4.6:
Role of Reduced Coenzymes NADH and FADH₂
Nicotinamide adenine dinucleotide or NAD+ and Flavin adenine dinucleotide or FAD are the two important coenzymes that act as electron carriers in various biochemical reactions.
During an oxidation reaction, these coenzymes absorb electrons and get reduced into NADH and FADH2.
Then during reduction, the reduced coenzymes release the electrons and convert back to their oxidized form.
In eukaryotic cells, glycolysis and the TCA cycle are two major sources of reduced coenzymes.
In the cytosol, glycolysis oxidizes glucose to pyruvate, reducing NAD+ to NADH.
In the mitochondria, the TCA cycle produces three molecules of NADH and one molecule of FADH2.
However, the NADH produced in the cytosol cannot directly enter the mitochondria. Therefore, two carrier pathways—the malate-aspartate shuttle and the glycerol phosphate shuttle transfer the electrons to NAD+ or FAD present inside the mitochondria.
Finally, the reduced coenzymes donate their electrons to the electron transport chain to generate a proton motive force for ATP production.
4.6:
Role of Reduced Coenzymes NADH and FADH₂
The energy released from the breakdown of the chemical bonds within nutrients can be stored either through the reduction of electron carriers or in the bonds of adenosine triphosphate (ATP). In living systems, a small class of compounds functions as mobile electron carriers, molecules that bind to and shuttle high-energy electrons between compounds in pathways. The principal electron carriers that will be considered originate from the B vitamin group and are derivatives of nucleotides; they are nicotinamide adenine dinucleotide, nicotine adenine dinucleotide phosphate, and flavin adenine dinucleotide. These compounds can be easily reduced or oxidized.
Nicotinamide adenine dinucleotide (NAD+/NADH) is the most common mobile electron carrier used in catabolism. NAD+ is the oxidized form of the molecule; NADH is the reduced form of the molecule. Nicotine adenine dinucleotide phosphate (NADP+), the oxidized form of an NAD+ variant that contains an extra phosphate group, is another important electron carrier; it forms NADPH when reduced. The oxidized form of flavin adenine dinucleotide is FAD, and its reduced form is FADH2. Both NAD+/NADH and FAD/FADH2 are extensively used in energy extraction from sugars during catabolism in chemoheterotrophs, whereas NADP+/NADPH plays an important role in anabolic reactions and photosynthesis. Collectively, FADH2, NADH, and NADPH are often referred to as having reducing power due to their ability to donate electrons to various chemical reactions.
This text is adapted from Openstax, Microbiology, Section 8.1: Energy, Matter and Enzymes