Enzymes

1. General Properties of Enzymes

• Biological Catalysts – Speed up reactions without being consumed.
• Highly Specific – Act on specific substrates.
• Work Under Mild Conditions – Function at physiological pH and temperature.
• Regulated Activity – Can be activated or inhibited.
• Lower Activation Energy – Facilitate reactions by stabilizing the transition state.

2. Classification of Enzymes (IUB System)

1. Oxidoreductases – Catalyze oxidation-reduction reactions (e.g., dehydrogenases).
2. Transferases – Transfer functional groups (e.g., kinases).
3. Hydrolases – Catalyze hydrolysis reactions (e.g., lipase, protease).
4. Lyases – Add or remove groups without hydrolysis (e.g., decarboxylases).
5. Isomerases – Catalyze isomerization (e.g., racemases).
6. Ligases (Synthetases) – Join molecules using ATP (e.g., DNA ligase).

3. Mechanism of Enzyme Action

• Step 1: Substrate Binding – Forms enzyme-substrate (ES) complex.
• Step 2: Catalysis – Conversion of substrate into product.
• Step 3: Product Release – Enzyme returns to original state.
• Models of Binding:
  • Lock and Key Model – Exact fit between enzyme & substrate.
  • Induced Fit Model – Enzyme changes shape for substrate binding.

4. Michaelis-Menten Equation & Plot

• Describes the enzyme kinetics of single-substrate reactions.
• Equation: v=Vmax?[S]Km+[S]v = \frac{V_{\max} [S]}{K_m + [S]}v=Km?+[S]Vmax?[S]? Where:
  • vvv = reaction velocity
  • Vmax?V_{\max}Vmax? = maximum velocity
  • [S][S][S] = substrate concentration
  • KmK_mKm? = Michaelis constant (substrate concentration at ½ Vmax?V_{\max}Vmax?)
• Plot: Hyperbolic curve showing the relationship between vvv and [S][S][S].

5. Lineweaver-Burk Equation & Plot

• Double reciprocal form of Michaelis-Menten equation: 1v=KmVmax?[S]+1Vmax?\frac{1}{v} = \frac{K_m}{V_{\max} [S]} + \frac{1}{V_{\max}}v1?=Vmax?[S]Km??+Vmax?1?
• Plot: Straight-line graph (inverse of velocity vs. inverse of substrate).
  • X-axis intercept: −1Km-\frac{1}{K_m}−Km?1?
  • Y-axis intercept: 1Vmax?\frac{1}{V_{\max}}Vmax?1?
  • Slope: KmVmax?\frac{K_m}{V_{\max}}Vmax?Km??

6. Introduction to Allosteric Enzymes

• Regulated by Effectors – Small molecules bind at sites other than the active site.
• Show Sigmoidal Kinetics (not hyperbolic like Michaelis-Menten enzymes).
• Types of Effectors:
  • Activators – Increase enzyme activity.
  • Inhibitors – Decrease enzyme activity.
• Example: Aspartate transcarbamoylase (ATCase).