Enzymes are biological molecules that play a role in living organisms. They act as biological catalysts, speeding up chemical reactions within cells. They lower the activation energy required for reactions, allowing processes to proceed at speeds necessary for life. Enzymes participate in reactions without being consumed, facilitating multiple reaction cycles.
Why Enzymes Are Classified
A systematic classification system for enzymes is important due to the vast number of enzymes discovered and their diverse functions. This organized approach helps scientists communicate effectively about specific enzymes and their roles, providing a common language that reduces ambiguity.
Classifying enzymes also aids in understanding how they work and predicting their behavior. By grouping enzymes with similar catalytic activities, scientists can infer properties of newly discovered enzymes or explore potential applications, including industrial uses or therapeutic developments.
The Main Enzyme Categories
Enzymes are categorized based on the type of chemical reaction they catalyze, following a system established by the International Union of Biochemistry and Molecular Biology. This system groups enzymes into six primary classes, each identified by a unique Enzyme Commission (EC) number. The first digit of this EC number indicates the enzyme’s main class.
Oxidoreductases
The first class, oxidoreductases, catalyzes oxidation-reduction reactions. These reactions involve the transfer of electrons from one molecule to another. One molecule loses electrons (is oxidized), while another gains them (is reduced).
An example is alcohol dehydrogenase, an enzyme that converts ethanol into an aldehyde. This process involves the transfer of electrons, making it a redox reaction. Many enzymes in this group utilize cofactors such as NADP+ or NAD+ to facilitate electron transfer.
Transferases
Transferases are enzymes that facilitate the movement of specific functional groups from one molecule to another. These functional groups can include methyl groups, phosphate groups, or amino groups.
A common example includes kinases, which transfer phosphate groups, often from ATP, to other molecules. Another instance is transaminases, which move amino groups between different molecules, playing a role in amino acid metabolism.
Hydrolases
Hydrolases catalyze hydrolysis reactions, breaking chemical bonds through the addition of water. Examples include lipases, which break down fats into fatty acids and glycerol, and proteases, which break down proteins into amino acids. Amylases, found in saliva, are also hydrolases that break down complex carbohydrates like starch.
Lyases
Lyases catalyze the breaking of various chemical bonds by mechanisms other than hydrolysis or oxidation. Their action often results in the formation of new double bonds or ring structures within molecules.
Aldolase, an enzyme involved in glycolysis, is an example. It cleaves fructose-1,6-bisphosphate into two smaller three-carbon sugar molecules.
Isomerases
Isomerases catalyze the rearrangement of atoms within a single molecule. This process converts a molecule from one isomeric form to another, meaning the product has the same molecular formula but a different structural arrangement.
Glucose isomerase is an example used in industrial processes to convert glucose into fructose. Another is triosephosphate isomerase, which interconverts dihydroxyacetone phosphate and glyceraldehyde 3-phosphate during glycolysis.
Ligases
Ligases are enzymes that catalyze the joining, or ligation, of two molecules. This process typically involves the formation of a new covalent bond and often requires energy, usually supplied by the hydrolysis of ATP. A well-known example is DNA ligase, which plays a role in DNA replication and repair by joining DNA fragments.
How Enzymes Get Their Names
Enzymes are typically named in a way that reflects their function, often incorporating the name of the molecule they act upon, known as the substrate, and the type of reaction they catalyze. Most enzyme names end with the suffix “-ase.”
For instance, the enzyme lactase breaks down lactose, while DNA polymerase is involved in building DNA strands. Some enzymes, such as pepsin or trypsin, retain older, less descriptive names from before a standardized system was widely adopted.