Enzyme Inhibition

Reversible Enzyme Inhibition

Competitive inhibitors

• Competitive inhibition: Inhibitor resembles the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes and thus competes with it for the binding site at the active center of the enzyme.
  • Inhibitor binds at the active center, blocking the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes from interacting with the binding site.
  • Inhibitor can be overcome through an excess of the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes; therefore, this inhibition is reversible.
  • Decreases affinity of the enzyme for the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes
• As affinity decreases, the K_m value increases, since an increased substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentration is required to obtain the half-maximal velocity.
• V_max is not changed, however.
• Changes in Michaelis-Menten curve:
  • The increase in K_m will cause the curve to shift to the right of the graph.
  • The reaction will eventually reach V_max but will require much higher substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes concentrations to do so.
  • The curve shifts to the right without changing the height of the curve.
• Changes in Lineweaver-Burk plot Lineweaver-Burk plot Enzyme Kinetics:
  • The increase in K_m will also lead to an increase in -1/K_m, shifting the x-intercept to the left.
  • The lack of change in V_max causes the y-intercept to remain unchanged. 
  • These changes cause the line to appear “more vertical” and to cross the original plot at the y-intercept.

Non-competitive inhibitors

• Non-competitive inhibition: Inhibitor does not have a similar shape to the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes because it binds to and inhibits the enzyme outside of the active center, usually at the allosteric site.
  • Substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes can thus continue to bind BIND Hyperbilirubinemia of the Newborn to the active center but is not converted due to the additional binding of the inhibitor.
  • Non-competitive inhibition can be reversible or irreversible.
  • Substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes excess does not displace the inhibitor.
• The number of functional enzyme-substrate complexes decreases.
• This decreases V_max, while K_m remains the same.
• Changes in Michaelis-Menten curve:
  • Decreasing V_max leads to a downward shift in the height of the curve.
  • K_m remains the same, so the curve does not shift on the x-axis.
• Changes in Lineweaver-Burk plot Lineweaver-Burk plot Enzyme Kinetics:
  • Decreasing the V_max leads to an increase in 1/V_max, causing the y-intercept to be higher.
  • Since K_m does not change, the x-intercept of -1/K_m will also remain the same.
  • These changes cause the curve to appear “more vertical” with the new line, creating a V shape with the original line, with the point at the x-intercept.

Uncompetitive inhibitors

• Uncompetitive inhibition: rare form of enzyme inhibition characterized by specific binding at the enzyme-substrate complex Enzyme-substrate complex Temporary molecule formed by the non-covalent binding of the enzyme and substrate. Basics of Enzymes
  • The inhibitor also binds outside the active center, but only if the enzyme-substrate complex Enzyme-substrate complex Temporary molecule formed by the non-covalent binding of the enzyme and substrate. Basics of Enzymes is already formed.
  • The result is a reversible conformational change and thus inactivation of the enzyme.
  • Prevents release of the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes from the binding site.
• K_m is reduced as the inhibitor makes the reaction favor the enzyme-substrate complex Enzyme-substrate complex Temporary molecule formed by the non-covalent binding of the enzyme and substrate. Basics of Enzymes, creating an initial increase in reaction rate.
• V_max is also reduced as the enzyme is prevented from forming products.
• Changes in Michaelis-Menten curve:
  • Decreasing K_m leads to a slight left shift Left Shift Yersinia pestis/Plague in the curve.
  • Decreasing V_max leads to a downward shift in the height of the curve.
• Changes in Lineweaver-Burk plot Lineweaver-Burk plot Enzyme Kinetics:
  • Decreasing K_m causes -1/K_m to also decrease and shifts the x-intercept to the right.
  • Decreasing the V_max leads to an increase in 1/V_max, causing the y-intercept to be higher.
  • The line will shift to the right and appear parallel to and above the initial line.

Irreversible Enzyme Inhibition

Suicide Suicide Suicide is one of the leading causes of death worldwide. Patients with chronic medical conditions or psychiatric disorders are at increased risk of suicidal ideation, attempt, and/or completion. The patient assessment of suicide risk is very important as it may help to prevent a serious suicide attempt, which may result in death. Suicide inhibitors

• Suicide Suicide Suicide is one of the leading causes of death worldwide. Patients with chronic medical conditions or psychiatric disorders are at increased risk of suicidal ideation, attempt, and/or completion. The patient assessment of suicide risk is very important as it may help to prevent a serious suicide attempt, which may result in death. Suicide inhibitors irreversibly bind BIND Hyperbilirubinemia of the Newborn to the active site Active site Area of an enzyme that binds to specific substrate molecules in order to facilitate a reaction. Basics of Enzymes of the enzyme via covalent binding.
• Bind BIND Hyperbilirubinemia of the Newborn at the same site as competitive inhibitors, but have permanent results in a noncompetitive manner
• V_max drops to zero and no amount of substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes is allowed to bind BIND Hyperbilirubinemia of the Newborn.
• Example: penicillin Penicillin Rheumatic Fever drugs acting on bacterial penicillin Penicillin Rheumatic Fever binding proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis

Allosteric Effects

• Allosteric regulation: most common form of regulation; performed by allosteric ligand/effectors
• Ligands bind BIND Hyperbilirubinemia of the Newborn outside the active center of the enzyme, namely at the allosteric center, leading to conformational change and activation or deactivation of the enzyme, depending on the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes‘s ability to fit the new shape.
• In allosteric enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes, 2 state forms can be distinguished:
  • The inactive T-form (tensed), stabilized by allosteric inhibitors
  • The active R-form (relaxed), stabilized by allosteric activators
• Often, the substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes itself represents an allosteric activator and promotes the R-condition for its own implementation.
  • Positive cooperativity: The first substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes facilitates the binding of the second substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes.
  • Negative cooperativity: The binding of the first substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes complicates the binding of the second substrate Substrate A substance upon which the enzyme acts. Basics of Enzymes.
• Depending on the nature of the allosteric effector, V_max, K_m, or both values can be changed.
• Allosteric regulation is most important for rate-limiting enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes.
• Changes in Michaelis-Menten curve:
  • Allosteric enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes have a sigmoidal curve when plotted in this way.
  • K_m values are usually significantly higher for allosteric enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes.
  • Allosteric inhibitors move the reaction toward the right side of the curve.
  • Allosteric activators move the reaction toward the left side of the curve.
• Changes in Lineweaver-Burk plot Lineweaver-Burk plot Enzyme Kinetics with activation:
  • Activation can increase V₀ of the reaction leading to a decrease in 1/V_max and causing a downward shift of the y-intercept.
  • Activation can also lead to a decrease in K_m and therefore a decrease in -1/K_m, causing the x-intercept to shift to the right.
  • Either way, the new line will appear “more horizontal” compared with the original line.