# Classification of Enzymes 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 ($\mathrm{H}^{+}$). | *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**: 1. **Hemotoxic**: Affects the heart and cardiovascular system. 2. **Neurotoxic**: Affects the nervous system and brain. 3. **Cytotoxic**: Acts locally at the site of the bite.