Coenzyme Definition and Examples

Understanding Coenzymes, Cofactors, and Prosthetic Groups

Heme is an example of a coenzyme or cofactor that contains both an organic and inorganic component.
Heme is an example of a coenzyme or cofactor that contains both an organic and inorganic component. MOLEKUUL/SCIENCE PHOTO LIBRARY, Getty Images

An enzyme is a macromolecule that catalyzes a chemical reaction. In other words, it makes an unfavorable reaction able to occur. Enzymes are built from smaller molecules to make an active subunit. One of the most important parts of an enzyme is the coenzyme.

Coenzyme Definition

A coenzyme is a substance that works with an enzyme to initiate or aid the function of the enzyme. It may be considered a helper molecule for a biochemical reaction.

Coenzymes are small, nonproteinaceous molecules that provide a transfer site for a functioning enzyme. They are intermediate carriers of an atom or group of atoms, allowing a reaction to occur. Coenzymes are not considered part of an enzyme's structure. They are sometimes referred to as cosubstrates.

Coenzymes cannot function on their own and do require the presence of an enzyme. Some enzymes require several coenzymes and cofactors.

Coenzyme Examples

The B vitamins serve as coenzymes essential for enzymes to form fats, carbohydrates and proteins.

An example of a non-vitamin coenzyme is S-adenosyl methionine, which transfers a methyl group in bacteria as well as in eukaryotes and archaea.

Coenzymes, Cofactors, and Prosthetic Groups

Some texts consider all helper molecules that bind to an enzyme to be types of cofactors, while others divide the classes of chemicals into three groups:

  • Coenzymes are non-protein organic molecules that bind loosely to an enzyme. Many (not all) are vitamins or are derived from vitamins. Many coenzymes contain adenosine monophosphate (AMP). Coenzymes may be described as either cosubstrates or prosthetic groups.
  • Cofactors are inorganic species or at least non-protein compounds that aid enzyme function by increasing the rate of catalysis. Typically cofactors are metal ions. Some metallic elements have no nutritional value, but several trace elements function as cofactors in biochemical reactions, including iron, copper, zinc, magnesium, cobalt, and molybdenum. Some trace elements that appear to be important for nutrition do not appear to act as cofactors, including chromium, iodine, and calcium.
  • Cosubstrates are coenzymes that bind tightly to a protein, yet will be released and bind again at some point.
  • Prosthetic groups are enzyme partner molecules that bind tightly or covalently to the enzyme (remember, coenzymes bind loosely). While cosubstrates bind temporarily, prosthetic groups permanently bond with a protein. Prosthetic groups help proteins bind other molecules, act as structural elements, and act as charge carriers. An example of a prosthetic group is heme in hemoglobin, myoglobin, and cytochrome. The iron (Fe) found at the center of the heme prosthetic group allows it to bind and release oxygen in the lung and tissues, respectively. Vitamins are also examples of prosthetic groups.

An argument for using the term cofactors to encompass all types of helper molecules is that many times both organic and inorganic components are necessary for an enzyme to function.

There are a few related terms also related to coenzymes:

  • Apoenzyme is the name given to an inactive enzyme that lacks its coenzymes or cofactors.
  • Holoenzyme is the term used to describe an enzyme that is complete with its coenzymes and cofactors.
  • Holoprotein is the word used for a protein with a prosthetic group or cofactor.

    A coenzyme binds to a protein molecule (the apoenzyme) to form an active enzyme (the holoenzyme).

    Key Points

    • You can think of a coenzyme or cosubstrate as a helper molecule that aids an enzyme in catalyzing a chemical reaction.
    • A coenzyme requires the presence of an enzyme in order to function. It is not active on its own.
    • While enzymes are proteins, coenzymes are small, non-protein molecules. Coenzymes hold an atom or group of atoms, allowing an enzyme to work.
    • Examples of coenzymes include the B vitamins and S-adenosyl methionine.

    References

    • Cox M, Lehninger AL, Nelson DR (2000). Lehninger principles of biochemistry (3rd ed.). New York: Worth Publishers.
    • Farrell SO, Campbell MK (2009). Biochemistry (6th ed.). Pacific Grove: Brooks Cole.
    • Hasim, Onn (2010). Coenzyme, Cofactor and Prosthetic Group - Ambiguous Biochemical Jargon. Kuala Lumpur: Biochemical Education. pp. 93–94.
    • Nelson D (2008). Lehninger Principles of Biochemistry. New York: W.H. Freeman and Company. p. 184.
    • Palmer T (1981). Understanding enzymes. New York: Horwood.
    • Sauke DJ, Metzler DE, Metzler CM (2001). Biochemistry: the chemical reactions of living cells (2nd ed.). San Diego: Harcourt/Academic Press.