Chapter 9: Enzyme Mechanisms and Inhibitors

Basic Catalytic Strategies Used by Many Enzymes

• Enzymatic Function: Enzymes catalyze a vast array of chemical reactions primarily by facilitating the formation of the transition state.

• Common Strategies: Despite the diversity in reactions, most enzymes commonly employ one or more of the following strategies to catalyze specific reactions:

  1. Covalent catalysis

  2. General acid–base catalysis

  3. Metal ion catalysis

  4. Catalysis by approximation and orientation

1. Covalent Catalysis

• Mechanism: The active site contains a highly reactive group, typically a powerful nucleophile.

• Modification: This nucleophilic group becomes temporarily covalently modified during the catalytic process.

2. General Acid–Base Catalysis

• Mechanism: A molecule other than water plays a crucial role as either a proton donor (general acid) or a proton acceptor (general base).

• Example: Chymotrypsin, a digestive enzyme, utilizes a histidine residue as a general base catalyst. This action enhances the nucleophilic power of a nearby serine residue, which is essential for its catalytic mechanism.

3. Metal Ion Catalysis

• Diverse Roles: Metal ions can function catalytically in several significant ways:

  • Electrophilic Catalyst: A metal ion can serve as an electrophilic catalyst, effectively stabilizing a negative charge that develops on a reaction intermediate during catalysis.

  • Nucleophile Generation: By increasing the acidity of a nearby molecule, such as water, a metal ion can facilitate the generation of a potent nucleophile.

  • Substrate Binding Enhancement: A metal ion may bind directly to the substrate, thereby increasing the number and strength of interactions with the enzyme. This enhances the binding energy and helps position the substrate optimally for catalysis.

  • Cofactor: Many enzymes rely on metal ions to function as essential cofactors.

4. Catalysis by Approximation and Orientation

• Mechanism: Many reactions involve two distinct substrates. In such instances, the reaction rate can be considerably enhanced by the enzyme physically bringing these two substrates into close proximity and orienting them precisely on a single binding surface. This increases the effective concentrations and correct angles for reaction.

Enzyme Activity Can Be Modulated by Temperature, pH, and Inhibitory Molecules

Environment Affects Catalytic Rates

Effect of Temperature

• Enzymatic activity is highly sensitive to temperature. Generally, reaction rates increase with temperature (due to increased kinetic energy), but beyond an optimal temperature, enzymes can undergo denaturation, leading to a loss of activity.

• Example: Enzymes from thermophilic archaea are adapted to function optimally at very high temperatures, showcasing natural adaptations to extreme environments.

Effect of pH

• The $pH$ of the environment significantly influences enzyme activity.

• Basis: The $pH$ dependence of enzymes is primarily due to the ionization states of the ionizable R groups (side chains) located within the enzyme's active site. These groups must be in a specific protonation state for proper substrate binding and catalytic function.

Enzyme Inhibition

• Inhibitors: These are compounds that decrease an enzyme's activity.

Irreversible Inhibitors (Inactivators)

• Mechanism: Irreversible inhibitors react with the enzyme, forming stable covalent bonds or extremely tight non-covalent associations with specific functional groups, often within or near the active site.

• Effect: A single molecule of an irreversible inhibitor can permanently shut off one molecule of enzyme.

• Significance: While often powerful toxins (e.g., nerve agents), some irreversible inhibitors are invaluable as drugs (e.g., certain antibiotics) due to their potent and specific action.

Reversible Inhibitors

• Mechanism: Reversible inhibitors bind to the enzyme through non-covalent interactions and can readily dissociate from it.

• Nature: They are frequently structural analogs of the enzyme's natural substrates or products.

• Outcome: They can prevent the substrate from binding to the enzyme, or, if they bind to the enzyme–substrate complex, they can prevent the catalytic reaction from proceeding.

• Application: Reversible inhibitors are commonly employed as drugs to modulate or slow down the activity of specific enzymes for therapeutic purposes.

