Enzyme Properties and Kinetics

Properties of Enzymes and Enzyme Kinetics

• Enzymes are biological catalysts that accelerate reaction rates without changing the final equilibrium of the reactants and products.
• Catalase exemplifies enzyme efficiency, breaking down hydrogen peroxide (H2O2) to water at a rate 10^14 times faster than the uncatalyzed reaction at 30°C.
• Enzyme structure consists of one or more folded polypeptide chains stabilized by weak bonds such as hydrogen bonds, electrostatic interactions, Van der Waals forces, and hydrophobic interactions.

Enzyme Activity and Mechanisms

• Enzymes reduce activation energy barriers, facilitating reactions.
• Lock and key model: Proposed by Emil Fisher, where the enzyme is complementary to the substrate.
• Induced fit theory, introduced by Daniel Koshland, suggests that enzyme conformation changes upon substrate binding, optimizing the active site for catalysis.

Kinetics of Enzyme Reactions

• Initial velocity (v0), Km (Michaelis constant), and Vmax (maximum velocity) are crucial kinetic parameters.
• Michaelis-Menten equation describes the rate of enzyme-catalyzed reactions based on substrate concentration.
• Reaction rate is calculated by measuring the increase of product or decrease of substrate over time.
• Many enzymes exhibit hyperbolic kinetics: at low substrate concentrations, velocity is directly proportional to substrate concentration; at high concentrations, the rate approaches Vmax and becomes independent of substrate concentration.

Enzyme Inhibition

• Competitive inhibitors bind to the active site, increasing Km (the concentration of substrate needed to reach half of Vmax) without changing Vmax.
• Non-competitive inhibitors bind elsewhere on the enzyme, reducing Vmax while leaving Km unchanged.
• Reversible inhibition can be reversed by removing the inhibitor, whereas irreversible inhibition permanently inactivates the enzyme.

Enzyme Regulation

• Allosteric modulation involves changes in enzyme activity due to binding at sites other than the active site. Allosteric activators enhance activity while inhibitors decrease it.
• Cooperativity refers to the phenomenon where the binding of a substrate to one active site influences the binding properties of other active sites on the enzyme.

Substrate Specificity and Classification

• Enzymes demonstrate specificity, often catalyzing one type of reaction and acting on related molecules (group specificity).
• Classification by the International Union of Biochemistry (I.U.B.) categorizes enzymes into six main classes based on their catalytic activities:
  • Oxidoreductases: Catalyze oxidation/reduction reactions (e.g., lactate dehydrogenase).
  • Transferases: Transfer functional groups between substrates (e.g., alanine aminotransferase).
  • Hydrolases: Catalyze hydrolytic reactions (e.g., trypsin).
  • Lyases: Catalyze non-hydrolytic additions or removals of groups.
  • Ligases: Join molecules through the synthesis of new bonds, coupled with ATP hydrolysis.

Key Terms

• Enzyme-substrate complex (ES): The intermediate formed when a substrate binds to the enzyme.
• Transition state: A high-energy state during a reaction that enzymes help stabilize, lowering activation energy.
• Km: Reflects the substrate concentration at which the reaction rate is half of Vmax; important for assessing enzyme efficiency.
• Vmax: Maximum reaction rate achieved by an enzyme at saturation with substrate.