Learning Objectives
- Describe the characteristics of enzymatic reactions.
- Explain enzyme structure, cofactors, and factors affecting enzyme activity.
- Explain substrate specificity and catalytic mechanisms.
- Describe enzyme kinetics using the Michaelis–Menten model.
- Explain regulatory mechanisms of enzymes.
- Differentiate types of enzyme inhibition.
5.1 General Information and Importance of Enzymes
Definition
Enzymes are biological catalysts—mostly proteins—that accelerate biochemical reactions in living systems without being consumed.
Key Characteristics
- High specificity
- High catalytic efficiency
- Regulatory control
- Produced by living cells
Biological and Practical Importance
- Metabolism and cellular function
- Medicine and disease diagnosis
- Food processing and agriculture
- Industrial biotechnology
Examples of Plant Enzymes
- Papaya: Papain, Chymopapain
- Pineapple: Bromelain
Microbial Sources of Enzymes
| Source |
Microorganism |
Enzyme |
Application |
| Bacteria |
Bacillus subtilis |
α-Amylase |
Starch modification |
| Fungi |
Aspergillus niger |
Pectinase |
Fruit juice processing |
| Fungi |
Rhizopus oryzae |
Lipase |
Lipid modification |
| Yeast |
Saccharomyces cerevisiae |
Invertase |
Confectionery |
5.2 Components of Enzymes
Major Components
- Apoenzyme: Protein part of enzyme
- Cofactor: Non-protein component
- Metal ions (e.g., Mg²⁺, Zn²⁺)
- Coenzymes (organic molecules, often vitamin-derived)
- Prosthetic groups (tightly bound cofactors)
- Holoenzyme: Apoenzyme + cofactor
5.3 Enzyme Active Site and Catalysis
Active Site
- Region where substrate binds
- Determines specificity
- Facilitates chemical transformation
Models of Enzyme Action
1. Lock-and-Key Model: Rigid active site fits substrate exactly.
2. Induced Fit Model: Active site changes shape during substrate binding.
Steps of Enzyme-Catalyzed Reaction
E + S ⇌ ES ⇌ EP ⇌ E + P
Role of Enzymes in Energy Reduction
- Lower activation energy
- Increase reaction rate
- Remain unchanged after reaction
5.4 Enzyme Kinetics (Michaelis–Menten Model)
Key Concepts
- Vmax: Maximum reaction velocity
- Km: Substrate concentration at half Vmax
- Enzyme–substrate complex formation
Factors Affecting Enzyme Activity
- Temperature
- pH and buffers
- Substrate concentration
- Enzyme concentration
- Ionic strength
5.5 Enzyme Inhibition
Types of Inhibitors
1. Reversible Inhibition
- Competitive: Competes with substrate for active site
- Uncompetitive: Binds only to ES complex
- Noncompetitive: Binds to enzyme or ES complex at different site
2. Irreversible Inhibition
- Covalent modification of enzyme
- Permanent loss of activity
5.6 Regulation of Enzyme Activity
Major Regulatory Mechanisms
- Gene expression control
- Proteolytic activation (zymogens)
- Covalent modification (e.g., phosphorylation)
- Allosteric regulation
- Enzyme degradation
Conceptual Insight:
In metabolic pathways, regulation often targets the rate-limiting enzyme to efficiently control overall metabolic flux.