Enzymes, Cofactors & Coenzymes – Comprehensive Exam Notes

Cofactor (general)

Any non-protein substance that must associate with an apoenzyme to permit catalysis.

Prosthetic group

A tightly bound organic cofactor that is a permanent component of the enzyme complex.

Coenzyme

A loosely bound organic cofactor that binds and releases each catalytic cycle.

Metalloenzyme

An enzyme with a metal ion tightly or covalently integrated into its structure; removal causes loss of structure/function.

Metal-activated enzyme

An enzyme whose metal ion is loosely bound; removal is usually reversible by dialysis followed by re-addition of the metal.

Apoenzyme

The inactive protein portion of an enzyme, lacking its required cofactor.

Holoenzyme

The catalytically competent complex of an apoenzyme plus its required cofactor(s).

Enzymes

Biological catalysts (usually proteins; exceptions are ribozymes) that increase reaction rate without altering equilibrium.

Active site

The three-dimensional cleft or pocket formed by specific amino-acid side chains that complements the substrate in geometry and chemical properties.

Michaelis constant (Km)

The substrate concentration required to reach half-maximal velocity (V_{max}/2); an indicator of an enzyme's affinity for its substrate.

Oxidoreductases

Enzymes that catalyze oxidation-reduction (electron/hydrogen/oxygen transfer) reactions.

Transferases

Enzymes that transfer a functional group (excluding electrons) between molecules.

Hydrolases

Enzymes that cleave bonds by adding water (hydrolysis).

Lyases

Enzymes that remove groups leaving double bonds or add groups to double bonds, without using water or oxidation.

Isomerases

Enzymes that catalyze intramolecular rearrangements to yield isomers.

Enzymes that couple two molecules using energy from ATP hydrolysis