Lecture 11: Enzyme Regulation Flashcards
Elevated enzyme levels in the blood can indicate what?
Tissue damage.
What enzyme is used to measure liver disease?
ALT, or alanine transferase.
What enzyme is used to measure lung damage?
α-1 antitrypsin
Zymogen
Inactive protein (or enzyme) precursors.
How are zymogens activated?
Proteolytic cleavage.
In the blood coagulation cascade, what zymogen is activated and what does it become?
Prothrombin —> thrombin.
True or false?
The two proteolytic cleavages of thrombin are reversible.
False.
What is the role of gamma carboxylation of glutamate residues on prothrombin?
This modification of prothrombin allows the binding of Ca 2+.
What is the role of Ca 2+ in the maturation of prothrombin to thrombin?
Ca 2+ binding to prothrombin allows it to expose its hydrophobic portion in order to associate with the cell membrane.
Why does prothrombin need to associate with the cell membrane in order to mature?
The protease that activates prothrombin to thrombin exists on the cell membrane.
What happens to prothrombin once it is cleaved by the membrane-bound protease?
It is release from the membrane so it can then exert its effects.
Once released from the membrane, how does thrombin exert its effects?
It exerts its effects through catalyzing the conversion of SOLUBLE fibrinogen into INSOLUBLE fibrin, which ultimately begins to clot.
(Fibrinogen/Fibrin) is insoluble.
Fibrin.
(Fibrinogen/Fibrin) is soluble.
Fibrinogen.
True or false?
Gamma carboxylation is a post-translation modification that regulates enzyme activity.
False.
It does not regulate enzyme activity, but is instead required for formation of the active enzyme for blood coagulation.
What is the co-factor required for gamma carboxylation of prothrombin?
Vitamin K.
Although zymogen activation is efficient, it is _______.
Irreversible.
How is thrombin inactivated?
Through the high-affinity binding of antithrombin.
Antithrombin
Binds to thrombin with very high affinity, serving to eliminate its ability to catalyze further blood clotting in the blood clotting cascade.
In what example tissue is the importance of enzyme activation demonstrated?
The lungs.
What type of enzyme is elastase?
A serine protease.
What protein does elastase degrade?
Elastase.
What is the etiology of emphysema?
The unregulated degradation of elastin by elastase in the lungs due to a dysfunctional a-1-antitrypsin, an inhibitor of elastase.
What cells release elastase and why?
Neutrophils release elastase in order to degrade foreign particles.
In what patients would one expect to find a-1-antitrypsin mutations?
Homozygous individuals who carry two defective genes and heavy smokers.
What is the treatment for a-1-antitrypsin deficiency?
Infusion of a-1-antitrypsin, though it is highly expensive.
Where is a-1-antitrypsin normally secreted from?
The liver into the blood stream.
What are two common mechanisms of enzyme regulation?
Phosphorylation and methylation.
A phosphorylating enzyme.
Kinase.
A dephosphorylating enzyme.
Phosphatase.
What are the residues that kinases and phosphatases act upon?
- Serine
- Threonine
- Tyrosine
Specifically, how do phosphorylation and dephosphorylation change the activity of an enzyme?
They change the charge of an amino acid, which, if near the active site, can change it’s activity dramatically.
Half of commonly prescribed drugs are ______ ______.
Enzyme inhibitors.
True or false?
Competitive inhibitors are irreversible.
False.
They are reversible and can be overcome by increasing [substrate].
True or false?
Competitive inhibitors usually resemble the enzyme’s normal substrate.
True.
Ki
k2 (E + I EI)
The smaller the Ki, the greater the _______.
Effectiveness of the inhibitor.
The less effective the inhibitor is, the ______ .
Greater the Ki.
In regards to Michaelis-Menten enzyme kinetics, competitive inhibitors alter _____, but not _____.
Km, but not Vmax.
True or false?
Non-competitive inhibitors usually resemble the enzyme’s normal substrate.
False.
They often do not bear any resemblance to the enzyme’s normal substrate.
Where and when do non-competitive inhibitors bind?
They bind to a location other than the active site, irrespective of whether substrate is present or not.
In regards to Michaelis-Menten enzyme kinetics, non-competitive inhibitors alter _____, but not _____.
Vmax, but not Km.
True or false?
Non-competitive inhibition can be overcome by increasing [substrate].
False.
No matter how much substrate is added, the inhibition cannot be overcome. It is not about competition for the active site.
What are the two types of non-competitive inhibitors?
Reversible and irreversible.
What are two examples of irreversible non-competitive inhibitors?
- a-1-antitrypsin
2. antithrombin
What is DIFP and what is its clinical significance?
Diisopropyl fluorophosphate.
This is a chemical poison (used as a component in mustard gas) that is an irreversible inhibitor of serine proteases.
In nerve cells, acetylcholinesterase is not degraded normally, resulting in interneuron communication disruption due to increased [acetylcholine].
In regards to irreversible non-competitive inhibition, what does the half-life of its effect have to do with?
The time of re-synthesis of new proteins/enzymes.
Many enzymes have [substrate] in the range of their _______, where they are most effective.
Km.
What is product inhibition?
Where the product of the enzyme-catalyzed reaction inhibits that specific enzyme.
What enzyme is an example of a product-inhibition?
Hexokinase. (Glucose + ATP —-> Glucose-6-phosphate
Allosteric Enzyme
These enzymes change their activity from binding an effector, which may be another substrate molecule or a different effector molecule.
Allosteric enzymes alternate between what two different conformations?
Relaxed and constrained.
Binding of substrate to an allosteric enzyme increases the likelihood that it will enter its _____ conformation.
Relaxed.
Allosteric enzymes can have multiple subunits that work together in a cooperative fashion.
On a velocity vs. [substrate] graph, what kind of shape would one of these enzymes assume?
Sigmoidal, as in that of hemoglobin.
Positive Effector
Activator; promotes relaxed conformation.
Negative Effector
Inhibitor; promotes constrained conformation.
Why would allosteric enzymes be the enzyme to catalyze the first step of a reaction sequence?
They are easily regulated, especially by their own products, products later on in the reaction sequence, or substrates in related sequences.
What is feedback inhibition?
When enzyme is inhibited by a substrate down the reaction sequence.
What three things can normally be said about the first committed step in any given pathway?
- It is functionally irreversible.
- It is early in the pathway.
- It is committed solely to one pathway.