• Three Common Types:

  1. Competitive inhibition

  2. Uncompetitive inhibition

  3. Noncompetitive inhibition

1. Competitive Inhibition

• Mechanism: The inhibitor is structurally similar to the enzyme's natural substrate and directly competes with the substrate for binding to the active site.

• Effect: By occupying the active site, the inhibitor prevents the actual substrate from binding, thus impeding catalysis.

• Overcoming Inhibition: This type of inhibition can be overcome by significantly increasing the concentration of the substrate, outcompeting the inhibitor for active site binding.

2. Uncompetitive Inhibition

• Mechanism: The inhibitor binds exclusively to the enzyme-substrate ($ES$) complex, not to the free enzyme. It binds at a distinct site that becomes accessible only after the substrate has bound.

• Effect: This binding effectively locks the substrate in place within the $ES$ complex, preventing the conversion of substrate to product.

• Substrate Binding: It does not hinder the initial binding of the substrate to the free enzyme.

3. Noncompetitive Inhibition

• Mechanism: The inhibitor can bind to either the free enzyme ($E$) or the enzyme-substrate ($ES$) complex. Its binding site is distinct from the active site.

• Effect: Binding of the inhibitor decreases the enzyme's overall catalytic efficiency by reducing the number of functional active enzyme molecules and/or their turnover number ($k_{cat}$). This inhibition cannot be overcome by simply adding more substrate.

• Types of Noncompetitive Inhibition:

  • Pure Noncompetitive Inhibition: In this specific scenario, the inhibitor binds equally well to the free enzyme and the enzyme-substrate complex ($K<em>I = K</em>{I'}$).

    • Kinetics: Only the turnover number ($k<em>{cat}$ or $V</em>{max}$) is decreased; the enzyme's affinity for the substrate ($K_M$) remains unchanged.

  • Mixed Noncompetitive Inhibition: Here, the inhibitor binds preferentially to either the free enzyme or the enzyme-substrate complex ($K<em>I eq K</em>{I'}$).

    • Kinetics: This results in a more complex pattern of inhibition, simultaneously altering both the apparent substrate binding affinity ($K<em>M^{app}$ which may increase or decrease) and the apparent maximal reaction rate ($V</em>{max}^{app}$ which always decreases).

Reversible Inhibitors are Kinetically Distinguishable (Using Lineweaver–Burk Plots)

Competitive Inhibition Kinetics

• Effect on $V<em>{max}$: There is no change in the apparent maximal reaction velocity ($V</em>{max}^{app}$ remains equal to $V_{max}$).

• Effect on $K<em>M$: The apparent Michaelis constant ($K</em>M^{app}$) is increased, meaning a higher substrate concentration is required to reach half the maximal velocity.

• Lineweaver-Burk Plot: When $1/V<em>0$ is plotted against $1/[S]$, the lines representing inhibited and uninhibited reactions intersect at the y-axis, indicating that $1/V</em>{max}$ is the same. The slope of the inhibited line ($K<em>M^{app}/V</em>{max}$) is greater than that of the uninhibited line ($K<em>M/V</em>{max}$).

Uncompetitive Inhibition Kinetics

• Effect on $V<em>{max}$: A decrease in the apparent maximal reaction velocity ($V</em>{max}^{app}$).

• Effect on $K<em>M$: An apparent decrease in the Michaelis constant ($K</em>M^{app}$). This is because both $V<em>{max}$ and $K</em>M$ are decreased by the same factor, resulting in no change in the ratio $K<em>M/V</em>{max}$. Consequently, the $K<em>M^{app}/V</em>{max}^{app}$ ratio remains unchanged.

• Lineweaver-Burk Plot: The lines representing inhibited and uninhibited reactions are parallel, indicating that the slope ($K<em>M/V</em>{max}$) is the same. The y-intercept ($1/V_{max}^{app}$) is higher in the presence of the inhibitor.

Pure Noncompetitive Inhibition Kinetics

• Effect on $V<em>{max}$: A decrease in the apparent maximal reaction velocity ($V</em>{max}^{app}$).

• Effect on $K<em>M$: There is no change in the Michaelis constant ($K</em>M$).

