Coenzyme definitions and examples

Understanding Coenzymes, Cofactors, and Pseudogroups

Coenzyme definition

Coenzyme A substance that works with an enzyme to initiate or support the function of the enzyme. It can be thought of as a helper molecule for a biochemical reaction. Coenzymes are small, proteinless molecules that provide a transfer site for an active enzyme. They are intermediate carriers of an atom or group of atoms, allowing a reaction to take place. Coenzymes are not considered part of the structure of enzymes, they are sometimes referred to as . cosubstrates .

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

Coenzyme example

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The B vitamins act as coenzymes necessary for enzymes to form fats, carbohydrates and proteins.

An example of a nonvitamin coenzyme is methionine S-adenosyl, which transfers a methyl group in bacteria as well as in eukaryotes and plant organisms.

Coenzymes, Cofactors and Pseudogroups

Some texts consider all helper molecules bound to an enzyme as cofactors, while other molecules divide classes of chemicals into three groups:

  • Coenzyme are nonprotein organic molecules loosely bound to an enzyme. Many (not all) are vitamins or are derived from vitamins. Many coenzymes contain adenosine monophosphate (AMP). Coenzymes can be described as cosubstrates or prosthetic groups.

  • Cofactors are inorganic species or at least non-protein compounds that function to assist enzymes by increasing the catalytic rate. Usually the cofactors are metal ions. Some metallic elements have no nutritional value, but some trace elements function as activators in biochemical reactions, including iron, copper, zinc, magnesium, cobalt, and molybdenum. Several trace elements that appear to be important for nutrition do not appear to act as cofactors, including chromium, iodine, and calcium.

  • Cosubstrates is a coenzyme that is tightly bound to a protein, but will be released and re-associated at some point.

  • Fake groups are enzyme partner molecules that are tightly or covalently bound to the enzyme (remember, coenzymes are loosely bound). While cosubstrates bind temporarily, pseudogroups bind permanently to a protein. Prosthetic groups help proteins bind other molecules, act as structural elements, and act as charge carriers. An example of a pseudogroup is the heme in hemoglobin, myoglobin, and cytochrome. Iron (Fe) is found at the center of the pseudoheme group allowing it to bind and release oxygen in the lungs and tissues, respectively. Vitamins are also examples of pseudogroups.

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

There are a few related terms that are also related to coenzymes:

  • Apoenzyme is the name given to an inactive enzyme that lacks its coenzymes or cofactors.

  • Holoenzymes is the term used to describe an enzyme complete with its coenzymes and cofactors.

  • Holoproteins is the word used for a protein with a pseudo-group or cofactor.

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

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