Electron Transport Chain (ETC) (Terminal oxidation) - Plant Respiration

Electron Transport Chain (ETC) (Terminal oxidation)

During glycolysis, link reaction and Krebs cycle the respiratory substrates are oxidised at several steps and as a result many reduced coenzymes NADH + H⁺ and FADH₂ are produced. These reduced coenzymes are transported to inner membrane of mitochondria and are converted back to their oxidised forms produce electrons and protons. In mitochondria, the inner membrane is folded in the form of finger projections towards the matrix called cristae. In cristae many oxysomes (F₁ particles) are present which have electron transport carriers are present. According to Peter Mitchell’s Chemiosmotic theory this electron transport is coupled to ATP synthesis. Electron and hydrogen(proton) transport takes place across four multiprotein complexes(I-IV). They are

1.     Complex-I (NADH dehydrogenase).

It contains a flavoprotein(FMN) and associated with non-heme iron Sulphur protein (Fe-S). This complex is responsible for passing electrons and protons from mitochondrial NADH (Internal) to Ubiquinone(UQ).

In plants, an additional NADH dehydrogenase (External) complex is present on the outer surface of inner membrane of mitochondria which can oxidise cytosolic NADH + H⁺.

Ubiquinone (UQ) or Coenzyme Quinone(Co Q) is a small, lipid soluble electron, proton carrier located within the inner membrane of mitochondria.

2.           Complex-II (Succinic dehydrogenase) It contains FAD flavoprotein is associated with non-heme iron Sulphur (Fe-S) protein. This complex receives electrons and protons from succinate in Krebs cycle and is converted into fumarate and passes to ubiquinone.

Succinate + UQ → Fumarate + UQH₂

3.           Complex-III (Cytochrome bc₁ com-plex) This complex oxidises reduced ubi-quinone (ubiquinol) and transfers the elec-trons through Cytochrome bc₁ Complex (Iron Sulphur center bc₁ complex) to cy-tochrome c. Cytochrome c is a small pro-tein attached to the outer surface of inner membrane and act as a mobile carrier to transfer electrons between complex III to complex IV.

4. Complex IV (Cytochome c oxidase)

This complex contains two copper centers (A and B) and cytochromes a and a_3. Complex IV is the terminal oxidase and brings about the reduction of 1/2 O₂ to H₂O.Two protons are needed to form a molecule of H₂O (terminal oxidation).

The transfer of electrons from reduced coenzyme NADH to oxygen via complexes I to IV is coupled to the synthesis of ATP from ADP and inorganic phosphate (Pi) which is called Oxidative phosphorylation. The F₀F₁-ATP synthase (also called complex V) consists of F₀ and F₁ . F₁ converts ADP and Pi to ATP and is attached to the matrix side of the inner membrane. F ₀ is present in inner membrane and acts as a channel through which protons come into matrix.

Oxidation of one molecule of NADH + H⁺ gives rise to 3 molecules of ATP and oxidation of one molecule FADH₂ produces 2 molecules of ATP within a mitochondrion. But cytoplasmic NADH + H⁺ yields only two ATPs through external NADH dehydrogenase. Therefore, two reduced coenzyme (NADH + H⁺) molecules from glycolysis being extra mitochondrial will yield 2 x 2 = 4 ATP molecules instead of 6 ATPs (Figure 14.10) . The Mechanism of mitochondrial ATP synthesis is based on Chemiosmotic hypothesis. According to this theory electron carriers present in the inner mitochondrial membrane allow for the transfer of protons (H⁺). For the production of single ATP, 3 protons (H+) are needed. The terminal oxidation of external NADH bypasses the first phosphorylation site and hence only two ATP molecules are produced per external NADH oxidised through mitochondrial electron transport chain. However, in those animal tissues in which malate shuttle mechanism is present, the oxidation of external NADH will yield almost 3 ATP molecules.

Abnormal rise in respiratory rate of ripening in fruits is called Climacteric. Examples are apple, banana, mango, papaya, pear.

Complete oxidation of a glucose molecule in aerobic respiration results in the net gain of 36 ATP molecules in plants as shown in table 14.2. Since huge amount of energy is generated in mitochondria in the form of ATP molecules they are called ‘power house of the cell’. In the case of aerobic prokaryotes due to lack of mitochondria each molecule of glucose produces 38 ATP molecules.

Recent view

When the cost of transport of ATPs from matrix into the cytosol is considered, the number will be 2.5 ATPs for each NADH + H⁺ and 1.5 ATPs for each FADH₂ oxidised during electron transport system. Therefore, in plant cells net yield of 30 ATP molecules for complete aerobic oxidation of one molecule of glucose. But in those animal cells (showing malate shuttle mechanism) net yield will be 32 ATP molecules.

Electron transport chain inhibitors

1.     2,4 DNP (Dinitrophenol) - It prevents synthesis of ATP from ADP, as it directs electrons from Co Q to O₂

2.     Cyanide - It prevents flow of electrons from Cytochrome a₃ to O₂

3.     Rotenone - It prevents flow of electrons from NADH + H+/FADH₂ to Co Q

4.     Oligomycin – It inhibits oxidative phosphorylation