What Are Enzymes?

Enzymes are biological catalysts—typically proteins—that accelerate chemical reactions in living organisms without being consumed. They lower the activation energy of reactions, increasing reaction rates by factors of up to millions.

Characteristics of Enzymes

• Specificity: Each enzyme binds specific substrates at its active site.
• Efficiency: Can increase reaction rates dramatically.
• Regulation: Activity modulated by inhibitors, activators, and environmental conditions.
• Reusability: Remain unchanged after the reaction and can be used repeatedly.
• Temperature & pH Sensitivity: Have optimal temperature and pH ranges.

Enzyme Classification

Enzymes are categorized by the type of reaction they catalyze (EC number system):

Enzyme Kinetics

Describes how reaction rate varies with substrate concentration. Key concepts:

• V_max: Maximum reaction velocity.
• K_m (Michaelis constant): Substrate concentration at half V_max.
• Michaelis–Menten Equation: v = (V_max[S])/(K_m + [S])

Mechanism of Enzyme Action

Enzymes facilitate reactions by stabilizing the transition state, reducing activation energy, and positioning substrates precisely in the active site.

Models of Enzyme Action

Lock-and-Key Model

Substrate fits exactly into the enzyme’s rigid active site, like a key fits a lock.

Induced Fit Model

Active site changes shape upon substrate binding, enhancing catalytic efficiency.

Factors Affecting Enzyme Activity

• Temperature: Activity rises to an optimum, then denatures enzyme.
• pH: Each enzyme has an optimal pH.
• Substrate Concentration: Higher [S] increases rate until V_max.
• Inhibitors: Competitive, noncompetitive, and uncompetitive inhibitors modulate activity.
• Cofactors & Coenzymes: Non-protein helpers (e.g., metal ions, vitamins) required for activity.