NADPH is a cofactor, used to donate electrons and a hydrogens to reactions catalyzed by some enzymes. Typically enzymes involved in anabolic pathways that create large molecules use NADPH, while enzymes involved in the breakdown of molecules use the analog NADH. Both plants and animals use NADPH and NADH, and they are typically segregated into organelles and cytosol.
Mitochondria use NADH during oxidative phosphorylation, while many enzymes in the cytosol synthesize large biomolecules using NADPH. Chloroplasts in plants also use NADPH as part of the pathway to synthesize sugars from sunlight and carbon dioxide. As in other reactions, NADPH helps carry electrons and protons driven by sunlight into new carbon-carbon bonds, creating sugar molecules.
NADPH is often kept at higher concentration in the cytosol than NADP+, to allow for the easy reduction of small molecules into larger macromolecules. The NADPH is more likely to lose its hydrogen and electrons when it is in high abundance. This can be contrasted to NADH, which is often found in lower concentration than NAD+. NADH is often used in catabolic pathways, the opposite of anabolic pathways. This favors anabolic reaction in the cytosol.
The ratios of these chemicals in the mitochondria is reversed, and catabolic oxidative reactions are favored. This ensures that fatty acids can be synthesized in the cytosol while the mitochondria can continue producing ATP for energy. The concentrations of NADPH and NADH are regulated by special enzymes and pathways in the mitochondrial membranes, as well as through the shuttling of molecules from one side of the membrane to the other, which often involves NADPH.
Function of NADPH
NADPH is the typical coenzyme used in reduction reactions, seen in the anabolic pathways of organisms. For example, when sugars are created during photosynthesis, carbon molecules are chained together using the energy from sunlight. NADPH function in transferring electrons and hydrogen displaced by the energy of sunlight. The NADPH first accepts the electrons and hydrogen when special enzymes transfer these particles to the molecule NADP+.
In this reaction the NADP+ becomes reduced when it accepts the electrons and hydrogen, going from a positive electrical state to a more negative neutral state as a NADPH molecule. Then, the NADPH molecule is oxidized by another enzyme. NADPH works with a wide variety of enzymes and is considered one of the universal electron carriers.