Knowt
lecture recording on 11 March 2025 at 12.15.23 PM
Enzyme Kinetics and Catalysis
Enzyme-Product Complex: Enzymes bind substrates to form an enzyme-product complex, which then leads to the release of the product.
Effect of Substrate Concentration: Increasing substrate concentration raises the reaction rate by increasing chances for enzyme-substrate interaction.
Limitations of Reaction Rates: For many reactions, non-catalyzed rates are slow; thus, kinetic graphs show how enzyme reactions depend on substrate concentration.
Rate of Reaction and Maximum Velocity
Maximum Rate (Vmax): As substrate concentration increases, a maximum rate is reached when all enzyme active sites are occupied.
Beyond saturation, additional substrate doesn't increase the rate; enzymes become 'busy'.
Analogy to Traffic: Like cars on a road, when all enzyme sites are occupied, adding more substrates leads to waiting, not increased speed.
Michaelis-Menten Kinetics
Michaelis Constant (Km): The substrate concentration at which reaction velocity is half of Vmax (50% saturation).
Interpretations: Lower Km indicates higher affinity of the enzyme for its substrate.
Relationship to Dissociation Constant: Km can approximate the dissociation constant for the enzyme-substrate complex.
Physiological Relevance: Km values are usually within the physiological range, allowing enzymes flexibility in metabolic processes.
High Km means an enzyme needs high substrate concentration to reach Vmax, potentially slow for cell metabolism.
Low Km could lead to constant saturation, limiting regulation and alternative substrate usage.
Enzyme Catalysis and Transition States
Transition State: Enzymes reduce the activation energy by stabilizing the transition state, which is the highest energy state during the conversion of substrates to products.
Three Mechanisms of Stabilization:
Orientation: Enzymes hold substrates in the proper orientation for reaction.
Electrostatic Stabilization: Enzymes provide favorable charge interactions that stabilize the transition state.
Strain Induction: Enzymes strain substrate molecules, encouraging formation of the transition state.
Example: A transition state analog can inhibit an enzyme by mimicking this high-energy state.
Cofactors and Prosthetic Groups
Definition: Non-protein components that assist enzymes, termed cofactors when they assist enzymes and prosthetic groups when they are tightly bound.
Examples:
Heme: A prosthetic group in hemoglobin crucial for oxygen transport.
Magnesium Ions: Used in ATP hydrolysis as a cofactor to stabilize transition states.
Coenzymes: Organic cofactors, such as coenzyme A, serve in metabolic reactions and are derived from vitamins.
Multi-Enzyme Complexes
Importance of Assembly: Some enzymes work efficiently in complexes to pass intermediate products quickly between active sites, preventing loss to diffusion.
Example: Carbamoyl phosphate synthetase in the urea cycle catalyzes three different reactions through internal channels.
Benefit: Ensures the correct sequence of reactions and prevents potential escape or reactivity of unstable intermediates.
Regulation of Enzymes
Need for Regulation: Enzymes require regulation to adapt to changing cellular conditions, ensuring substrate metabolism is appropriate for the cell’s needs.
Feedback Control: Often, downstream products can inhibit earlier enzymes within a pathway, like how ATP inhibits phosphofructokinase in glycolysis when energy is sufficient.
Inhibition Types:
Irreversible Inhibitors: Form permanent bonds (e.g., aspirin with cyclooxygenase), reducing Vmax.
Reversible Inhibitors: Allow temporary inhibition through competitive or non-competitive binding.
Summary
Enzymes enhance the rate of reactions by lowering activation energy and stabilizing transition states. With mechanisms such as orientation, electrostatic effects, and strain, enzymes achieve this effectively. Enzyme activity is finely tuned through cofactors, complex formations, and cellular regulations, enabling organisms to efficiently respond to metabolic needs.