• Lineweaver-Burk Plot: The lines intersect on the x-axis (meaning $1/K<em>M$ is the same). The y-intercept ($1/V</em>{max}^{app}$) is increased, and the slope ($K<em>M/V</em>{max}^{app}$) is also increased, by the same factor.

Mixed Noncompetitive Inhibition Kinetics

• Nature: In mixed noncompetitive inhibition, the inhibitor binds preferentially to either the free enzyme ($E$) or the enzyme–substrate complex ($ES$), but still binds to both. Thus, its kinetic behavior is a blend of noncompetitive and either competitive or uncompetitive inhibition characteristics.

• Effect on $V<em>{max}$: The apparent maximal velocity ($V</em>{max}^{app}$) always decreases.

• Effect on $K<em>M$: The apparent Michaelis constant ($K</em>M^{app}$) may either increase or decrease, depending on the inhibitor's preference for $E$ or $ES$.

• Lineweaver-Burk Plot: The plot resembles that of a pure noncompetitive inhibitor, but the lines converge (intersect) either above or below the x-axis, not directly on it, reflecting the complex changes in both $V<em>{max}$ and $K</em>M$.

Lineweaver–Burk of Reversible Inhibition Summary

• Competitive Inhibition: The intercept on the y-axis ($1/V<em>{max}$) is the same in the presence and absence of inhibitor. The slope ($K</em>M/V_{max}$) is increased for the inhibited enzyme.

• Uncompetitive Inhibition: The slope of the line is the same as that for the uninhibited enzyme. The intercept on the y-axis ($1/V_{max}$) is increased.

• Noncompetitive Inhibition:

  • Pure Noncompetitive: The values of both the y-intercept ($1/V<em>{max}^{app}$) and the slope ($K</em>M/V_{max}^{app}$) are larger by the same factor compared to the uninhibited reaction.

  • Mixed Noncompetitive: In this case, $V<em>{max}$ always decreases, and $K</em>M$ may increase or decrease. The plot shows lines converging either above or below the x-axis, rather than on it. (Note: The slide summary here states $K<em>M$ is not affected for mixed, but the detailed explanation for mixed kinetics indicated $K</em>M$ 'may increase or decrease'. We follow the detailed kinetic explanation for greater accuracy).

Example: Penicillin Is an Irreversible Inhibitor of a Peptidase

• Structure: Penicillin is an antibiotic characterized by a thiazolidine ring fused to a highly reactive $\beta$-lactam ring.

• Mechanism as an Antibiotic: Penicillin acts by irreversibly inhibiting glycopeptide transpeptidase, an enzyme crucial for the formation of bacterial cell walls (e.g., in Staphylococcus aureus). By targeting this enzyme, penicillin disrupts bacterial cell wall synthesis, leading to bacterial lysis and death.

Example: Chymotrypsin Illustrates Basic Principles of Catalysis and Inhibition

• Chymotrypsin, a protease, employs a combination of the basic catalytic strategies: covalent catalysis, general acid–base catalysis, and catalysis by approximation and orientation.

Chymotrypsin Action Proceeds in Two Steps

• Fundamentals: The enzymatic reaction of chymotrypsin proceeds through the formation of a transient, covalently bound enzyme–substrate intermediate.

• Step 1: Acyl-enzyme Formation (Rapid Phase)

  • The acyl group of the substrate becomes covalently attached to the active site Serine $195$ of the enzyme.

  • During this process, the first product, p-nitrophenolate (or the amino component of the peptide), is released.

  • This intermediate complex, where the substrate's acyl group is covalently linked to the enzyme, is termed the acyl-enzyme intermediate.

  • One molecule of p-nitrophenolate is produced rapidly for each enzyme molecule as the acyl-enzyme intermediate is formed.

• Step 2: Acyl-enzyme Hydrolysis (Slower Phase)

  • The acyl-enzyme intermediate undergoes hydrolysis (reaction with water).

  • This hydrolysis releases the carboxylic acid component of the original substrate.

  • Crucially, this step regenerates the free, active enzyme, effectively