Biochemistry Lecture: Enzymes and Kinetics

Introduction to Enzymes

• Definition: Enzymes are biological catalysts that speed up reactions without affecting the thermodynamics (enthalpy or Gibbs free energy) of the reaction.
• Function: They affect reaction kinetics, not thermodynamics, by increasing the rate of reaction.

Classification of Enzymes

1. Oxidoreductases: Catalyze oxidation-reduction reactions.
   • Example: Alcohol dehydrogenases convert alcohols to aldehydes or ketones.
2. Transferases: Transfer functional groups between molecules.
   • Example: Aminotransferases in amino acid degradation.
3. Hydrolases: Catalyze hydrolysis reactions.
4. Lyases: Break chemical bonds by means other than hydrolysis and oxidation.
   • Example: Pyruvate decarboxylase removes CO2 from pyruvate.
5. Isomerases: Catalyze structural shifts in molecules.
   • Example: Ribulose phosphate epimerase.
6. Ligases: Catalyze the ligation (joining) of two substrates.
7. Synthases: Join two molecules by forming new bonds.

How Enzymes Work

• Enzymes provide favorable conditions (charge, pH) and stabilize transition states.
• Enzyme-Substrate Complex: Formation reduces activation energy, essential for catalytic activity.
• Specificity: Enzymes are highly specific to substrates due to the shape of their active sites.
• Enzymes can be regulated by inhibitors or activators, as well as post-translational modifications.

Theories of Enzyme and Substrate Interaction

• Lock and Key Theory: Active site is in perfect conformation for substrate binding.
• Induced Fit Model: Enzyme and substrate undergo conformational changes to achieve binding.

Enzyme Kinetics

• Michaelis-Menten Equation: Describes reaction rates based on substrate and enzyme concentration.
  • Vmax: Maximum reaction rate.
  • Km (Michaelis constant): Substrate concentration at which reaction rate is half of Vmax.
  • Importance of Km: Indicates enzyme affinity for substrate.
• Lineweaver-Burk Plot: Double reciprocal graph for determining kinetic properties like Vmax and Km.

Effects of Local Conditions on Enzyme Activity

• Temperature: Reaction rate doubles with every 10°C increase until the optimal temperature.
• pH: Affects ionization and denaturation; optimal pH for human blood enzymes is 7.4.
• Salinity: High salt levels can disrupt bonds, affecting enzyme conformation and activity.

Regulation of Enzyme Activity

• Feedback Regulation: Controls enzyme activity based on product levels.
  • Feedback Inhibition: Turning off pathways when enough product is made.
  • Reversible Inhibitors: Can be competitive, non-competitive, uncompetitive.
    • Competitive Inhibition: Inhibitors compete with substrate for active site.
    • Non-Competitive Inhibition: Inhibitors bind to allosteric sites, reducing Vmax.
    • Uncompetitive Inhibition: Inhibitors bind to enzyme-substrate complex, affecting both Vmax and Km.
• Regulatory Mechanisms:
  • Allosteric Activation: Involves activators binding to allosteric sites.
  • Phosphorylation: Covalent modification altering activity.
  • Zymogens: Inactive precursors activated by cleavage.

Conclusion

• Enzymes are crucial for metabolic functions, acting as catalysts that are not used up in reactions.
• Understanding enzyme kinetics and regulation is essential for biochemical studies.

• Questions & Further Study: Encouraged to explore enzyme kinetics experiments and regulatory mechanisms further.