are proteins containing derivatives of riboflavin. The flavin portion which is bounded to be a protein is the prosthetic group i.e alternately reduced as it expects electrons and oxidized when electrons are passed on to another component of ETC.
• Two flavins are known i.e FAM and FAD
• It participates in either one or two electron transfer and plays the intermediate role in which two electrons are donated and only one electron is accepted.

4.Co-enzymes:

• It is a hydrophobic quinone with long isoprenoid side chain and the closely related compounds plastoquinone (in the chloroplast of green plants) menaquinone of bacteria play the analogous role in membrane-associated electron transfer system.
• It can accept one or two electrons like flavoproteins and act junction between two electrons donor and one electron acceptor.

5.Iron-containing protein

• Fe-s protein: It is associated with electron transfer within various places of electron transfer system. It is also called as no-one protein because iron in the proteins is not in a cytochrome. Eg;ferredoxin. (found in photosynthetic)
• Cytochrome b,c₁,c,a,a₃: These are iron red or brown protein that in a sequence to carry electrons from ubiquinone to molecular oxygen. They undergo oxidation and reduction through loss or gain of the single electron. The different cytochromes are designed by the letters cyt a,b,c as distinguished by light absorption spectra and depending upon type.

Sequential transfer of electrons Transfer chain

When a pair of electrons or hydrogen atom containing electrons from an oxidized able substrate is coupled with the reduction of ultimate electron acceptor like oxygen. There is large free energy change,some of which are conserved in a form of ATP at several steps in the chain.

• The first reaction is the oxidation of NADH where two electrons are transformed from NADH to FMN or FAD reducing them to FMN, FAD or FADH₂.
• electron are transferred from FMNH₂ moiety of NADH dehydrogenase to coenzyme Q
• The high potential electron of FADH₂ from Krebs cycle is transformed directly to Co Q. Similarly, glycerol phosphate dehydrogenase and fatty acyl Co a dehydrogenase also transfers their electrons to Co.Q
• The iron atom of cytochromes is one electron carrier in contrast to NADH flavoprotein and Co Q. Which are two electron carriers, therefore a molecule of reduced Co. Q.
• There are five cytochromes between Co Q and oxygen in the electron transfer chain.
• Cytochrome and cyt a₃ are the terminal members of respiration chain and it exists as a complex called cytochrome oxidase.

Electron carriers junction in multienzyme complex for generation of proton motive force

Electron carrier of the respiratory chain is organized into membrane embedded supramolecular complex i.e integral membrane proteins that are typically transmembrane proteins.Electron associated transmembrane proteins are categorized into four complexes.

• Complex I

1. NADH-ubiquinone oxidoreductase or NADH Dehydrogenase.
2. In includes flavo protein and iron sulphur centre.
3. It catalyses the transfer of two hydrogens to the outside of the cell resulting in oxidation of NADH₂ to NAD

Complex II

1. It consists of succinate dehydrogenase complex and its iron sulphur centre. It bypasses the steps in complex I.
2. It feeds the electrons and protons from FADH₂, FADH, FMNH₂, FMN directly to Co. Q

Complex III

1. It is also called as ubiquinone Cyt C₁ oxidoreductase. It consists of Cyt b, Cyt C₁, and its iron sulphur centre .It receives electrons from reducing CoQH₂.
2. Its major function is to transfer electrons to cytochrome C₁ and transport two protons out of the cell.It separates the portion from entering the electron transport chain by interacting with quinone pool to pump additional proton out of the cell.

Complex IV

1. It is called as cytochrome oxidase and consist of Cyt a,Cyt a₃ and Fe-s centre.The complex IV reduces oxygen to water in aerobic respiration.In anaerobic respiration, the terminal electron acceptor is two than oxygen.
2. It pumps two portions for every two electrons consume in the reaction.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.