Oxidative phosphorylation is the synthesis of energy-rich ATP molecules with the help of energy liberated during oxidation of reduced coenzymes ( i.e.,NADH2 and FADH2), produced in respiration. The synthesis of ATP requires an enzyme i.e., ATP synthase. The enzyme ATP synthase is considered to be the 5th complex of electron transport chain. ATP synthase is present in the F1 or headpiece of F0– F1 or elementary particles. The particles are present in the inner mitochondrial membrane.
Mechanism of oxidative phosphorylation in mitochondria
ATP synthase becomes active in ATP formation only when there is a proton gradient having higher concentration of H+ or protons on the F0 side as compared to F1 side. Pushing of protons with the help of energy liberated by passage of electrons from one carrier to another results in the increase in concentration of protons in the outer chamber or outer surface of inner mitochondrial membrane.
Transport of electrons from NADH2 over electron transport chain helps in pushing 3 pairs of proton to the outer chamber while 2 pairs of protons are sent outwardly during electron flow from FADH2 (as FADH2 donates its electron further down to the electron transport chain).
There is a higher proton concentration in the outer chamber . As a result, the protons pass inwardly into matrix or inner chamber through the inner membrane. The inner membrane possesses special proton channels in the region of F0 (base) of the F0- F1 particles. The flow of protons through the F0 channel thus induces the F1 particle to function as ATP synthase. The energy of proton gradient is used in attaching a phosphate radical to ATP by high-energy bond. As a result, it produces ATP. Oxidation of one molecule of NADH2 thus produces 3 ATP molecules while a similar oxidation of FADH2 forms only 2 ATP molecules.
Two ATP molecules are produced during glycolysis and 2 ATP molecules are produced during double Krebs cycle. Glycolysis also produces 2 molecules of NADH2. Its reducing power is transferred to mitochondria for ATP synthesis. For this, a shuttle system operates at the inner mitochondrial membrane.
In aerobic respiration, complete oxidation of one glucose molecule produces 38 ATP molecules. But the number of ATP molecules so produced may vary depending upon the mode of entry of NADH2 in mitochondria. The two molecules of NADH2 formed in glycolysis cannot directly enter the membranes of mitochondria. These enter into the mitochondria through a shuttle or indirect pathway.
The mitochondrial shuttles are systems used to transport reducing agents across the inner mitochondrial membrane. NADH cannot cross the membrane, but as a reducing agent it can definitely reduce another molecule that can pass or cross the membrane. As result of which its electrons can easily reach the electron transport chain.
The two main shuttle systems in humans are-Glycerol-phosphate shuttle and the other is malate -aspartate shuttle. In humans Glycerol-phosphate shuttle is primarily present in brown adipose tissue, kidneys, back of neck. The malate -aspartate shuttle is present in rest of the body.
This mechanism is present in animal tissues such as brown adipose tissue, kidneys, back of neck, insect flight muscle, white muscle and liver. It involves the reduction of dihydroacetone phosphate to glycerol-3-phosphate by cytoplasmic NADH. The process also requires an enzyme glycerol-3-phosphate dehydrogenase. Inside the mitochondrion oxidation of glycerol-3-phosphate takes place. As a result it forms dihydroacetone phosphate. FAD accepts the electrons from NADH2 . Reduced FAD (i.e., FADH2) yields only 2 ATP molecules through ETS. Dihydroacetone phosphate thus returns back to the cytoplasmic matrix or cytosol. As such, in this shuttle only 36 ATP molecules are produced. Such shuttle occurs in most of the eukaryotic cell.
This shuttle occurs in animal tissues such as heart muscle, liver etc. In this mechanism cytoplasmic NADH reduces oxalo acetic acid to malic acid which is carried across the inner mitochondrial membrane by a specific malate ketoglutarate transporter. As a result , oxidation of malate or malic acid to oxaloacetic acid takes place inside the mitochondrion.
In this shuttle, malate accepts electrons from NADH2 . This NADH enters the electron transport chain in order to yield ATP molecules. Each molecule of NADH2 yields 3 ATP molecules and as such, complete oxidation of one molecule of glucose would yield 38 ATP molecules.
The shuttle continuously transfers electron from cytoplasm to mitochondria and does not allow the accumulation of hydrogen molecules in the cytoplasm.
Uncouplers of Oxidative phosphorylation
Uncouplers of Oxidative phosphorylation in mitochondria inhibit the coupling between the electron transport and phosphorylation reactions and thus inhibit ATP synthesis without affecting the respiratory chain and ATP synthase. Therefore, blocking oxidative phosphorylation effectively decreases ATP concentrations in the cell.
Uncoupling agents are compounds which dissociate the synthesis of ATP from the transport of electrons through the cytochrome system. The electron transport continues to function, leading to oxygen consumption, but it inhibits the phosphorylation of ADP . Following are few uncoupling agents or inhibitors:
- CCCP and 2,4-Dinitrophenol: It disrupts the proton gradient by carrying protons across a membrane. This ionophore (i.e., ion-carrier) uncouples proton pumping from ATP synthesis because it carries protons across the membrane.
- Oligomycin: It blocks the flow of protons through the F0 subunit . Thus inhibiting ATP synthase.
- Rotenone: It blocks the ubiquinone binding site thus preventing the transfer of electrons from complex-I to ubiquinone
- Malonate and oxaloacetate: These are the inhibitors of succinate dehydrogenase (i.e., complex II).
Comparision between Oxidative phosphorylation and Photophosphorylation
Following are the differences between oxidative phosphorylation and photophosphorylation
|1. The process occurs during respiration.||1. It occurs during photosynthesis.|
|2. It occurs within mitochondria.||2. It occurs in the chloroplast.|
|3. No pigment system.||3. Pigment systems(i.e., PSI and PSII) are present.|
|4. Synthesis of ATP takes place in the F1 particles present on the cristae in the inner mitochondrial membrane.||4. Synthesis of ATP takes place on the lamellae of the chloroplast.|
|5. Oxidative phosphorylation requires molecular oxygen for terminal oxidation.||5. Photophosphorylation does not require oxygen.|
|6. Phosphorylation occurs with electron transport system.||6. Synthesis of ATP takes place during cyclic and non-cyclic electron transport.|
|7. ADP and inorganic phosphate produces ATP by utilizing energy released during electron transport.||7. Sunlight is the external energy source for photophosphorylation|
|8. The ATP molecules produced during oxidative phosphorylation are released into the cytoplasm and this energy molecules are used to carry out various metabolic reactions of the cell.||8. The ATP molecules produced during photophosphorylation are used to fix CO2 to carbohydrates in dark reaction.|