• Reducing equivalent NADH, FADH₂ generated during glycolysis and the link between glycolysis and Kreb’s cycle are used to synthesize ATP by a process called oxidative phosphorylation (OP).
• Oxidative phosphorylation involves two components-
  • Electron transport chain
  • ATP synthase.
• The flow of electrons from the reducing equivalence across the electron transport chain generates proton motive force (PMF).
• The energy stored in proton motive force is used to drive the synthesis of ATP.
• ATP synthase utilizes this proton motive force to drive the synthesis of ATP.

Electron transport chain:

• Electron transport chain consists of the series of electron carriers arranged asymmetrically in the membrane.
• The membrane may be either cytoplasmic membrane as in the case of bacteria or inner mitochondrial membrane as in case of eukaryotes.
• The electron carriers are sequentially arranged and get reduced as they accept electron from the previous carrier and oxidized as they pass electron to the succeeding carrier.
• The different electron carriers are:
  • NADH dehydrogenase
  • Flavoproteins (FMN and FAD)
  • Ubiquinone
  • Iron sulfur (Fe) center
  • Cytochrome

1. NADH dehydrogenase:

• Two types of NAD dependent dehydrogenase can feed electron transport chain.
• They are NADH and NADPH.
• NADPH is less common as it is involved in anabolic reactions (biosynthesis).
• NADH dehydrogenase removes two hydrogen atoms from the substrate and donates the hydride ion (H^–) to NAD⁺ forming NADH and H⁺ is released in the solution.
• NAD⁺ accepts two e^– and two protons from the substrate during catabolic reaction and transfers to the electron transport chain.
• NAD⁺ is then reduced to NADH+ H⁺.
• Reduced NADH+ H⁺ transfers its e^– and proton to FMN which in turn is reduced to FMNH₂.
  • AH₂+ NAD⁺   <——————–>A + NADH + H⁺
  • (Reduced substrate)                 (oxidized substrate)
  • NADH + H⁺ + FMN  <———–> FMNH₂+ NAD⁺

2. Flavoproteins:

• Flavoproteins are derived from Vitamin B₂ (Riboflavin).
• These are the protein containing FMN and FAD as the prosthetic group which may be covalently bound with the protein.
• They are capable of accepting electrons and protons but can only donate electrons.
• The protons are expelled outside the membrane.
• FMN accept electron and proton from NADH and get reduced to FMNH₂ which in turn channel only e^– through to ubiquinone.
• FAD is the component of succinate dehydrogenase complex.
• It accepts two electron and two protons from succinate and gets reduced to FADH₂, in the process succinate is converted to fumarate.
• FADH₂ channels its electron only to FeS center through ubiquinone.
• Succinate+ FAD ^{____________________}> Fumarate + FADH₂

3. Ubiquinone:

• Ubiquinone are omnipresent in nature.
• These are similar in structure and property with Vitamin K.
• In plants, these are found as plastoquinone and in bacteria, these are found as menaquinone.
• These are lipid soluble (hydrophobic) and can diffuse across the membrane and channel electrons between carriers.
• Ubiquinone can accept electrons as well as protons but transfer only electrons.
• They accept electron from complex 1 and 2.
• They can accept one e^– and get converted into semiquinone or two e^–s to from quinone.

4. FeS center:

• These are non-heme Fe (iron) containing proteins in which the Fe-atom is covalently bonded to Sulphur of cysteine present in the protein and to the free Sulphur atoms.
• Less commonly found FeS centers known as Reiske iron sulphur centers have iron bonded to Histidine residue of the proteins.
• There are different types of iron Sulphur center, simplest type consists of an iron atom, another type known as 2Fe-2S (Fe₂S₂) and the third one (most commonly found) is 4Fe-4S (Fe₄-S₄) and comprises the ferredoxin.
• FeS center consists of Fe-atoms which can interconnect between ferrous and ferric form as they accept and donate electrons respectively.
• They are capable of receiving and donating electrons only.
• They form the components of all four complexes.

5. Cytochromes:

• Cytochromes are the proteins with characteristic absorption of visible lights due to the presence of heme containing Fe as co-factor.
• There are three different types of cytochrome a, b and c.
• Cytochrome a and b are tightly but not covalently linked with their proteins whereas cytochrome c is covalently bonded with its protein through cysteine.
• Cytochrome ‘a’ has the maximum absorption spectra at 600nm.
• Cytochrome ‘b’ has maximum absorption spectra at 560nm and cytochrome ‘c’ has maximum absorption spectra at 550nm.
• Cytochromes are capable of accepting and transferring only one e^– at a time during which the Fe^– atoms interconvert between ferrous and ferric.
• Cytochrome- Fe²⁺ <————> Cytochrome- Fe³⁺ + e^–
• Cytochromes are arranged in the order cytochrome ‘b’, cytochrome c₁, cytochrome ‘c’ and cytochrome a/a₃.
• a/a₃ is also known as cytochrome oxidase.

