What Are Coenzymes?

The main function of coenzymes is to act as metabolic intermediaries. The best known coenzymes are NAD + and NADH, coenzyme A and ATP.
What Are Coenzymes?

Written by Equipo Editorial

Last update: 16 December, 2022

Coenzymes are small non-protein organic molecules that transport chemical groups between the different enzymes in the body in order for them to carry out their functions. They’re also known as cosubstrates.

As this second name indicates, coenzymes are not part of enzymes, but are substrates that bind to them. Due to this, the coenzymes of the non-prosthetic groups of the enzymes are differentiated, if they bind closely to them.

Both coenzymes and prosthetic groups belong to a larger group, the so-called cofactors. They’re non-protein molecules that need enzymes for their activity.

Coenzyme molecules are often vitamins, or are made from vitamins. Many coenzymes contain the nucleotide adenosine as part of their structure . An example of this is ATP, coenzyme A or NAD +.

A little history about coenzymes

The first coenzyme discovered was NAD +, by Arthur Harder and William Youdin in 1906. They observed that by adding a boiled and filtered yeast extract, alcoholic fermentation into unboiled yeast extract was accelerated.

At the beginning of the 20th century, other coenzymes were also identified, such as ATP, by Karl Lohmann, and coenzyme A, by Fritz Albert Lipmann in 1945.

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Functions of coenzymes

The main function of coenzymes is to act as metabolic intermediaries. During metabolism, numerous biochemical reactions take place. However, most of them correspond to basic types of reactions that involve the transfer of functional groups.

Each kind of group transfer reaction is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it and another set of enzymes that consume it.

An example of all this are dehydrogenase enzymes. They use NADH as a cofactor. Hundreds of different enzymes remove electrons from their substrates and reduce NAD + to NADH. This coenzyme is ultimately reduced to a substrate for any of the reductase enzymes present in the cell that need to reduce their substrates.

nad nadh coenzymes

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Best known coenzymes

Here are some of the best-known coenzymes:

NAD + and NADH

Nicotinamide adenine dinucleotide, or NAD +, is a coenzyme found in all living cells. During metabolism, it participates in redox reactions, transporting electrons from one reaction to another.

Thus, the coenzyme is found in two forms in cells: NAD + and NADH. The first form is the oxidizing agent, since it accepts electrons from other molecules and becomes reduced, forming NADH, which can be used as a reducing agent to donate electrons.

Coenzyme A or CoA

The main function of this coenzyme is to transfer acyl groups that participate in various metabolic pathways, such as the Krebs cycle.

It is made up of three parts:

  • Body: made up of vitamin B5.
  • Head: composed of ADP or adenosine bisphosphate.
  • Tail: composed of beta-mercaptoethylamine.

Coenzyme A helps enzymes work, but it also acts as a kind of link for other molecules.

ATP or adenosine triphosphate

This is a molecule used by all living organisms to provide energy in chemical reactions. In addition, ATP is the precursor of other coenzymes, such as the previous two.

It’s a very important coenzyme, as it forms one of the 4 monomers used in the synthesis of cellular RNA.

SAM or S-adenosyl methionine

This coenzyme participates in the transfer of methyl groups in chemical reactions. It was discovered by an Italian in 1952, Giulio Cantoni.

In its structure, you can find ATP and methionine. More than 40 metabolic reactions have been determined that involve the transfer of a methyl group from SAM to different substrates, such as nucleic acids, proteins, and lipids.


Glutathione, or GSH, is a tripeptide that contains a rare peptide bond between the amino group of cysteine and the carboxyl group of the glutamate side chain.

It is an antioxidant, so it protects cells from toxins and free radicals. Glutathione is almost always in its reduced form. The reason for this is that the enzyme that converts it from its oxidized form, glutathione reductase, is constitutively active and inducible under oxidative stress.

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