Enzyme Inhibition
Enzymes: Key Concepts and Takeaways
Understanding Enzyme Inhibition
Need to know different types of enzyme inhibition:
Competitive Inhibition
Non-competitive Inhibition
Uncompetitive Inhibition
Feedback Inhibition
Impact on Michaelis-Menten kinetics:
Understanding how each type of inhibition affects the parameters $Km$ (Michaelis constant) and $V{max}$ (maximum velocity).
Must be able to distinguish different types of inhibition using Lineweaver-Burk Plots.
Allostery:
Understanding what allostery is, along with its implications for allosteric inhibition and activation of enzymes.
Impact of allostery on Michaelis-Menten equation leading to the Hill equation:
Cooperativity scenarios:
$n < 1$ (negative cooperativity)
$n > 1$ (positive cooperativity)
$n = 1$ (no cooperativity)
For reference, material is covered in the online book on page 355.
Enzyme Inhibition Overview
Common Drug Targets
Many drugs function as enzyme inhibitors. Here are a few examples:
Statins:
Drug Examples: Atorvastatin, Simvastatin
Target Enzyme: HMG-CoA reductase
Therapeutic Use: Decrease plasma cholesterol levels (Antihyperlipidemic agents).
Allopurinol:
Target Enzyme: Xanthine oxidase
Therapeutic Use: Gout treatment.
Methotrexate:
Target Enzyme: Dihydrofolate reductase
Therapeutic Use: Cancer chemotherapy.
Captopril & Enalapril:
Target Enzyme: Angiotensin-converting enzyme
Therapeutic Use: High blood pressure management.
Dicoumarol:
Target Enzyme: Vitamin K epoxide reductase
Therapeutic Use: Anticoagulant.
Types of Enzyme Inhibition
Competitive Inhibition
Definition: Competitive inhibitors compete with the substrate for the active site of the enzyme, forming an enzyme-substrate complex.
Characteristics:
Typically structurally similar to the normal substrate, allowing competition for the active site.
Inhibition occurs because the enzyme can bind either the substrate or the inhibitor, but not both simultaneously.
Competitive inhibitor binds reversibly to the active site.
High substrate concentration can overcome the competitive inhibition, as it will outcompete the inhibitor for binding.
Applications: Many drugs act as competitive inhibitors due to their structural mimicry of a target enzyme's substrate.
Non-Competitive Inhibition
Definition: Non-competitive inhibitors bind to either the enzyme or the enzyme-substrate complex at a different site than the active site, reducing enzyme activity.
Characteristics:
Binding is reversible and causes a change in the enzyme's three-dimensional structure.
Since binding can occur with substrate and or the enzyme-substrate complex, its effects cannot be overcome by increasing substrate concentration.
Results in a decrease in $V{max}$ but does not affect $Km$ (Michaelis constant).
Example: The action of pepstatin on the enzyme renin is a classic example of non-competitive inhibition.
Uncompetitive Inhibition
Definition: Uncompetitive inhibitors bind to the enzyme only after the substrate has bound, affecting the enzyme-substrate complex.
Characteristics:
Inhibition occurs after substrate binding, leading to the substrate remaining associated with the enzyme.
The inhibitor decreases both substrate affinity (lowering $Km$) and $V{max}$.
Key Point: The effect of uncompetitive inhibition cannot be overcome by adding excess substrate.
Graphical Analysis of Enzyme Inhibition
Lineweaver-Burk Plot Analysis
Utilized to analyze the effects of enzyme inhibitors on kinetic parameters.
Impact of various inhibitors on Lineweaver-Burk Plots:
Competitive Inhibition:
$V_{max}$ is unchanged.
$K_m$ is increased, leading to lines intersecting on the Y-axis.
Non-competitive Inhibition:
$V_{max}$ is decreased.
$K_m$ is unchanged, leading to lines intersecting on the X-axis.
Uncompetitive Inhibition:
Both $V{max}$ and $Km$ decrease, resulting in two parallel lines.
Feedback Inhibition
Mechanism: Pathways are inhibited by their end products. Feedback inhibition helps in regulating metabolic pathways.
General Scheme:
Pathway consists of multiple enzymes converting an initial substrate (e.g., threonine) through intermediates.
Example: As levels of isoleucine rise, it binds to an allosteric site on enzyme 1, inhibiting its activity.
Allostery and Hill Coefficient
Allosteric Modulation
Allostery refers to the regulation of an enzyme's activity through the binding of molecules at sites other than the active site.
Two forms of allostery:
Allosteric Activation: Enhances enzyme activity.
Allosteric Inhibition: Reduces enzyme activity.
Hill Coefficient Quantification
Cooperativity (n):
$n = 1$: No cooperativity
$n > 1$: Positive cooperativity
$n < 1$: Negative cooperativity
The Hill equation is used to quantify the relationship and is expressed as:
Implications for Michaelis-Menten behavior under different conditions of cooperativity.
Graphical representation of the Hill equation shows changes in fraction bound based on cooperative binding effects across varying substrate concentrations, providing insights into the dynamics of enzyme activity.