Enzymes Note M.N. Chatterjea For Nurses

Enzymes Note M.N. Chatterjea Enzymes are biological catalysts that play a crucial role in facilitating various biochemical reactions within living organisms. These macromolecules are predominantly proteins, though some RNA molecules, known as ribozymes, also exhibit enzymatic activity. Enzymes increase the rate of reactions by lowering the activation energy required for the reactions to occur, thereby significantly enhancing the efficiency of metabolic processes. This paper will delve into the classification of enzymes, their mechanisms of action, factors affecting their activity, and the chemical composition of enzymes.

Enzymes

Enzymes are defined as colloidal, thermolabile proteins that function as biocatalysts. They can be classified based on their structure into monomeric and oligomeric enzymes. Monomeric enzymes consist of a single polypeptide chain, such as ribonuclease, while oligomeric enzymes are composed of multiple polypeptide chains, like lactate dehydrogenase and hexokinase.

When multiple enzyme-catalyzing reaction sites are present on the same macromolecule, this is referred to as a multienzyme complex. The function of such complexes can be diminished if they are fractionated into smaller units, each exhibiting individual enzyme activity. Common examples include fatty acid synthetase and pyruvate dehydrogenase.

Substrates

The specific molecules on which enzymes act are known as substrates. Enzymes bind to their substrates at specific sites, facilitating the conversion of substrates into products through biochemical reactions.

Factors Affecting Catalytic Activity of Enzymes

Enzymes possess remarkable catalytic power, accelerating reactions by factors of up to a million times by lowering the energy of activation. Several factors influence the catalytic activity of enzymes:

1. Temperature

Temperature plays a crucial role in enzyme activity. Each enzyme has an optimal temperature range where it is most active. An increase in temperature typically enhances enzymatic activity, up to a certain point. However, extreme temperatures can lead to denaturation, rendering the enzyme inactive.

2. pH Levels

The pH of the environment also significantly affects enzyme activity. Each enzyme has an optimal pH at which its activity is maximized. Deviations from this optimal pH can lead to decreased activity or denaturation of the enzyme.

3. Substrate Concentration

As substrate concentration increases, the rate of enzymatic reaction also increases, reaching a maximum velocity (Vmax) when all active sites of the enzyme are occupied. This relationship is described by the Michaelis-Menten equation.

4. Enzyme Concentration

Increasing the concentration of the enzyme, while keeping substrate concentration constant, typically leads to an increase in the rate of reaction, provided there is sufficient substrate available.

5. Inhibitors and Activators

Certain molecules can inhibit or enhance enzyme activity. Competitive inhibitors bind to the active site, preventing substrate binding, while non-competitive inhibitors bind elsewhere, altering enzyme function. Activators can increase enzymatic activity by enhancing the enzyme’s ability to bind to its substrate.

Chemical Composition of Enzymes

Enzymes are primarily composed of proteins, which are polymers of amino acids. The sequence and arrangement of these amino acids determine the enzyme’s structure and function. Most enzymes are conjugated proteins, meaning they may contain additional non-protein components, known as cofactors or coenzymes, which assist in enzymatic reactions.

Cofactors and Coenzymes

• Cofactors: Inorganic ions, such as magnesium or zinc, that assist enzymes in catalyzing reactions.
• Coenzymes: Organic molecules, often derived from vitamins, that serve as temporary carriers of specific atoms or functional groups during the reaction.

Nomenclature and Classification of Enzymes

Enzymes are typically named by adding the suffix “ase” to the name of the substrate on which they act. For example:

• Enzymes acting on nucleic acids are called nucleases.
• Enzymes that hydrolyze dipeptides are referred to as dipeptidases.

Enzyme Classification

The International Union of Biochemistry (IUB) classifies enzymes into six main categories based on the type of reaction they catalyze:

1. Oxidoreductases: Enzymes involved in oxidation-reduction reactions (e.g., lactate dehydrogenase).
2. Transferases: Enzymes that transfer functional groups between substrates (e.g., aspartate transaminase).
3. Hydrolases: Enzymes that catalyze hydrolysis reactions (e.g., pepsin).
4. Lyases: Enzymes that add or remove groups to form double bonds (e.g., fumarase).
5. Isomerases: Enzymes that catalyze the rearrangement of molecular structures (e.g., triosephosphate isomerase).
6. Ligases: Enzymes that catalyze the joining of two substrates (e.g., DNA ligase).

Each enzyme is assigned a unique enzyme code number (EC number) based on its classification, which helps in identifying and categorizing enzymes systematically.

Specificity of Enzymes

Enzyme specificity refers to the ability of an enzyme to selectively bind to a specific substrate among a range of similar compounds. This property is essential for the precise regulation of biochemical pathways. Specificity can be categorized into three types:

1. Stereochemical Specificity

This type of specificity pertains to an enzyme’s ability to distinguish between stereoisomers. For example, enzymes may act on only one optical isomer of a substrate, such as D- or L-amino acids, which have different three-dimensional arrangements.

2. Reaction Specificity

Enzymes may catalyze only one specific reaction among several possible reactions that a substrate can undergo. For example, one enzyme may catalyze the deamination of an amino acid, while another enzyme would be responsible for its transamination.

3. Substrate Specificity

Substrate specificity is categorized into absolute specificity and relative specificity:

• Absolute Specificity: Enzymes that act on only one substrate, such as urease, which hydrolyzes urea.
• Relative Specificity: Enzymes that can act on a group of related substrates. For instance, trypsin specifically cleaves peptide bonds at the carboxyl side of lysine and arginine residues.

Mechanism of Enzyme Action

The mechanism of enzyme action describes how enzymes facilitate biochemical reactions. The most widely accepted model for enzyme action is the Michaelis-Menten model. According to this model, an enzyme (E) binds to a substrate (S) to form an enzyme-substrate (ES) complex. This complex then converts to a product (P) while regenerating the enzyme (E):

$E+S\rightleftharpoons ES\to E+P$

Active Site

The active site of the enzyme is a specific region where substrate binding occurs. This site is composed of amino acid residues that create a unique environment conducive to the reaction. The interaction between the substrate and the active site is often described by the lock-and-key or induced fit models.

• Lock-and-Key Model: This model posits that the enzyme’s active site is complementary in shape to the substrate, allowing for a precise fit.
• Induced Fit Model: This model suggests that the active site undergoes a conformational change upon substrate binding, optimizing the fit and enhancing catalysis.

Conclusion

Enzymes are vital biological catalysts that facilitate a wide range of biochemical reactions essential for life. Understanding their classification, mechanism of action, and factors affecting enzymatic activity is crucial for numerous applications, including medicine, biotechnology, and pharmacology. Continued research in enzymology will enhance our comprehension of enzyme dynamics and pave the way for novel therapeutic approaches to combat various diseases and health challenges. The intricate interplay between enzymes, substrates, and their environment underscores the complexity and elegance of biochemical systems in living organisms.