Oxidative Phosphorylation

Oxidative Phosphorylation – ATP Synthesis

Oxidative phosphorylation occurs in mitochondria here transfer of electrons is coupled with ATP formation from ADP and Pi. It is driven by the strong thermodynamics tendency of O₂ to become reduced, therefore, called oxidative phosphorylation.

Oxidative phosphorylation accounts for only a small fraction of ATP generated in living cells of higher plants. The number of ATP molecules synthesized depends upon the nature of electron donor. Electrons derived from oxidation of NADH₂ molecules produced in the matrix results in generation of three ATP molecules. FADH₂ enters the respiratory chain at a different point then NADH₂, therefore only two molecules of ATP are generated for each molecules of succinic acid oxidized to fumaric acid in the respiratory chain. The ATP generation takes place at three sites that exist along electron transport chain. The currently accepted mechanism of mitochondrial ATP synthesis is based on the chemiosmotic hypothesis, proposed by Peter Mitchell.

Elementary Particles or Fo-F1 ATP Synthase Complex

The ATP synthesis takes place by a protein complex associated with the inner membrane, the Fo-F1 ATP synthase. It consists of two major components Fi and F. Fi is a peripheral membrane protein complex that is composed of at least five different sub-units and contains the catalytic site for converting ADP and Pi to ATP. This complex is attached to the matrix side of the inner membrane. F_o is an integral protein complex that consists of at least three different polypeptides that form channel through which protons are able to cross the inner membrane. The passage of proton through the channel activates the ATP synthase to synthesize ATP and simultaneously dissipates the proton electrochemical gradient.

Energy Balance of Respiration – ATP Yield Per Molecule of Glucose

Complete oxidation of glucose molecule through glycolysis. Conversion of pyruvic acid to acetyl Co A and Krebs cycle leads to net formation of four molecules of ATP by substrate level phosphorylation (two during glycolysis and two in Krebs cycle); two molecules of NADH₂ in the cytosol (during glycoysis), eight molecules of NADH₂ and two molecules of FADH₂ in the mitochondrial matrix.

1. The oxidation of 8 NADH₂ molecules produced in mitochondrial matrix lead to generation of 24 ATP molecules, i.e., three per NADH₂
2. Four molecules of ATP are generated in the oxidation of succinic acid fumaric acid via FADH₂. These make the total 28 ATP molecules.
3. Addition of two ATP molecules formed during substrate level phosphorylation in Krebs cycle via GTP brings the total ATP synthesized to 30.
4. Two ATP molecules are generated during substrate level phosphorylation in glycolysis and four from oxidation of NADH₂ produced during glycolysis. Addition these six ATP molecules brings the total ATP synthesis to 36 molecules.

Thus, there is a total yield of 36 ATP molecules per glucose molecule completely oxidized to CO₂ and water.

In the absence of oxygen, the Krebs cycle and electron transport chain cannot function, therefore oxidation of NADH₂ stops and supply of NAD⁺ necessary for dehydrogenase reactions is limited. So, continued operation of glycolysis stops. To overcome this problem plants (especially fungi) and other organisms metabolize pyruvates by a process known as fermentation.

Fermentation

Fermentation is a sequential series of reactions that occurs in the absence of oxygen. The pyruvate is converted to ethanol and CO₂ or to lactic or other organic acids depending the organisms are involved. Under extreme low O₂ concentrations, for example in plants growing in waterlogged or flooded conditions, the plant tissues are forced to carry out fermentative metabolism.

Fermentation liberates a fraction of energy available in each molecule of glucose, i.e., approximately 53 Kcal per mole. There is net gain of two ATP molecules during fermentation.

Types of Fermentations

The fermentation may be: Lactic Acid Fermentation Or Alcoholic Fermentation

Lactic Acid Fermentation

This type of fermentation is common to mammalian muscle as well as plants (fungi). These organisms use NADH₂ in the presence of enzyme lactate dehydrogenase to reduce pyruvate to lactate. NAD⁺ is regenerated during this process. Carbon dioxide is not released during lactic acid fermentation.

Lactate dehydrogenase

Pyruvic Acid + NADH₂ ————————————————— Lactic Acid

Alcoholic Fermentation

This type of fermentation is more widely known to occur in brewer’s yeast but also common in plants. The pyruvate is acted upon by two enzymes, pyruvate decarboxylase and alcohol dehydrogenase producing ethanol and CO₂. The NADH₂ is oxidized to NAD⁺.

Pyruvate decarboxylase

Pyruvic Acid ————————————————— Acetaldehyde + CO₂

Alcohol dehydrogenase

Acetaldehyde + NADH ————————————————— Ethyl alcohol + NAD⁺

Ethanol is thought to be a less harmful end product of fermentation than lactic acid, because accumulation of lactic acid promotes acidification of the cytosol.