The complicated processes of metabolism wouldn't be possible without the help of certain high-energy molecules. The main purpose of these molecules is to transfer either inorganic phosphate groups (Pi) or hydride (H-) ions. The inorganic phosphate groups are used to make high energy bonds with many of the intermediates of metabolism. These bonds can then be broken to yield energy, thus driving the metabolic processes of life. Hydride ions can be transferred from one intermediate to another resulting in a net oxidation or reduction of the intermediate. Oxidation corresponds to a loss of hydride and reduction to the gaining of hydride. Certain reduced forms of high energy molecules such as NADH and [FADH2] can donate their electrons to the electron carriers of the electron transport chain (ETC) which results in the production of ATP (only under aerobic conditions).
ATP (Adenosine Triphosphate) contains high energy bonds located between each phosphate group. These bonds are known as phosphoric anhydride bonds.
There are three reasons these bonds are high energy:
- The electrostatic repulsion of the positively charged phosphates and negatively charged oxygen stabilizes the products (ADP + Pi) of breaking these bonds.
- The stabilization of products by ionization and resonance. As the bonds are broken there is an increased stability due to the resonance of that product's structure.
- The entropy increases. There is a greater stability in the products because there exists a greater entropy; i.e. more randomness. 1 mole of reactants has a higher energy than 2 moles of products. Disorder is favored over order according to the 2nd law of thermodynamics.
ADP (Adenosine Diphosphate) also contains high energy bonds located between each phosphate group. It has the same structure as ATP, with one less phosphate group. The same three reasons that ATP bonds are high energy apply to ADP's bonds.
NAD+ (Nicotinamide adenine dinucleotide (oxidized form)) is the major electron acceptor for catabolic reactions. It is strong enough to oxidize alcohol groups to carbonyl groups, while other electron acceptors (like [FAD]) are only able to oxidize saturated carbon chains from alkanes to alkenes. It is an important molecule in many metabolic processes like beta-oxidation, glycolysis, and TCA cycle. With out NAD+ the aforementioned processes would be unable to occur.
NADH (reduced form) is an NAD+ that has accepted electrons in the form of hydride ions. NADH is also one of the molecules responsible for donating electrons to the ETC to drive oxidative phosphorolation and also pyruvate during fermentation processes.
NADP+ (Nicotinamide adenine dinucleotide phosphate (oxidized form)) is the major electron donator for anabolic reactions.
Nicotinamide adenine dinucleotide phosphate (reduced form)
- Garrett, H., Reginald and Charles Grisham. Biochemistry. Boston: Twayne Publishers, 2008.
- Raven, Peter. Biology. Boston: Twayne Publishers, 2005.
- What is the name of the high energy bond in ATP and ADP?
- What is the major electron donator in anabolic reactions.
- Without looking draw the structures of ATP, NAD+, NADH, NADP+, NADPH.
- What properties make the phosphoric anhydride bond a high energy bond? (Hint: There are three reasons)
- Think about all of the metabolic pathways, find similarities and differences between the steps that use NADH and the ones that use NADPH.
Contributors and Attributions
- Darik Benson (UCD)