Enzymes: The Biological Catalysts

Study Notes

Enzymes Overview

Enzymes are biological catalysts made of proteins that accelerate biochemical reactions without being consumed.
They are classified into six main groups: Hydrolases, Oxidoreductases, Lyases, Transferases, Ligases, and Isomerases.

Hydrolases

Catalyze hydrolysis reactions by adding water to split bonds, such as:
- Carbohydrases (e.g., Sucrase, Diastase)
- Proteases (e.g., Pepsin, Trypsin)
- Esterases (e.g., Lipase, Chlorophyllase)

Oxidoreductases

Catalyze oxidation/reduction reactions, transferring hydrogen, oxygen, or electrons between substrates. Key examples include:
- Oxidases
- Peroxidases
- Dehydrogenases
- Catalase
- Reaction example: 2 H2O2 → 2 H2O + O2

Lyases

Catalyze bond breakage by removing groups from substrates without water addition. Example:
- Zymase transforms sugar into ethanol and carbon dioxide.
- Aldolase splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Isomerases

Facilitate geometric or structural changes within a molecule, forming isomers. Example:
- Phosphoglucoisomerase converts glucose-6-phosphate to fructose-6-phosphate.

Ligases

Catalyze the joining of two molecules, forming new C-O, C-S, C-N, or C-C bonds while breaking down ATP.
Example: Asparagine synthetase and RNA Ligase facilitate the ligation process.

Transferases

Transfer chemical groups from one compound to another, with examples including:
- Transaminases transfer amino groups.
- Hexokinases transfer phosphate groups.

Enzyme Nomenclature

Different naming methods include:
- Trivial names (e.g., Pepsin)
- Substrate-based names (e.g., Sucrose → Sucrase)
- Reaction-based names (e.g., Carboxylation → Carboxylase)
- Enzyme Commission Codes (EC) classify enzymes by their reaction type, functional group, coenzyme, and substrate (e.g., E.C. 1.1.1.1 = alcohol dehydrogenase).

Structure of Enzymes

Enzymes are primarily proteins classified into:
- Simple Protein Enzymes: Composed solely of protein.
- Complex (Conjugated) Protein Enzymes: Consist of protein part (apoenzyme) and non-protein part (cofactor).
- The complete enzyme is referred to as holoenzyme.

Cofactors

Three types of cofactors:
- Prosthetic groups: Tightly bound (e.g., FAD).
- Coenzymes: Temporarily bind during catalysis (e.g., NAD+).
- Activators: Inorganic ions (e.g., Zn²⁺, Ca²⁺) firmly attached to enzymes.

Enzyme Characteristics

Enzymes act in small quantities and are sensitive to temperature and pH.
They exhibit specificity, catalyzing specific reactions without being consumed.
Enzymes possess a colloidal nature.

Mechanism of Enzyme Action

Substrate binds to the enzyme’s active site to form an enzyme-substrate complex (ES).
The complex breaks down to yield products while regenerating the original enzyme.

Factors Affecting Enzyme Activity

Temperature: Reaction rates increase with temperature until a maximum point, after which activity declines.
pH: Each enzyme has an optimal pH, where its activity is maximized; extreme pH levels can deactivate enzymes.
Substrate Concentration: Increased substrate concentration raises reaction rates until the enzyme becomes saturated.
Enzyme Concentration: Higher enzyme amounts generally enhance reaction rates until substrate limitation occurs.
Inhibitors:
- Competitive Inhibitors block the active site, requiring the substrate to compete for binding.
- Non-competitive Inhibitors bind elsewhere on the enzyme, reducing its effectiveness.

Description

This quiz covers the fundamental aspects of enzymes, showcasing their role as biological catalysts that accelerate biochemical reactions. It includes classifications such as hydrolases, oxidoreductases, and others. Test your understanding of how enzymes function without being consumed in the reaction.