Arrangement of five electron carriers in the form of four respiratory enzyme complex

• The five electrons carriers are arranged in the form of four complexes.
  • Complex I: NADH Quinone oxidoreductase complex (NADH to Quinone)
    Note: NADH——->FMN——> FeS—–> Q
  • Complex II: Succinate dehydrogenase complex (Succinate to Quinone)
    Note: Succinate——> FAD—–> FeS—-> Q
  • Complex III: cytochrome bc₁ (Ubiquinone to cytochrome c)
    Note: UQ₂——> cyt bc₁—->cyt c
  • Complex IV: Cytochrome oxidase (cytc to O₂)
    Note: cyt c—-> cyt a—–> cyt a₃—-> O₂

Complex I: NADH dehydrogenase complex

• This complex is also known as NADH dehydrogenase complex, consists of 42 different polypeptides, including FMN containing flavoprotein and at least six FeS centers.
• Complex I is ‘L’ shaped with its one arm in the membrane and another arm extending towards the matrix.
• During catabolic reaction, NAD⁺ is reduced to NADH+ H⁺ and this NADH + H⁺ feeds electrons and protons at the point of origin in the ETC.
• Both e^– and protons are transported to FMN which is then reduced to FMNH₂.
• FMNH₂ transfers only e^– to FeS center whereas protons are extruded outside the membrane (intermembrane space), in the process FMNH₂ is oxidized back to FMN.
• Electrons flow through FeS centers which alternate between reduced (Fe²⁺) and oxidized (Fe³⁺) froms.
• Electrons are finally transferred to ubiquinone, which along with protons obtained by the hydrolysis of water in the matrix site of the membrane is reduced to UQH₂.

Complex II: Succinate dehydrogenase complex.

• Complex II is also known as succinate dehydrogenase complex.
• Succinate dehydrogenase complex is located towards the matrix side of the membrane.
• Succinate is oxidized to fumarate as it transfers two e^–s and two protons to FAD.
• FAD is reduced to FADH₂.
• FAD transfers only electrons through FeS center to quinone.
• Quinone (Q) in presence of protons is reduced to QH₂.
• Complex II consists of covalently linked FAD containing flavoprotein and two FeS centers.

Complex III: Cytochrome bc1

• Ubiquinone are hydrophobic, lipid soluble molecules capable of diffusing across the membrane.
• Because of this property, ubiquinones can channel electrons between less soluble electron carriers.
• Electrons are channeled from complex I and complex II to cytochrome bc₁.
• The figure shows the stoichiometry for two ubiquinone (UQH₂).
• Ubiquinones undergo two rounds of oxidation, one towards the enzyme site on the inner membrane site of the membrane where two electrons are transferred across cyt c₁ to cyt c.
• Another oxidation occurs towards the site of membrane containing cyt b where again 2 electrons are passed to cyt bc and cyt b_H.
• During these two oxidation reactions, four protons are expelled outside the membrane and 2UQH₂ is oxidized to 2UQ.
• One of the UQ diffuse towards the matrix site of the membrane where it receives two electrons flowing through cytochrome b₁.
• This UQ along with two protons obtained from the hydrolysis of water in the matrix site of the membrane is reduced to UQH₂, thus completing the Q-cycle.

Complex IV: Cytochrome Oxidase

• It is also called as cytochrome oxidase.
• Cytochrome c undergoes oxidation in the side of the membrane facing the intermembrane space and O₂ is reduced in the matrix side of the membrane to H₂O.
• Complex IV consists of iron containing heme-a and heme-a₃.
• Along with iron atoms, cytochrome oxidase also consists of Cu A and Cu B.
• Cu A is closely but not intimately associated with heme ‘a’ and Cu B is intimately associated with heme a₃.
• Electrons from cytochrome c flows to Cu A and then to heme ‘a’ and then to heme a₃ and then to Cu B and then finally to Oxygen.
  • Cytochrome c —> Cu A —–> Heme a—–> heme a₃—->Cu B—> O₂
• The copper atoms interconvert between cuprous (reduced) and cupric (oxidized).
• Electrons from Cu B and heme a₃ is transferred to O₂ forming O^–-O^– bridge.
• Two more electrons are pass through O^–-O^– resulting in breakage of O^–-O^– bridge forming O₂^– and O^2-.
• Two protons are supplied from the matrix side forming OH^– and OH^–.
• Now, addition of two more proton from matrix side resulting in formation of two molecule of water (2H₂O).

ATP synthesis:

• Chemiosmotic theory given by Peter Mitchell (1961) in the widely accepted mechanism of ATP generation.
• According to this theory electron and proton channel into the membrane from the reducing equivalence flows through a series of electron carriers, electrons flow from NADH through FMN, Q, cytochrome and finally to O₂.
• However, proton as they flow through the membrane are extended at different position in the intermembrane space.
• The extension of protons creates a slight positivity/acidity to the outerside of membrane.
• Reduction of quinones and O₂ to water requires protons which are provided by the hydrolysis of water in the matrix side of the membrane.
• This results in accumulation of hydroxyl ion in the inner (matrix) side of membrane resulting in slight negativity/alkalinity in the inner side of the membrane.
• This creates a charge difference between outer side of the membrane, and inner side of membrane which energizes the membrane.
• This is electrochemical potential, and this potential along with the pH gradient generates the proton motive force (PMF).
• This proton motive force tends to drive the proteins through ATP synthase in to the inner side of the membrane, the consequence of which is ATP production.
• ATP synthase consists of two components, transmembrane ion conducting subunit called F_o and cytoplasmic multiprotein subunit called F₁ which is responsible for ATP production.
• F₁ catalyzes the reversible reaction in which ADP is phosphorylated to ATP.
  • ADP + P_i <————->ATP
• Proton motive force driven H⁺ through F_o causes the rotation of C-protein of the subunit.
• Rotation of c generates torque.
• This torque is transmitted through  gamma (γ) and epsilon (ε) subunit to β-subunit of F₁ resulting in its conformational change.
• This conformational change in β-subunit allows binding of ADP with inorganic phosphate (P_i).
• Binding of ADP and P_i results in production of ATP and β-subunit original conformation is regained.

Oxidative phosphorylation: Electron transport chain and ATP synthesis

The post Oxidative phosphorylation: Electron transport chain and ATP synthesis appeared first on Online Biology Notes.