Introduction to Oxidation & Electron Transport Chain
- We require energy for various activities e.g. muscle contraction, nerve conduction, synthetic reactions, active transport etc.
- The ultimate source of energy is the food that we Consume.
- Carbohydrates, lipids and proteins present in food provide us energy.
- The complex molecules present in food are digested in the gastrointestinal tract to yield smaller molecules e.g. monosaccharides, fatty acids and amino acids
- These, in turn, are oxidised in various catabolic Pathways
- Their carbon atoms are oxidised to carbon dioxide while the hydrogen atoms are transferred to coenzymes e.g. NAD+, FMN, FAD.
- The reduced coenzymes transfer the hydrogen atoms to the mitochondrial respiratory chain wherein these are oxidised to water
- The energy released during this oxidation is used to phosphorylate adenosine diphosphate (ADP) to adenosine triphosphate (ATP)
High Energy Compounds
- The energy released during the oxidation of monosaccharides, fatty acids and amino acids may not be required immediately
- Therefore, there must be some way of storing the energy so that it may be readily available when needed
- The energy released during catabolism is captured in the form of a group of compounds known as “high-energy phosphates”
Synthesis of ATP (Electron Transport Chain)
- Oxidation coupled with phosphorylation of ADP is known as “oxidative phosphorylation”
- Oxidative phosphorylation, thus, is the mechanism by which the energy present in various nutrients is captured in an easily utilisable form
Substrate level phosphorylation
- Here energy from a high energy compound is directly transferred to nucleoside diphosphate to form a triphosphate without the help of ETC e.g.,
- Bisphospho glycerate kinase
- Pyruvate kinase
- Succinate thiokinase
Storage of High Energy Phosphates
- Phosphocreatine provides a high energy reservoir of ATP to regenerate ATP rapidly and catalyzed by creatine kinase
- ATP + Creatine –> Phosphocreatine + ADP + ΔG0
- 43.1 kJ/mol (-10.5 kcal/mol)
- The reaction is mitochondrial and of special significance in the myocardium which has a high energy requirement, about 6kg of ATP per day
ENZYMES CONCERNED WITH BIOLOGICAL OXIDATION
- The enzymes concerned with biological oxidation are oxidoreductases
- They can be sub-divided into:
- These enzymes remove hydrogen from a substrate and transfer it to oxygen forming water .
- e.g. cytochrome oxidase and tyrosinase
- The enzymes forming hydrogen peroxide are flavoproteins containing FMN or FAD
- e.g. L- amino acid oxidase (containing FMN) and xanthine oxidase (containing FAD)
- Examples of dehydrogenases are:
- Those containing NAD – Lactate dehydrogenase, malate dehydrogenase, isocitrate dehydrogenase, Glutamate dehydrogenase, Pyruvate dehydrogenase, alpha keto glutarate dehydrogenase etc
- b. Those containing NADP – Glucose-6- phosphate dehydrogenase, 6-phosphogluconate dehydrogenase etc
- c. Those containing flavin nucleotides – Succinate dehydrogenase, fatty acyl CoA dehydrogenase etc
- d. Those containing iron-porphyrin – . All the cytochrome accept cytochrome oxidase. Cytochrome a, cytochrome b, c,c1, P-450 etc
- These enzymes convert hydrogen peroxide into water
- These include:
- a. Peroxidases
- b. Catalase
- These enzymes incorporate oxygen into a substrate and the other oxygen is reduced to water.
- e.g.,homogentisate oxidase , tryptophan pyrrolase hydroxylases
Location of enzymes in mitochondria
Oxidation and Reduction
- Oxidation is defined as the loss of electrons and reduction as the gain in electrons.
- When a substance exists both in the reduced state and in the oxidized state, the pair is called a redox couple.
ORGANIZATION OF ELECTRON TRANSPORT CHAIN
- In the ETC, the electrons are transferred from NADH to a chain of electron carriers
- All the components of ETC are located in the inner membrane of mitochondria.
- There are four distinct multi-protein complexes; these are named as complex-I,II, III and IV. These are connected by two mobile carriers, co-enzyme Q and cytochrome C
ETC Complex – I (NADH-CoQ reductase)
- It contains a flavoprotein, consisting of FMN as a prosthetic group and FeS protein.
- NADH is the donor of electrons, FMN accepts them and gets reduced to FMNH2.
- 2e- and 1 hydrogen ion are transferred from NADH to FMN.
- The electrons from FMNH2 are then transffered to Fe-S.
- The electrons are then transferred to coenzymeQ.
- The energy released is 12kcal/mol.
- This is utilized to drive 4 protons out of the mitochondria.
Complex II or Succinate-Q-Reductase
- The electrons from FADH2 enter the ETC at the level of CoQ
- This step does not liberate enough energy to act as proton pump.
