Lecture 2 - Enzymes, Enzyme Kinetics, Regulation of Enzyme Activity Flashcards
Which is NOT a form of regulating enzyme activity?
A. Binding of regulatory molecules to sites on the enzyme distinct from catalytic site
B. Covalent modifications
C. Localization within different compartments of the cell
D. All of the above are forms of regulation
D
RNA molecules that possess catalytic function are known as ________.
Ribozymes
What is the Vmax for an enzyme?
Vmax = maximum velocity at a point where the enzyme is saturated with substrate
What is the Km of an enzyme?
Km = substrate concentration requried to attain 1/2 Vmax
Which is false regarding enzyme kinetics?
A. Addition of additional substrate does not increase the Vmax.
B. A lower Km value is generally reflective of tighter substrate binding
C. Km and Vmax are fixed for a given enzyme
D. Km and Vmax can vary with structure of substrate
C - this is false because Km and Vmax can vary with structure of substrate, temperature and pH.
Kd, the dissociation constant, can be determined by:
measuring the ratio of free and bound species at equilibrium
Which is true regarding Km vs. Kd?
A. Kd is generally more difficult to measure experimentally
B. Both Km and Kd reflect the strength of interaction between enzymes and their substrates
C. The units for Km is in concentration (moles/liter)
D. All of the above are true regarding Km vs. Kd
D
The slope of a Lineweaver-Burke Plot is equal to the:
Km/Vmax
An particular enzyme inhibitor increases the Km of a reaction and is NOT allosteric. Therefore, it has which of the following properties?
A. Binds to an enzyme but not necessarily at an active site
B. Inhibitor’s effects cannot be overcome by high substrate concentrations
C. It also changes the Vmax
D. It resembles a substrate
D - A compeititive enzyme inhibitor leaves the Vmax unchanges but increases the Km. It also resembles a substrate and competes with the substrate for binding to an active site on the enzyme.
An irreversible enzyme inhibitor decreases the _____ by decreasing the amount of active enzyme in solution.
A. Vmax
B. Km
A - An irreversible enzyme inhibitor decreases the Vmax and has no effect on Km.
Which two enzyme inhibitors are kinetically indistinguishable?
A. Competitive and non-competitive
B. Non-competitive and irreversible
C. Irreversble and competitive
D. Allosteric activator and competitive enzyme
B. Both non-competitive and irreversible inhibitors decrease the Vmax and have no effect on the Km.
The effects of irreversible enzyme inhibitors can only be overcome by:
the synthesis of new enzyme
Fluorouracil, allopurinol, and aspirin are examples of what type of enzyme inhibitors?
Irreversible enzyme inhibitors
Which type of enzyme inhibitor reacts covalently with some functional group on the enzyme, thereby resulting in inactivation (“poisoning”) of the enzyme?
Irreversible enzyme inhibitor
Inhibition of COX-I by aspirin is caused by the what type of enzyme inhibitor?
Irreversible enzyme inhibitor - irreversible acetylation of the hydroxyl group of serine 530 in the COX-I active site
What are the 3 characteristics of regulatory enzymes?
- Catalyze irreversible reactions
- Catalyze the rate-limiting step in a pathway
- Catalyze the committed step in a pathway
________ act to alter kinetic properties of enzymes.
A. Isozymes
B. Covalent Modification
C. Larger Catalytic Units
D. Allosteric Effectors
D
Pyruvate deydrogenase complex is an example of which (of the 3 types) of catalytic units?
A. Macromolecular complex
B. Multifunctional protein
C. Metabolon
D. None of these
A
The CAD protein in the pathway of pyrimidine synthesis is an example of which (of the 3 types) of catalytic units?
A. Macromolecular complex
B. Multifunctional protein
C. Metabolon
D. None of these
B - CAD protein catalyzes four different reactions
Many enzymes require “cofactors” and thus their activity will be influenced by cofactor availability. In addition, the Km of many enzymes for their substrates is within the range of concentration of that substrate in the cell.
At concentrations around the Km, a change in the concentration of the substrate will:
A. Not affect the rate of the reaction
B. Slightly affect the rate of the reaction
C. Moderately affect the rate of the reaction
D. Dramatically affect the rate of the reaction
D
______ are two or more different enzymes that catalyze the same reaction, but usually have characteristic tissue specificity.
Isozymes
Which is FALSE regarding isozymes glucokinase and hexokinase?
A. Glucokinase is specific for liver and pancreatic B cells
B. Both catalyze the conversion of glucose to glucose-6-phosphate
C. Hexoinase has a much lower Km and Vmax than glucokinase
D. Conversion of glucose to G6P is minimal in tissues expressing hexokinase, even at fasting blood glucose levels.
D - Conversion of glucose to G6P is MAXIMAL in tissues expressing hexokinase, even at fasting blood glucose levels.
What is an example of the diagnostic value of isozymes?
Appearance of the CK2 isoform of creatine kinase in the blood is an indicator of a myocardial infarction
Allosteric _______ either decrease the Km, increase the Vmax, or both.
activators
Allosteric _______ either increase the Km, decrease the Vmax, or both.
inhibitors
Phosphofructokinase-1 is an example of what type of enzyme?
Allosteric enzyme - serves as the rate limiting step in Glycolysis
Phosphofructokinase-1 mainly efects what kinetic parameter?
Km
_________ catalyzes the phosphorylation of fructose-6-P to fructose-1,6-P2,
Phosphofructokinase-1
What is the primary allosteric inhibitor of phosphofructokinase-1?
ATP (Note: citrate further enhances this effect)
Which does NOT relieve the allosteric inhibition of PFK-1 by ATP?
A. AMP
B. ADP
C. fructose-1,6-P2
D. fructose-2,6-P2
C (F-1,6-P2 is the phosphorylated form of fructose-6-P)
The most important type of reversible covalent modification of enzymes in metabolism is: ______.
phosphorylation
The modifying enzymes required to catalyze phosphorylation are _________.
protein kinases
The reverse reaction, dephosphorylation, is catalyzed by _______.
protein phosphatases
_______ are a family of enzymes that catalyze the transfer of phosphate from the gamma-position of ATP to a hydroxyl group on serine, threonine or tyrosine residues of proteins to form a phosphate ester linkage.
Protein kinases
Which is FALSE concerning the consequences of phosphorylation/de-phosphorylation?
A. In many cases, the phosphorylation state of an enzyme is determined by specific hormones
B. Phosphorylation does not alter an enzyme’s Km and Vmax
C. Phosphorylation can alter an enzyme’s response to allosteric regulatory molecule
D. Phosphorylation can activate or inactivate an enzyme
B (Phosphorylation of enzymes CAN alter their kinetic properties.)
If an enzyme is in a _______ pathway, phosphorylation will tend to activate it.
catabolic pathway
If an enzyme is in a _________ pathway, phosphorylation will tend to inhibit it.
anabolic pathway
___________ use water to hydrolyze the phosphate ester and return the enzyme/protein to its de-phosphorylated form.
Protein phosphatases
_________ is an irreversible covalent modification that is commonly used in biological systems for activation of cellular processes.
Limited proteolysis
What happens as a result of conformational change during limited proteolysis?
A new function is acquired
Overall catalytic efficiency in a multi-step reaction is often facilitated by the formation of multi-enzyme complexes that enhance ______.
substrate availability
In limited proteolysis, cleavage of ______ bonds occurs at a limited number of sites.
peptide bonds
