Chapter 6 Flashcards
(22 cards)
What are enzyme inhibitors?
Substances that decrease enzyme activity
Enzyme inhibitors can be classified as reversible or irreversible.
What is the difference between reversible and irreversible enzyme inhibitors?
Reversible inhibitors bind temporarily; irreversible inhibitors form permanent bonds
Reversible inhibitors can be further categorized into competitive, uncompetitive, and mixed inhibitors.
How does competitive inhibition work?
Inhibitor competes with substrate for the active site
Enzyme isn’t shut off, and can shift equilibrium to favor P (by adding more S)
This type of inhibition can be overcome by increasing substrate concentration.
What is the effect of competitive inhibition on Michaelis-Menten kinetics?
Vmax remains unchanged; Km increases by factor alpha (called apparent Km)
Competitive inhibition makes it appear as if less substrate is needed to reach half of Vmax.
What does the Lineweaver-Burk plot look like for competitive inhibition?
Lines intersect at the y-axis
The slope increases due to an increase in Km, while Vmax remains constant.
Equation becomes: slope = (aKm)/Vmax
How does uncompetitive inhibition work?
Inhibitor binds to a different location rather than the active site, only binding to the enzyme-substrate complex
This does not affect substrate binding and only prevents the complex from converting into product or having catalytic function.
What is the effect of uncompetitive inhibition on Michaelis-Menten kinetics?
Vmax decreases by a’ (called apparent Vmax); Km decreases by a’
This type of inhibition results in a lower apparent Km and a lower Vmax. Both Vmax and Km can be divided by a’ to find the value.
What does the Lineweaver-Burk plot look like for uncompetitive inhibition?
Lines are parallel
Both the slope and the y-intercept increase, indicating changes in Vmax and Km.
How does mixed inhibition work?
Inhibitor can bind to either the enzyme or the enzyme-substrate complex. This can impact substrate binding, and prevent catalytic function
This can affect both Vmax and Km in different ways depending on the binding preference.
What is the effect of mixed inhibition on Michaelis-Menten kinetics?
Vmax decreases due to lowering [E]; Km can increase or decrease.
The specific effects depend on the relative affinities for the inhibitor. Find a’ first, then a. Effects written as Vmax/a’ and aKm/a’
What does the Lineweaver-Burk plot look like for mixed inhibition?
Lines intersect left of the y-axis
This indicates changes in both Vmax and Km.
When does noncompetitive inhibition occur?
When an inhibitor binds to an allosteric site, not the active site. Rare case of a = a’.
This type of inhibition decreases Vmax without affecting Km. Will only appear as Vmax/a’ and Km x (1)
How can changes to Michaelis-Menten kinetics be used?
To identify the type of enzyme inhibition
By analyzing changes in Vmax and Km, the specific inhibition type can be inferred. Start by observing the change in Vmax, then find a’.
How does a suicide inactivator work?
It covalently binds to the enzyme and is converted into a reactive form that permanently inactivates the enzyme
This type of irreversible inhibition is often used in drug design. Is unreactive until bound to the active site
How does a transition state analog work?
It mimics the transition state of the substrate, binding noncovalently to the enzyme more tightly than the substrate itself
This prevents the enzyme from catalyzing the reaction.
How are enzymes regulated?
Through various mechanisms including allosteric modulation, reversible noncovalent modifications and irreversible covalent modifications
Regulation can enhance or inhibit enzyme activity. Affects rate of entire pathway.
What are allosteric modulators?
Molecules that bind to an enzyme at a site other than the active site, causing a change in activity. Can cause a conformation change to the enzyme
Generally metabolites or cofactors. They can be activators (positive) or inhibitors (negative). Can be homotropic or heterotropic.
What is the mechanism for reversible covalent modifications?
Addition or removal of functional groups that can change enzyme activity. Often modifies one or more amino acids
Common modifications include phosphorylation, methylation, and acetylation.
What is the most common covalent modification of enzymes?
Phosphorylation: the adding of a phosphoryl group.
This modification plays a critical role in regulating enzyme activity and signaling pathways.
What are reversible covalent modifications?
Covalent modifications to one or more amino acids. Change the local properties of the enzyme (conformational change, binding to a membrane)
Different enzymes, such as protein kinase or phosphoprotein phosphotase can mediate modifications and can regulate enzymes
What are protein kinase and phosphotase?
Protein kinase is the enzyme that adds a phosphoryl group to a protein. Phosphoprotein phosphotase is the enzyme that removes a phosphoryl group.
Addition of a phosphoryl group often activates an enzyme, while removal often deactivates, or terminates the signal.
What are irreversible covalent modifications?
Proteolytic cleavage, ubiquitnation, or covalent attachment of large groups
ex. Chymotrypsin and trypsin.
