Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process. Most enzymes are proteins, though some RNA molecules (ribozymes) also exhibit catalytic activity.

Classification (IUBMB System)

Enzymes are classified into six major classes based on the type of reaction they catalyze:

The active site is a specific region on the enzyme where the substrate binds and the reaction occurs. It is usually a small, 3D pocket or cleft.

Mechanism of Enzyme Action

Enzymes work by lowering the activation energy required for a reaction to proceed. Two main models describe substrate binding:

• Lock-and-Key Model: Suggests the enzyme's active site is a rigid, perfect match for the substrate.
• Induced-Fit Model: Suggests the active site is flexible and changes shape to wrap more tightly around the substrate upon binding.

Enzyme kinetics is the study of the rates of enzyme-catalyzed reactions.

Michaelis-Menten Equation

This fundamental equation relates the reaction velocity (v) to the substrate concentration ([S]):

v = (Vmax * [S]) / (Km + [S])

• Vmax: The maximum velocity reached when the enzyme is saturated with substrate.
• Km (Michaelis Constant): The substrate concentration at which the velocity is half of Vmax. A low Km indicates high affinity for the substrate.

Enzyme activity is highly sensitive to the environment:

• Temperature: Each enzyme has an optimum temperature; high heat causes denaturation.
• pH: Changes in pH can alter the ionization state of the active site.
• Substrate Concentration: Increasing [S] increases velocity until Vmax is reached.

Allosteric Regulation

Allosteric enzymes have a "regulatory site" (other than the active site) where an effector binds. Binding can either increase (activator) or decrease (inhibitor) the enzyme's activity.

Inhibitors are substances that reduce the rate of enzyme-catalyzed reactions.

• Competitive Inhibition: The inhibitor resembles the substrate and competes for the active site. Km increases, but Vmax remains unchanged.
• Non-competitive Inhibition: The inhibitor binds to a site other than the active site, changing the enzyme's shape. Vmax decreases, but Km remains unchanged.
• Uncompetitive Inhibition: The inhibitor binds only to the Enzyme-Substrate (ES) complex. Both Vmax and Km decrease.

Some enzymes require non-protein components to function:

• Apoenzyme: The protein part of the enzyme (inactive).
• Holoenzyme: The complete, active enzyme (Apoenzyme + Cofactor).
• Cofactors: Can be inorganic metal ions (e.g., Mg2+, Zn2+) or organic molecules (Coenzymes).
• Prosthetic Group: A cofactor that is tightly or covalently bound to the enzyme.
• Ribozymes: RNA molecules that act as enzymes, such as those in the ribosome involved in peptide bond formation.

7. Exam Focus: Critical Concepts & FAQs

Common Mistakes

• Mistake: Confusing Km with Vmax. Correction: Km is a concentration; Vmax is a rate.
• Mistake: Thinking all enzymes are proteins. Correction: Ribozymes are non-protein RNA catalysts.

Frequently Asked Questions

Q: What is activation energy?
A: It is the minimum energy required to start a chemical reaction. Enzymes decrease this energy barrier, allowing reactions to happen faster at body temperature.

Q: How does an allosteric inhibitor differ from a competitive one?
A: A competitive inhibitor fights for the active site, while an allosteric inhibitor binds elsewhere to change the active site's shape.