- The 3 major enzyme system that transfer their e- directly to ubiquinone from the FAD prosthetic group
- I.Succinate dehydrogenase
- II.Fatty acyl CoA dehydrogenase
- III.Mitochondrial glycerol phosphate dehydrogenase
- It accepts a pair of electrons from NADH or FADH2 through complex I and II.
- The Q cycle thus facilitates the switching from the two electron carrier ubiquinol to the single electron carrier cytochrome c
Complex III or Cytochrome Reductase
- This is a cluster of Fe-S proteins, cyt b and cyt c1, both contain heme prosthetic group.
- During this process of transfer of electron, the iron in heme group shuttles between Fe3+ and Fe2+ forms.
- The free energy change is -10kcal/mol and 4 protons are pumped out.
- It is a peripheral membrane protein containing one heme prosthetic group.
- It collects electrons from Complex III and delivers them to complex IV.
Complex IV or Cytochrome Oxidase
- It contains different proteins including cytochrome a and a3.
- 4 electrons are accepted from cytochrome c and passed on to molecular oxygen.
- 2 protons are pumped out to the inter membrane space.
- According to the chemiosmotic hypothesis proposed by Mitchell, the energy released during the transport of electrons in the respiratory chain is used to actively eject the hydrogen ions (H+) or protons from the matrix through the inner mitochondrial membrane which is otherwise impermeable to hydrogen ions.
- This ejection establishes an electrochemical gradient across the membrane.
- The concentration of hydrogen ions on the outer side becomes higher as compared to the inner side
- The outer side also becomes electropositive as compared to the inner side
- This electrochemical gradient increases upto a certain limit
- The complexes I, III and IV in the respiratory chain act as proton pumps ejecting hydrogen ions from the mitochondrial matrix to the intermembrane space
- The energy released during influx of protons is used to activate a membrane-bound enzyme, ATP synthase which converts ADP and Pi into ATP.
Summary of ATP Synthesis
Regulation of Oxidative Phosphorylation
- Oxidative phosphorylation in the respiratory chain results in consumption of oxygen.
- This is also known as tissue respiration.
- Under the usual physiological conditions when oxidisable substrates and oxygen are available, the rate of tissue respiration is regulated mainly by the concentration of ADP.
- Increased utilisation of ATP raises the concentration of ADP which increases tissue respiration resulting in increased conversion of ADP into ATP.
Inhibitors and Uncouplers of Oxidative Phosphorylation
- Site specific Inhibitors
- Inhibitors of Oxidative Phosphorylation
- b-i. oligomycin- inhibits the Fo
- b-ii. Ionophores (eg valinomycin)
- b-iii. Cyanide
- Uncouplers of Oxidative Phosphorylation
- i- 2,4-dinitrophenol
- ii- 2,4-dinitrocresol
- iii- chlorocarbonylcyanidephenyl hydrazone (CCCP)
- Thyroxin is also act as a physiological uncoupler
Thermogenin in brown adipose tissue
2,4 DINITROPHENOL POISONING
- Dinitrophenol is currently used industrially in the manufacture of dyes, explosives, herbicides, insecticides and lumber preservatives.
- DNP kills bacteria and fungi by uncoupling phosphorylation
- Unfortunately DNP has resurfaced as an illegal weight loss product.
- It radically increases consumption of oxygen and metabolic fuels and nearly all metabolic energy is wasted as heat.
- Cells die because of both excess temperature and lack of ATP
CYANIDE AND CARBON MONOXIDE ARE MITOCHONDRIAL POISONS
- Both cyanide and carbon monoxide bind to hemoglobin and inhibit oxygen transport.
- They also inhibit electron transport and production of ATP.
- Cells respond to cyanide or CO poisoning by switching to anaerobic metabolism, resulting in lactic acidosis and ultimate death, unless immediate measures are taken.
- CO poisoning is treated with oxygen.
- In both cyanide and CO poisoning, methylene blue can be administered: it alleviates the inhibition of complex IV by accepting electrons from complex III, allowing both complex I and complex III to pump protons, so that ATP can continue to be synthesized.
- Cyanide can also be converted to the relatively harmless thiocyanate ion by the administration of thiosulfate
- Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke like episodes (MELAS) is one of a group of conditions which is caused by defects in oxidative phosphorylation and characterized by defects in the process whereby NADH drives electrons along the mitochondrial respiratory chain complex and generates ATP.
- The electron transport system consists of electron carriers located in the innermitochondrial membrane
- Electron from four major flavoproteins feed electrons to ubiquinone
- Energy derived from the conductance of electrons is used by 3 complexes to pump protons and generates proton motive force
- This proton gradient is used for the synthesis of ATP by ATP synthase
- Numerous toxins can severely impair the electron transport system
- Some chronic diseases have metabolic links to dysregulation of oxidative phosphorylation
Other Biochemistry Notes
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