8.8:
Electron Carriers
Electron carriers are compounds that shuttle around high energy electrons, via redox reactions – the coordinating states of oxidation and reduction, respectively, losing and gaining these negatively charged particles.
For instance, one principal electron carrier is nicotinamide adenine dinucleotide, or NAD+ named so because, at the first carbon position, one ribose ring has an adenine base, while the other has a nicotinamide. At the fifth carbon position, two phosphate groups join these two nucleotides.
During metabolism, NAD+, as a coenzyme, binds to an enzyme, such as dehydrogenase, and acts as an oxidizing agent, removing two hydrogen protons, along with a pair of electrons from the reactant.
The electrons are then transferred to the positively charged nitrogen, and the hydrogen attaches to the opposite carbon, reducing NAD+ into NADH.
In the end, the extra hydrogen proton is left in the cytoplasm, and NADH shuttles its electrons to the mitochondrial membranes, where they enter the electron transport chain, and the carriers can continue to undergo redox reactions.
8.8:
Electron Carriers
Electron carriers can be thought of as electron shuttles. These compounds can easily accept electrons (i.e., be reduced) or lose them (i.e., be oxidized). They play an essential role in energy production because cellular respiration is contingent on the flow of electrons.
Over the many stages of cellular respiration, glucose breaks down into carbon dioxide and water. Electron carriers pick up electrons lost by glucose in these reactions, temporarily storing and releasing them into the electron transport chain.
Two such electron carriers are NAD+ and FAD, both derived from B vitamins. The reduced forms of NAD+ and FAD, NADH, and FADH2, respectively, are produced during earlier stages of cellular respiration (glycolysis, pyruvate oxidation, and the citric acid cycle).
The reduced electron carriers NADH and FADH2 pass electrons into complexes I and II of the electron transport chain, respectively. In the process, they are oxidized to form NAD+ and FAD.
Additional electron carriers in the electron transport chain are flavoproteins, iron-sulfur clusters, quinones, and cytochromes. With the assistance of enzymes, these electron carriers eventually transfer the electrons to oxygen molecules. The electron carriers become oxidized as they donate electrons and reduced as they accept them, and thus alternate between their oxidized and reduced forms.
Electron carriers provide a controlled flow of electrons that enables the production of ATP. Without them, the cell would cease to function.