Enzymes: Biological Catalysis

Chapter Overview

This chapter introduces the fundamental concepts of enzyme function, mechanisms of catalysis, and the kinetic principles underlying enzyme activity. Enzymes are essential biological catalysts that accelerate and regulate biochemical reactions in living cells.

Learning Objectives

• Understand the physicochemistry of enzyme catalysis

• Define and explain activation energy

• Describe general catalysis mechanisms

• Explain enzyme/substrate recognition

• Detail the catalytic mechanisms of lysozyme and chymotrypsin, including the function of key residues

• Apply enzyme kinetics concepts: Michaelis-Menten equation, Km, kcat, Vmax

• Interpret Lineweaver-Burk plots and calculate kinetic parameters

• Understand the meaning of kcat/Km

• Distinguish enzyme inhibition mechanisms (competitive, uncompetitive, noncompetitive, mixed) and their effects on kinetic plots

Introduction to Enzymes

General Properties of Enzymes

Enzymes are biological catalysts, primarily proteins, that accelerate thermodynamically favorable reactions, allowing them to proceed at rates compatible with life.

• Biological Catalysts: Enzymes increase reaction rates without being consumed.

• Specificity: Enzymes are highly selective for their substrates and reactions.

• Kinetic Control: Enzymes enable cells to control the rates of metabolic reactions.

• Temperature and pH Sensitivity: Enzyme activity depends on optimal conditions for their three-dimensional structure.

Physicochemistry of Enzyme Catalysis

Activation Energy and Reaction Rates

Enzymes function by lowering the activation energy required for a reaction to proceed, thereby increasing the rate of product formation.

• Activation Energy (ΔG‡): The energy barrier that must be overcome for reactants to convert to products.

• Transition State Stabilization: Enzymes stabilize the transition state, reducing ΔG‡ and enhancing reaction rates.

• Arrhenius Equation: The rate constant k increases with temperature and decreases with higher activation energy.

Equation:

General Catalysis Mechanisms

Enzymes employ several mechanisms to catalyze reactions:

• General Acid/Base Catalysis: Transfer of protons to stabilize intermediates.

• Covalent Catalysis: Formation of transient covalent bonds with substrates.

• Electrostatic Stabilization: Stabilization of charged intermediates.

• Proximity Effects: Bringing reactants close together to facilitate reaction.

Enzyme/Substrate Recognition

Models of Enzyme Action

Enzyme specificity arises from precise substrate recognition, explained by two models:

• Lock-and-Key Model: Substrate fits exactly into the enzyme's active site.

• Induced Fit Model: Enzyme undergoes conformational change upon substrate binding, optimizing interaction.

Catalytic Mechanisms of Lysozyme and Chymotrypsin

Lysozyme

Lysozyme cleaves polysaccharide chains in bacterial cell walls. Its catalytic mechanism involves acid/base catalysis and stabilization of a carbocation intermediate.

• Key Residues: Glu35 (acts as acid), Asp52 (acts as base)

• Mechanism: Glu35 donates a proton, Asp52 stabilizes the transition state and attacks the carbocation.

• pH Dependence: Activity depends on the protonation state of Glu35 and Asp52.

Chymotrypsin

Chymotrypsin is a serine protease that hydrolyzes peptide bonds. Its mechanism involves a catalytic triad and formation of a tetrahedral intermediate.

• Catalytic Triad: Ser195 (nucleophile), His57 (base), Asp102 (stabilizes His57)

• Mechanism: Ser195 attacks the peptide bond, His57 accepts a proton, Asp102 stabilizes the charge.

• Oxyanion Hole: Stabilizes the tetrahedral intermediate during catalysis.

Enzyme Kinetics

Michaelis-Menten Equation

The Michaelis-Menten equation describes the rate of enzymatic reactions as a function of substrate concentration.

• Initial Rate (v): Rate of product formation when substrate is in excess.

• Km (Michaelis constant): Substrate concentration at which the reaction rate is half of Vmax.

• Vmax: Maximum rate achieved when enzyme is saturated with substrate.

• kcat (Turnover Number): Number of substrate molecules converted per enzyme per second.

Equation:

Lineweaver-Burk Plot

A double reciprocal plot used to determine Km and Vmax from experimental data.

Equation:

• X-intercept:

• Y-intercept:

kcat/Km: Enzyme Efficiency

The ratio kcat/Km is a measure of catalytic efficiency and substrate specificity. It is especially useful under physiological conditions where substrate concentration is low.

• High kcat/Km: Indicates high efficiency; approaches the diffusion limit (~108–109 M-1s-1).

• Low Km: Indicates tight substrate binding.

Enzyme Inhibition Mechanisms

Types of Inhibition

Enzyme inhibitors reduce or abolish enzyme activity. They are classified as reversible or irreversible.

• Competitive Inhibition: Inhibitor competes with substrate for active site. Increases apparent Km, Vmax unchanged.

• Uncompetitive Inhibition: Inhibitor binds only to ES complex. Decreases both Km and Vmax.

• Noncompetitive Inhibition: Inhibitor binds to enzyme or ES complex at a site other than the active site. Vmax decreases, Km unchanged.

• Mixed Inhibition: Inhibitor binds to both enzyme and ES complex, affecting both Km and Vmax.

• Irreversible Inhibition: Inhibitor covalently modifies the enzyme, permanently inactivating it.

Regulation of Enzyme Activity

Mechanisms of Regulation

Cells regulate enzyme activity to maintain homeostasis and respond to changing conditions.

• Substrate Level Control: Reaction rate increases with substrate concentration.

• Product Inhibition: Accumulation of product inhibits enzyme activity.

• Feedback Inhibition: End product of a pathway inhibits an earlier step.

• Allosteric Regulation: Effectors bind to sites other than the active site, causing conformational changes.

• Covalent Modification: Reversible (e.g., phosphorylation) or irreversible (e.g., zymogen activation).

Allosteric Enzymes

Allosteric enzymes are often multisubunit proteins that display cooperative substrate binding and are regulated by effectors.

• Sigmoidal Kinetics: v vs. [S] plots are sigmoidal, not hyperbolic.

• Homotropic Allostery: Substrate itself acts as an effector.

• Heterotropic Allostery: Other molecules act as effectors.

Cofactors and Coenzymes

Role in Catalysis

Many enzymes require non-protein molecules for activity.

• Cofactors: Inorganic ions or organic molecules required for enzyme function.

• Coenzymes: Complex organic molecules, often derived from vitamins.

• Metalloenzymes: Enzymes containing metal ions as cofactors.

Summary Table: Key Kinetic Parameters

Example Applications

• Drug Design: Many drugs are enzyme inhibitors (e.g., ACE inhibitors for hypertension).

• Metabolic Regulation: Feedback inhibition and allosteric control are crucial for metabolic homeostasis.

• Biotechnology: Enzyme kinetics guide the optimization of industrial and research applications.

Additional info: These notes expand on the brief points in the slides, providing definitions, equations, and context for each topic. Tables have been inferred and constructed to summarize inhibition types and kinetic parameters for clarity.