In metabolism, many redox reactions are involved. Redox reactions require that electrons can be transferred or removed to either reduce or oxidize a particular substrate or molecule. Therefore, we need intermediates capable of undergoing electron transfer. These are the coenzymes NAD/NADH and FAD/FADH2. These coenzymes can exist in their oxidized (NAD+ and FAD) or reduced forms (NADH and FADH2). NADPH is a close derivatives of NADH that also acts as a redox couple.
NAD+ and NADP+
NAD+ and NADP+ are derivatives of nicotinic acid or nicotinamide. They intervene in biological redox reactions.
Figure: NAD is a derivative of nicotinic acid or nicotinamide. NADP+ contains an additional phosphate group
Both NAD+ and NADP+ can undergo two electron redox steps, in which a hydride is transferred from an organic molecule to the NAD+ or NADP+, with the electrons flowing to the positively charged nitrogen of NAD+ which serves as an electron sink. All NAD+/NADH reactions in the body involve 2 electron transfers. The products of these reactions is indicated ad NADH or NADPH, respectively.
Figure: All NAD+/NADH reactions in the body involve 2 electron hydride transfers
Image by Fvasconcellos 19:44, 9 December 2007 (UTC). w:Image:NAD oxidation reduction.png by Tim Vickers. / Public domain. Wikimedia Commons
The main difference between NADH and NADPH is that NADH is mainly involved in catabolic reactions, such as respiration, whereas NADPH is involved in anabolic reactions, such as photosynthesis.
FAD and FADH2
FAD (or flavin mononucleotide-FMN) and its reduction product, FADH2, are derivatives of riboflavin, and can also undergo redox reactions:
Figure: derivatives of riboflavin
FAD/FADH2 differ from NAD+/NADH since they are bound tightly to enyzmes which use them. This is because FADH2 is susceptible to reaction with dioxygen while NADH is not. FAD/FADH2 is another redox pair that intervene in redox processes in biological systems
Figure: FAD/FADH2 electrons transfers. Image adapted from original by DMacks / Public domain on Wikimedia Commons
FAD/FADH2 are tightly bound to enzymes so as to control the nature of the oxidizing/reducing agent that interacts with them. (i.e. so dioxygen in the cell won't react with them in the cytoplasm.). FAD is usually involved in the oxidation of saturated carbon chains to form double bonds: