Enzymes

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being consumed in the process.

• They are nitrogenous organic compounds made up of amino acids linked by peptide bonds, forming long polypeptide chains.
• It catalyzes various biological processes such as metabolism, digestion, and cell signaling

Examples of Enzymes

1. Lipases are a group of enzymes that aid in the digestion of lipids in the intestine.
2. Amylase is a protein that aids in the conversion of carbohydrates to sugars. Saliva contains this enzyme.
3. Maltase is a sugar that breaks down maltose into glucose; it is found in saliva and various foods, including potatoes, pasta, and beer.
4. Trypsin is an enzyme that breaks down proteins into amino acids and is located in the small intestine.
5. Lactase is an enzyme present in the small intestine that aids in the breakdown of lactose, a sugar found in milk, into glucose and galactose.

Structure of Enzyme

Enzymes are globular proteins that act as biological catalysts. Their structure is highly specific and directly related to their function.

• Enzymes are proteins that are made up of several polypeptide chains, also known as amino acids, that have been folded and coiled numerous times.
• They have linear chains of amino acids in three-dimensional structures.
• The enzyme's catalytic activity is determined by the amino acid sequence. Only a small portion of an enzyme's structure participates in catalysis and is located around the binding sites.
• They have separate sites; the active site of an enzyme is made up of the catalytic and binding sites.

Classification of Enzymes

The International Union of Biochemistry and Molecular Biology (IUB) classifies enzymes into six major classes based on the type of chemical reactions they catalyze. These classes are:

• Oxidoreductases: It is an enzyme that catalyzes the oxidation and reduction reactions in which electrons are transferred from one form of a molecule (electron donor) to another (electron acceptor). Consider the enzyme pyruvate dehydrogenase. Cofactors for oxidoreductase enzymes are commonly NADP+ or NAD+.

AH₂+B→A+BH₂

• Transferases: These catalyze the transfer of a chemical group (functional group) from one compound (referred to as the donor) to another compound (referred to as the recipient). A transaminase, for example, is an enzyme that transfers an amino group from one molecule to another.

A–X+B↔B–X+A

• Hydrolases: They are hydrolytic enzymes that catalyze the hydrolysis reaction by cleaving the bond and hydrolyzing it with water molecules, i.e., they catalyze the hydrolysis of a bond. Pepsin, for example, breaks down peptide connections in proteins.

A–X+H₂O→X–OH+A–H

• Lyases: They are enzymes that catalyze bodywork by creating a double bond or adding a group to a double bond without involving hydrolysis or oxidation. Aldolase catalyzes the conversion of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, for example.

A–X+B–Y→A=B+X–Y

• Isomerases: Isomerases are a class of enzymes that catalyze the conversion of a molecule into its isomeric form. They facilitate intramolecular rearrangements by breaking and reforming bonds within the same molecule. For example, in glycogenolysis, the enzyme phosphoglucomutase catalyzes the conversion of glucose-1-phosphate to glucose-6-phosphate. In this reaction, the phosphate group is shifted from one position to another within the same molecule.

A_Cis→A′_Trans

• Ligases: They are catalytic enzymes that catalyze the ligation or connecting of two big molecules by establishing a new chemical link between them. DNA ligase, for example, catalyzes the formation of a phosphodiester bond between two DNA fragments.

A+B→AB

Enzyme Cofactor

Cofactors are chemical substances that are not proteins and are found in enzymes. A cofactor affects the action of an enzyme by acting as a catalyst. Apoenzymes are enzymes that do not require a cofactor. The holoenzyme is made up of an enzyme and its cofactor.

Three Kinds of Cofactors Present in Enzymes:

1. Prosthetic groups: These are cofactors that are always covalently or permanently linked to an enzyme. Many enzymes have a FAD (flavin adenine dinucleotide) prosthetic group.
2. Coenzyme: A coenzyme is a non-protein organic molecule that only interacts with an enzyme during catalysis. It is separated from the enzyme at all other times. NAD⁺ is a widely used coenzyme.
3. Metal ions: Certain enzymes require a metal ion in the active site to establish coordinate bonds during catalysis. A number of enzymes use the metal ion cofactor Zn²⁺.

Mechanism of Enzyme Action

The active site of an enzyme is a specific region that binds to the substrate and catalyzes the chemical reaction to form products. After the reaction is completed, the products are released (dissociate) from the enzyme surface, allowing the enzyme to be reused.

Two of the most well-known mechanisms of enzyme function are the induced fit hypothesis and the lock and key mechanism.

• Induced Fit Hypothesis: The active site of the enzyme does not have a firm shape. As a result, the substrate does not completely fit into the enzyme's active site. When the enzyme binds to the substrate, the active site changes form, becoming complementary to the substrate's shape. Because of the flexibility of the protein, this conformational shift is possible.
• Lock and Key Mechanism: It is known as Fisher's theory, which describes the enzyme-substrate interaction. Emil Fischer proposed the lock and key model in 1894. As a result, it's sometimes referred to as Fisher's theory. The enzyme-substrate interaction is described by the second model.

Enzymes as Biochemical Catalysts

Biochemical catalysts are also known as enzymes, and the phenomenon is known as biochemical catalysis. Enzymes are widely used to enhance or expedite the efficient preparation and effect of beverages, chocolates, curd, predigested infant food, washing powders, and other products.

Examples of Enzyme Catalysis

• Cane sugar inversion: Cane sugar is converted to glucose and fructose by the enzyme invertase.

C12H22O11(aq) + H2O(1) → C₆H₁₂O₆(aq) + C₆H₁₂O₆(aq)

• Conversion of milk to curd: The enzyme lactase, which is released by lactobacilli, is responsible for turning milk into curd.
• Conversion of glucose into ethyl alcohol: Glucose is converted to ethyl alcohol and carbon dioxide by the zymase enzyme.

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(aq)

• Conversion of starch into maltose: Starch is converted to maltose by the diastase enzyme.

Factors Affecting Enzyme Catalysis

1. Concentration of Substrate: In the presence of an enzyme, the rate of a chemical reaction increases as the substrate concentration rises until a limiting rate is achieved, after which additional increases in the substrate concentration have no effect on the reaction.
2. Concentration of Enzyme: When the enzyme concentration is much lower than the substrate concentration, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration. This is true for any catalyst; when the catalyst concentration rises, the reaction rate rises as well.
3. Temperature: For most chemical reactions, a temperature increase of 10°C about doubles the reaction rate, according to a well-known rule of thumb.
4. Hydrogen Ion Concentration (pH): Most enzymes are proteins, and they are sensitive to variations in pH or hydrogen ion concentration. The degree of ionization of an enzyme's acidic and basic side groups, as well as the substrate components, is affected by changes in pH.

Chemical Nature of Enzyme

• Enzymes are primarily composed of proteins.
• Amino acids are folded into specific three-dimensional shapes, determining the enzyme's structure.
• The structure includes an active site where the substrate binds and catalytic reactions occur.
• Enzymes can also be RNA molecules, although less common than protein enzymes.
• The sequence and structure of amino acids in enzymes dictate their function and specificity for particular substrates.