NADPH Definition

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 insures 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 a 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.

  • NADH – An analog of NADPH lacking a phosphate group, which functions in catabolic reactions.
  • Electron Carrier – Molecules uses as intermediates in the transfer of electrons in biological pathways.
  • Anabolic Reaction – A reaction which uses small monomers to build large polymer molecules.
  • Catabolic Reaction – A reaction which releases the energy from the bonds of large molecules and stores them in electron carriers.


1. Acetyl-CoA is a molecule used in processes both within the mitochondria, and in the cytosol. The only problem is it will not freely travel through the mitochondrial membranes. Acetyl-CoA can be converted to many other molecules, some of which have specific membrane transporters. To get acetyl-CoA in both spaces, it must be converted between molecules. Which of the following represents how NADPH can aid in this process?
A. NADPH will attach to the molecules, transporting them across the membrane.
B. NADPH will be used to oxidize the molecules, allowing them cross the double membrane.
C. NADPH will be created when these molecules are oxidized to traverse the membrane.

Answer to Question #1
C is correct. To reduce means to gain electrons, while the process of oxidation involves the loss of electrons. Reactions like this are typically called redox reactions, as both processes must happen, but to different molecules. The NADPH carries a hydrogen and two electrons, which it is more than willing to give up. NADPH is created by the reduction of NAD+, and can only occur when another molecule is oxidized. NADPH cannot accept any more electrons, and therefore answer B cannot take place. Remember that NADPH is a coenzyme, and must be used in conjunction with an enzyme. It does not act on molecules directly, as in answer A.