Enzymes definition easy
Enzymes: Definition, Classification, Chemical Properties, Mechanism of Action and Factors Affecting Activity
1. Definition of Enzymes
Enzymes are biological catalysts (biocatalysts) that accelerate biochemical reactions involved in digestion and metabolism. They are found throughout the body, including in cells, the cytoplasm, mitochondria, tissues, and body fluids.
Based on where they exert their catalytic activity, enzymes are classified into two main types:
Endoenzymes
Endoenzymes are enzymes that function inside cells. Most enzymes belong to this category. Examples include metabolic enzymes such as cytochrome oxidase.
Exoenzymes
Exoenzymes are released by cells and catalyze biochemical reactions outside the cell. Examples include digestive enzymes such as amylase, lipase, and protease.
2. Classification of Enzymes
Enzymes are named and classified according to several criteria, including their substrate, the type of reaction they catalyze, their synthesis, and their chemical nature.
2.1. Classification Based on the Substrate
Enzymes are often named by adding the suffix -ase to the name of their substrate.
| Substrate | Corresponding enzyme |
|---|---|
| Carbohydrates | Carbohydrases |
| Proteins | Proteinases |
| Lipids | Lipases |
| Nucleic acids | Nucleases |
| Maltose | Maltase |
| Lactose | Lactase |
| Sucrose | Sucrase |
| Urea | Urease |
2.2. Classification According to the Enzyme Commission
According to the Enzyme Commission of the International Union of Biochemistry, enzymes are classified into six main classes based on the reactions they catalyze:
| Enzyme class | Catalyzed reaction |
|---|---|
| Oxidoreductases | Oxidation-reduction reactions |
| Transferases | Transfer of functional groups or atoms |
| Hydrolases | Hydrolysis reactions |
| Lyases | Addition or removal of atoms or groups to or from double bonds |
| Isomerases | Rearrangement of atoms within molecules |
| Ligases | Joining of molecules using ATP |
3. Chemical Properties of Enzymes
Enzymes are large, specialized protein molecules composed of one or more polypeptide chains. They may consist entirely of proteins or combine a protein component with one or more non-protein components.
Based on their chemical composition, enzymes are divided into two types:
Simple Enzymes
Simple enzymes are composed entirely of protein molecules.
Conjugated Enzymes (Holoenzymes)
Holoenzymes consist of a protein component and a non-protein component, both of which contribute to enzymatic activity.
The main components include:
-
Apoenzyme: The protein component of a conjugated enzyme.
-
Cofactor: The non-protein component required for the activity of certain enzymes.
-
Prosthetic group: A cofactor that is firmly attached to the enzyme.
-
Coenzyme: A cofactor that is loosely bound to the enzyme.
4. Properties of Protein Enzymes
Protein enzymes possess several important characteristics:
-
Catalytic activity: They accelerate chemical reactions without undergoing permanent changes.
-
Specificity: They exhibit specificity in their action and interact with particular substrates.
-
Low concentration requirement: They are effective in relatively small quantities.
-
Denaturation: As proteins, they can undergo denaturation under unfavorable conditions.
-
Reversibility: Enzyme-catalyzed reactions may be reversible, depending on the reaction conditions.
5. Mechanism of Enzymatic Action
Michaelis and Menten proposed a hypothesis to explain the mechanism of enzyme action. According to this hypothesis, enzymatic reactions occur through the following stages:
Step 1: Enzyme-Substrate Combination
The substrate binds to the enzyme's active site, forming an enzyme-substrate complex.
Step 2: Breakdown of the Substrate
The active site facilitates the weakening and breaking of chemical bonds within the substrate, leading to the formation of reaction products.
Step 3: Enzyme Dissociation
The enzyme eventually dissociates from the final products and becomes available to bind another substrate molecule, allowing the catalytic cycle to continue.
6. Formation of the Enzyme-Substrate Complex
Two main hypotheses explain how enzymes interact with their substrates:
-
Lock-and-key hypothesis.
-
Induced-fit hypothesis.
6.1. Lock-and-Key Hypothesis
Proposed by Emil Fischer in 1914, the lock-and-key hypothesis suggests that an enzyme's active site has a rigid and specific three-dimensional conformation.
According to this model, the substrate fits directly into the enzyme's active site, much like a key fitting into a lock, forming an enzyme-substrate complex and allowing the reaction to proceed.
6.2. Induced-Fit Hypothesis
Proposed by Koshland in 1963, the induced-fit hypothesis suggests that enzyme active sites are flexible rather than completely rigid.
As the substrate approaches the enzyme, interactions between the two molecules induce a change in the enzyme's shape. This conformational adjustment allows the active site to adapt more closely to the substrate, facilitating the formation of the enzyme-substrate complex.
7. Factors Affecting Enzyme Activity
Enzyme activity is influenced by several environmental and chemical factors that can affect the rate of biochemical reactions.
7.1. Temperature
Temperature is an important factor influencing enzyme activity.
Enzyme activity generally increases as temperature rises, up to approximately 40°C, due to increased molecular movement and more frequent interactions between enzymes and substrates.
Most enzymes have an optimal temperature at which their catalytic activity reaches its highest level.