Electron Transport system (ETS) and Oxidative Phosphorylation
The electron transport "chain" or respiratory chain is a series of electron carriers in the membrane of the mitochondria. It is present in the inner mitochondrial membrane. Through a series of reactions, the "high energy" electrons are passed to oxygen. In the process, a gradient is formed, and ultimately ATP is produced. Electron transport requires oxygen directly. Electron transport takes place on cristae of mitochondria. Electrons are transferred through cytochromes (Cytbcyt c1, c2, a, a3) and finally reach molecular O2. Enzyme useful for terminal oxidation is cytochrome oxidase or terminal oxidase. Both cytochrome a and a3 form a system called cytochrome oxidase. Copper is also present in Cyt a3, in addition to iron. Synthesis of ATP during oxidation is called oxidative phosphorylation. 3 ATP and 1 H2O are flavoproteins, between Cyt b and Cyt c1, between Cyt a and a3. Chain level phosphorylation takes place in oxysomes.
The following steps in the respiratory process are to release and utilize the energy stored in NADH + H+ and FADH2. This is accomplished when they are oxidized through the electron transport system and the electron are passed on to O2 resulting in the formation of H2O.
Chemiosmotic mechanism of ATP synthesis has been postulated by Peter Mitchell in 1961. Electrons from NADH produced in the mitochondrial matrix during citric acid cycle are oxidized by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone located within the inner membrane. Ubiquinone also receives reducing equivalents via FADH2 (complex II) that is generated during oxidation os succinate in the citric acid cycle.
The reduced ubiquinone (ubiquino I) is then oxidized with the transrer of electrons to cytochrome c via cytochrome bc1 complex (complex III). Cytochrome c is a small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer of electrons between complex III and IV. Complex IV refers to cytochrome c oxidase complex containing cytochromes a and a3, and two copper centers. When the electrons pass from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate.
Glucose is the favoured substrate for respiration. All carbohydrates are usually first converted into glucose before they are used for respiration. Other substrates can also be respired, as has been mentioned earlier, but then they do not enter the respiratory pathway at the first step.
Fats would need to be broken down into glycerol and fatty acids first. If fatty acids were to be respired they would first be degraded to acetyl CoA and enter the pathway. Glycerol would enter the pathway after being converted to PGAL. The proteins would be degraded by proteases and the individual amino acids (after deamination) depending on their structure would enter the pathway at some stage within the Krebs' cycle or even as pyruvate or acetyl CoA.
Let us now look at another aspect of respiration. As you know, during aerobic respiration, O2 is consumed and CO2 is released. The ratio of the volume of CO2 evolved to the volume of O2 consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio.