Exam 2 Lecture 7
Chapter 1: Introduction to Enzyme Inhibition
Topics covered: Enzyme inhibitors and allosteric enzymes.
Objective to complete discussion on inhibitors today and begin allosteric enzymes.
Allosteric enzymes will be wrapped up on Friday, followed by a review of short chapter eight, which is favorable for exam preparation due to its straightforward nature.
Harder concepts from Chapter 9 will be postponed until after the exam.
Chapter 7: Enzyme Inhibitors
Division of enzyme inhibitors into two main categories:
Reversible inhibitors
Irreversible inhibitors
Reversible Inhibitors
Interactions between reversible inhibitors and enzymes occur through non-covalent interactions.
Reversible inhibitors allow for transient interactions, which can be visualized through an equilibrium model:
Example: Dihydrofolate reductase enzyme with the reversible inhibitor methotrexate (shown in green).
Key characteristics:
On and off binding of the inhibitor occurs; when bound, the enzyme-inhibitor complex is stabilized by non-covalent forces.
Irreversible Inhibitors
Irreversible inhibitors have a longer duration of action due to the formation of covalent bonds between the inhibitor and the enzyme.
Stability of the enzyme-inhibitor complex is higher compared to reversible inhibitors.
Risks associated with irreversible inhibitors:
Potential for unpredictable and toxic side effects limits their use in pharmaceuticals.
Increasing clinical relevance as strategies evolve to moderate these risks.
Example: Penicillin acts as an irreversible inhibitor but with a caveat of its reactive bond formation leading to covalent interactions.
Classes of Irreversible Inhibitors
Group Specific Reagents:
React with specific functional groups to form covalent enzyme-inhibitor bonds.
Affinity Labels:
Substrate analogs resemble the substrate, bind to the active site, and possess chemically reactive groups allowing the formation of covalent bonds.
Mechanism-Based (Suicide) Inhibitors:
Utilize the enzyme's own catalytic mechanism to inhibit itself.
Example: Penicillin targets glycopeptide transpeptidase, an enzyme that produces cross-links in bacterial cell walls.
Mechanism involves penicillin mimicking the substrate (D-ala-D-ala motif) leading to a nucleophilic attack by the serine in the active site.
Chapter 2: Covalent Enzyme Inhibitors
Transition state analogs:
Not classified as irreversible since they do not form covalent bonds with enzymes.
Bind with such high affinity that interactions resemble irreversible inhibitors due to long lifetimes in the active site.
Mechanism-Based Inhibitors
Known as suicide inhibitors because their inactivation results from the enzyme's execution of its own catalytic function.
Example: Interaction of penicillin with glycopeptide transpeptidase leads to irreversible inactivation of the enzyme.
Comparison between natural substrate and penicillin shows detailed mechanistic interactions leading to inhibitor bond formation.
Chapter 3: Types of Reversible Inhibition
Focus on four types of reversible inhibitors:
Competitive Inhibitors:
Bind directly to the free enzyme, competing with the substrate for the active site.
Look similar to the substrate allowing for effective competition.
Uncompetitive Inhibitors:
Bind exclusively to the enzyme-substrate (ES) complex after substrate interaction.
Mixed Inhibition:
Binds to either free enzyme or ES complex, impacting both types but with variances in affinity.
Pure Non-competitive Inhibition:
Has an equal affinity for both free enzyme and ES complex.
Significant distinctions between competitive, uncompetitive, mixed, and pure non-competitive inhibitors elucidated in detail.
Parameters and Kinetic Analysis
Discussion of kinetic parameters pertaining to enzyme kinetics and inhibitor effects, particularly:
Km (Michaelis constant)
Vmax (maximum velocity)
In the presence of inhibitors, new terms are introduced:
Km apparent reflects the altered conditions due to inhibitors.
Vmax apparent as altered maximum velocity in the presence of inhibitors.
Competitive Inhibition Effects
Competitive inhibitors increase Km, showing reduced affinity of substrate due to inhibition, while leaving Vmax unchanged:
Vmax apparent equals Vmax
Km apparent is relative to the concentration of the inhibitor and is described by factor alpha:
Uncompetitive Inhibition Effects
In contrast, uncompetitive inhibition leads to a decrease in both Km and Vmax:
Km apparent reduced by factor alpha.
Vmax apparent diminished, indicating fewer products can be formed due to inhibitor binding.
Non-competitive vs. Mixed Inhibition
Pure non-competitive inhibitors affect Vmax without impacting Km, whereas mixed inhibitors can elevate or reduce Km based on binding affinities.
Chapter 4: Graphical Representations
The various types of inhibitors and their effects can be depicted through:
Michaelis-Menten plot: Shows reaction velocity vs. substrate concentration; competitive inhibition shifts the curve but maintains the same Vmax.
Lineweaver-Burk plot: Displays reciprocal values, showing intersections at varied points based on inhibitor type, corroborating inhibitor actions mathematically.
Chapter 5: Summary of Inhibition Types
Reiterated the summary of the different types of reversible inhibition:
Competitive inhibition: Km increases, Vmax remains constant.
Uncompetitive inhibition: Both Km and Vmax decrease.
Mixed inhibition: Varies; Km may increase or decrease and Vmax always declines.
Pure non-competitive inhibition: Km stable, Vmax diminishes.
Closing remarks meliorating the connection of various enzyme inhibitors in practical applications and research.
Chapter 1: Introduction to Enzyme Inhibition
Topics covered: Enzyme inhibitors and allosteric enzymes.
Objective to complete discussion on inhibitors today and begin allosteric enzymes.
Allosteric enzymes will be wrapped up on Friday, followed by a review of short chapter eight, which is favorable for exam preparation due to its straightforward nature.
Harder concepts from Chapter 9 will be postponed until after the exam.
Chapter 7: Enzyme Inhibitors
Division of enzyme inhibitors into two main categories:
Reversible inhibitors
Irreversible inhibitors
Reversible Inhibitors
Interactions between reversible inhibitors and enzymes occur through non-covalent interactions, such as hydrogen bonds, ionic bonds, and hydrophobic interactions.
Reversible inhibitors allow for transient interactions, which can be visualized through an equilibrium model:
Example: Dihydrofolate reductase enzyme with the reversible inhibitor methotrexate (shown in green).
Key characteristics:
On and off binding of the inhibitor occurs; when bound, the enzyme-inhibitor complex is stabilized by non-covalent forces.
Irreversible Inhibitors
Irreversible inhibitors have a longer duration of action due to the formation of stable covalent bonds between the inhibitor and functional groups within the enzyme's active site.
Stability of the enzyme-inhibitor complex is higher compared to reversible inhibitors.
Risks associated with irreversible inhibitors:
Potential for unpredictable and toxic side effects limits their use in pharmaceuticals.
Increasing clinical relevance as strategies evolve to moderate these risks.
Example: Penicillin acts as an irreversible inhibitor but with a caveat of its reactive bond formation leading to covalent interactions.
Classes of Irreversible Inhibitors
Group Specific Reagents:
React with specific functional groups (e.g., serine, cysteine, histidine residues) to form covalent enzyme-inhibitor bonds, altering the enzyme's structure or catalytic activity.
Affinity Labels:
Substrate analogs resemble the substrate, bind to the active site, and possess chemically reactive groups (e.g., halomethyl ketones) allowing the formation of stable covalent bonds with nearby amino acid residues, leading to irreversible inactivation.
Mechanism-Based (Suicide) Inhibitors:
Utilize the enzyme's own catalytic mechanism to convert a relatively unreactive inhibitor precursor into a highly reactive species at the active site, which then forms a covalent bond, thereby inactivating itself. This process is often irreversible and highly specific.
Example: Penicillin targets glycopeptide transpeptidase, an enzyme that produces cross-links in bacterial cell walls.
Mechanism involves penicillin mimicking the D-ala-D-ala motif of the natural substrate, leading to a nucleophilic attack by the active site serine residue of the transpeptidase. This forms a stable, covalent penicilloyl-enzyme intermediate that is resistant to hydrolysis, thereby irreversibly inhibiting bacterial cell wall synthesis.
Chapter 2: Covalent Enzyme Inhibitors
Transition state analogs:
Not classified as irreversible since they do not form covalent bonds with enzymes, rather they mimic the unstable transition state of the substrate. They bind with such high affinity (due to their structural resemblance to the transition state) that their interactions resemble irreversible inhibitors due to long lifetimes (very slow dissociation rates) in the active site.
Mechanism-Based Inhibitors
Known as suicide inhibitors because their inactivation results from the enzyme's execution of its own catalytic function.
Example: Interaction of penicillin with glycopeptide transpeptidase leads to irreversible inactivation of the enzyme.
Comparison between natural substrate and penicillin shows detailed mechanistic interactions leading to inhibitor bond formation.
Chapter 3: Types of Reversible Inhibition
Focus on four types of reversible inhibitors:
Competitive Inhibitors:
Bind directly to the free enzyme, competing with the substrate for the active site. They often resemble the substrate structurally, preventing substrate binding and thus reducing the effective enzyme concentration available for catalysis.
Uncompetitive Inhibitors:
Bind exclusively to the enzyme-substrate (ES) complex, typically at an allosteric site formed only after substrate interaction. This binding distorts the active site, hindering catalysis and often preventing product release.
Mixed Inhibition:
Binds to either free enzyme or ES complex, but with different affinities. This can impact both substrate binding (Km) and catalytic turnover (Vmax).
Pure Non-competitive Inhibition:
A special case of mixed inhibition where the inhibitor has an equal affinity for both the free enzyme and the ES complex. This primarily affects the catalytic efficiency (Vmax) without altering substrate binding affinity (Km).
Significant distinctions between competitive, uncompetitive, mixed, and pure non-competitive inhibitors elucidated in detail.
Parameters and Kinetic Analysis
Discussion of kinetic parameters pertaining to enzyme kinetics and inhibitor effects, particularly:
Km (Michaelis constant): Represents the substrate concentration at which the reaction velocity is half of Vmax. It serves as an inverse measure of the enzyme's affinity for its substrate; a lower Km indicates higher affinity.
Vmax (maximum velocity): The maximum reaction rate achievable when the enzyme is saturated with substrate, reflecting the enzyme's catalytic efficiency.
In the presence of inhibitors, new terms are introduced:
Km apparent reflects the altered conditions due to inhibitors.
Vmax apparent as altered maximum velocity in the presence of inhibitors.
Competitive Inhibition Effects
Competitive inhibitors increase Km, showing reduced apparent affinity of substrate due to the need for higher substrate concentrations to outcompete the inhibitor, while leaving Vmax unchanged because, at very high substrate concentrations, the inhibitor can be effectively overcome:
Vmax apparent equals Vmax
Km apparent is relative to the concentration of the inhibitor and is described by factor alpha:
The factor alpha () quantifies the effect of the inhibitor on Km apparent relative to the free inhibitor concentration and its dissociation constant (Ki).
Uncompetitive Inhibition Effects
In contrast, uncompetitive inhibition leads to a decrease in both Km and Vmax. The inhibitor binding to the ES complex effectively removes active enzyme, reducing the total enzyme available for catalysis (lowers Vmax) and also appears to increase the enzyme's affinity for the substrate (lowers Km) as the ES complex is stabilized by inhibitor binding or effectively removed from the equilibrium.
Non-competitive vs. Mixed Inhibition
Pure non-competitive inhibitors affect Vmax without impacting Km, whereas mixed inhibitors can elevate or reduce Km based on binding affinities.
Chapter 4: Graphical Representations
The various types of inhibitors and their effects can be depicted through:
Michaelis-Menten plot: Shows reaction velocity vs. substrate concentration; competitive inhibition shifts the curve to the right (higher apparent Km) but maintains the same Vmax, whereas uncompetitive inhibition lowers both Vmax and Km, resulting in a different shape.
Lineweaver-Burk plot: Displays reciprocal values (1/V vs. 1/[S]), providing linear plots that clearly differentiate inhibition types. Competitive inhibition lines intersect at the Y-axis (same 1/Vmax), uncompetitive lines are parallel, and mixed inhibition lines intersect at a point not on either axis.
Chapter 5: Summary of Inhibition Types
Reiterated the summary of the different types of reversible inhibition:
Competitive inhibition: Km increases, Vmax remains constant.
Uncompetitive inhibition: Both Km and Vmax decrease.
Mixed inhibition: Varies; Km may increase or decrease and Vmax always declines.
Pure non-competitive inhibition: Km stable, Vmax diminishes.
Closing remarks meliorating the connection of various enzyme inhibitors in practical applications and research.