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The majority of the energy conserved during catabolism reactions occurs near the end of the metabolic series of reactions in the electron transport chain. The electron transport or respiratory chain gets its name from the fact electrons are transported to meet up with oxygen from respiration at the end of the chain.
The overall electron chain transport reaction is:
2 H+ + 2 e- + 1/2 O2 → H2O + energy
Notice that 2 hydrogen ions, 2 electrons, and an oxygen molecule react to form as a product water with energy released is an exothermic reaction. This relatively straight forward reaction actually requires eight or more steps. The energy released is coupled with the formation of three ATP molecules per every use of the electron transport chain.
The electron transport chain is initiated by the reaction of an organic metabolite (intermediate in metabolic reactions) with the coenzyme NAD+ (nicotinamide adenine dinucleotide). This is an oxidation reaction where 2 hydrogen atoms (or 2 hydrogen ions and 2 electrons) are removed from the organic metabolite. (The organic metabolites are usually from the citric acid cycle and the oxidation of fatty acids--details in following pages.) The reaction can be represented simply where M = any metabolite.
MH2 + NAD+ → NADH + H+ + M: + energy
One hydrogen is removed with 2 electrons as a hydride ion (H-) while the other is removed as the positive ion (H+). Usually the metabolite is some type of alcohol which is oxidized to a ketone. NAD+ is a coenzyme containing the B-vitamin, nicotinamide, shown on a previous page.
The purpose of the other seven steps in the electron transport chain is threefold:
Once the NADH has been made from a metabolite in the citric acid cycle inside of the mitochondria, it interacts with the first complex 1 enzyme, known as NADH reductase. This complex 1 contains a coenzyme flavin mononucleotide (FMN) which is similar to FAD.
The sequence of events is that the NADH, plus another hydrogen ion enter the enzyme complex and pass along the 2 hydrogen ions, ultimately to an interspace in the mitochondria. These hydrogen ions, acting as a pump, are utilized by ATP synthetase to produce an ATP for every two hydrogen ions produced. Three complexes (1, 3, 4) act in this manner to produce 2 hydrogen ions each, and thus will produce 3 ATP for every use of the complete electron transport chain.
In addition, NADH passes along 2 electrons to first FMN, then to an iron-sulfur protein (FeS), and finally to coenzyme Q. The net effect of these reactions are to regenerate coenzyme NAD+. This regeneration of reactants occurs in many of the reactions so that a cycling effect occurs. The NAD+ is ready to react further with metabolites in the citric acid cycle. Coenzyme Q, which also picks up an additional 2 hydrogen ions to make CoQH2, is soluble in the lipid membrane and can move through the membrane to come into contact with enzyme complex 3.
In summary, the very first enzyme complex in the electron transport chain is coupled with the formation of ATP. The coupled reaction may be written as:
MH2 + NAD+ → NADH + H+ + M + energy
ADP + P + energy → ATP + H2O
Coenzyme QH2 carrying an extra 2 electrons and 2 hydrogen ions now starts a cascade of events through enzyme complex 3, also known as cytochrome reductase bc.
Cytochromes are very similar to the structure of myoglobin or hemoglobin. The significant feature is the heme structure containing the iron ions, initially in the +3 state and changed to the +2 state by the addition of an electron. The CoQH2 (yellow)passes along the 2 electrons first to cytochrome (blue) b1 heme (magenta), then b2 heme , then to an iron-sulfur protein (green), then to cytochrome c1 (red with black heme), and finally to cytochrome c (not shown). Co Q is represented by the inhibitor antimycin (yellow) in the graphic.
In the meantime the 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP.
Refer to the middle graphic: Cytochrome c is a small molecule which is also able to move in the lipid membrane layer and diffuses toward cytochrome a complex 4. At this time it continues the transport of the electrons, and provides the third and final time that 2 hydrogen ions are channeled to the interspace of the mitochondria for ultimate conversion into ATP.
ATP synthetase is also found at numerous locations in the bilayer membrane of the mitochondria. Three ATP are produced by the pumping action of the re-entry of the hydrogen ions through the ATP synthetase.
Finally, oxygen has diffused into the cell and the mitochondria for the finally reaction of metabolism. Oxygen atom reacts with the 2 electrons and 2 hydrogens to produce a water molecule.