3.5 Classification of Enzymes | Enzymes | Biology 11

Enzymes can be categorized in two primary ways: based on the type of reaction they catalyze or based on the substrate they act upon.

Classification Based Upon Reaction Type

The International Union of Biochemistry (IUB) established a systematic classification in 1961, grouping all enzymes into six major classes based on the type of chemical reaction they catalyze.

Class	Name	Function	Example
1	Oxidoreductases	Catalyze oxidation-reduction reactions by adding or removing electrons or hydrogen ions ( $H^{+}$ ).	Cytochrome oxidase (oxidizes cytochrome).
2	Transferases	Catalyze the transfer of a specific functional group (other than hydrogen) from one substrate to another.	Hexokinase (transfers a phosphate group from ATP to glucose).
3	Hydrolases	Catalyze the breakdown (hydrolysis) of large molecules into smaller ones by adding water and breaking covalent bonds.	Pepsin, trypsin (digestive enzymes that break down proteins).
4	Lyases	Catalyze the breakdown of specific covalent bonds and removal of chemical groups without using water (hydrolysis).	Histidine decarboxylase (breaks bonds in histidine to form histamine and CO₂).
5	Isomerases	Catalyze the intramolecular rearrangement of atoms within a molecule, converting one isomer to another.	Phosphohexose isomerase (converts glucose 6-phosphate to fructose 6-phosphate).
6	Ligases (Synthetases)	Catalyze the joining of two molecules, using energy derived from the hydrolysis of ATP.	DNA Polymerase (links DNA monomers to form a polymer).

Classification Based Upon Substrate

Enzymes can also be classified more simply by the substrate they act upon. This naming convention often involves adding the suffix "-ase" to the name of the substrate.

Proteases: Act on proteins.

Pepsin and trypsin: Digest large polypeptides.
Aminopeptidases and carboxypeptidases: Digest peptones into dipeptides.
Erypsin: Digests dipeptides into amino acids.

Lipases: Act on lipids (fats), hydrolyzing them into fatty acids and glycerol.

Example: Pancreatic lipases.

Carbohydrases: Act on carbohydrates.

Amylase: Digests starch into maltose.
Cellulase: Digests cellulose into cellobiose.
Sucrase: Digests sucrose into glucose and fructose.
Lactase: Digests lactose into galactose and glucose.

Nucleases: Act on nucleic acids (DNA and RNA).

RNAases: Digest RNA into ribonucleotides.
DNAases: Digest DNA into deoxyribonucleotides.
ATPases: Catalyze the hydrolysis of ATP.

Venoms as Enzyme Inhibitors

Snake venom is a complex mixture of toxins and enzymes. Some of these components act as potent enzyme inhibitors.

Mechanism: Venom can inhibit essential enzymes, such as cholinesterase (leading to loss of muscle control) and cytochrome oxidase (disrupting cellular respiration).

Types of Venom Action:

Hemotoxic: Affects the heart and cardiovascular system.
Neurotoxic: Affects the nervous system and brain.
Cytotoxic: Acts locally at the site of the bite.

Classification Based Upon Reaction Type

The International Union of Biochemistry (IUB) established a systematic classification in 1961, grouping all enzymes into six major classes based on the type of chemical reaction they catalyze.

Class	Name	Function	Example
1	Oxidoreductases	Catalyze oxidation-reduction reactions by adding or removing electrons or hydrogen ions ( $H^{+}$ ).	Cytochrome oxidase (oxidizes cytochrome).
2	Transferases	Catalyze the transfer of a specific functional group (other than hydrogen) from one substrate to another.	Hexokinase (transfers a phosphate group from ATP to glucose).
3	Hydrolases	Catalyze the breakdown (hydrolysis) of large molecules into smaller ones by adding water and breaking covalent bonds.	Pepsin, trypsin (digestive enzymes that break down proteins).
4	Lyases	Catalyze the breakdown of specific covalent bonds and removal of chemical groups without using water (hydrolysis).	Histidine decarboxylase (breaks bonds in histidine to form histamine and CO₂).
5	Isomerases	Catalyze the intramolecular rearrangement of atoms within a molecule, converting one isomer to another.	Phosphohexose isomerase (converts glucose 6-phosphate to fructose 6-phosphate).
6	Ligases (Synthetases)	Catalyze the joining of two molecules, using energy derived from the hydrolysis of ATP.	DNA Polymerase (links DNA monomers to form a polymer).

Classification Based Upon Substrate

Enzymes can also be classified more simply by the substrate they act upon. This naming convention often involves adding the suffix "-ase" to the name of the substrate.

Proteases: Act on proteins.

Pepsin and trypsin: Digest large polypeptides.

Aminopeptidases and carboxypeptidases: Digest peptones into dipeptides.

Erypsin: Digests dipeptides into amino acids.

Lipases: Act on lipids (fats), hydrolyzing them into fatty acids and glycerol.

Example: Pancreatic lipases.

Carbohydrases: Act on carbohydrates.

Amylase: Digests starch into maltose.

Cellulase: Digests cellulose into cellobiose.

Sucrase: Digests sucrose into glucose and fructose.

Lactase: Digests lactose into galactose and glucose.

Nucleases: Act on nucleic acids (DNA and RNA).

RNAases: Digest RNA into ribonucleotides.

DNAases: Digest DNA into deoxyribonucleotides.

ATPases: Catalyze the hydrolysis of ATP.

Venoms as Enzyme Inhibitors

Snake venom is a complex mixture of toxins and enzymes. Some of these components act as potent enzyme inhibitors.

Mechanism: Venom can inhibit essential enzymes, such as cholinesterase (leading to loss of muscle control) and cytochrome oxidase (disrupting cellular respiration).

Types of Venom Action:

Hemotoxic: Affects the heart and cardiovascular system.

Neurotoxic: Affects the nervous system and brain.

Cytotoxic: Acts locally at the site of the bite.