02.09 - Enzymes Flashcards
These are specialised protein catalysts that accelerate chemical reactions
Enzymes
This refers to the protein part of a holoenzyme.
Apoenzyme
This refers to the non-protein component of a holoenzyme.
Cofactor
Which of the following is not a characteristic of an enzyme?
a. It lowers the activation energy needed for the reaction.
b. It increases the rate of a reaction.
c. It is changed in the reaction process.
d. It is highly specific, binding selectively to its substrate and catalysing only one type of reaction.
c. It is changed in the reaction process. (Enzymes are not changed by the reactions!)
Which of the following is not an oxidation-reduction coenzyme?
a. FAD
b. Vit E
c. Vit C
d. Coenzyme A
d. Coenzyme A
What enzyme cofactor is derived from pantothenic acid and takes part in the citric acid cycle and fatty acid synthesis?
Coenzyme A
A member of the Vitamin-B complex, this coenzyme is a carrier of activated CO2 and thus catalyses carboxylation reactions (e.g. reactions that involve carboxylases).
Biotin
It is a riboflavin-derived coenzyme of several dehydrogenases involved in oxidation-reduction reactions.
FAD
What is the cofactor of lactate dehydrogenase, the enzyme that converts L-lactate into pyruvate?
NAD+
It is a Vitamin B6-derived coenzyme involved in carbohydrate, amino acid and neurotransmitter synthesis.
Pyridoxal phosphate (PLP)
What coenzyme assists enzymes involved in glycogenolysis, heme synthesis, and histamine synthesis?
PLP
What enzyme cofactor assists hexokinase/glucokinase in the conversion of glucose to glucose-6-phosphate?
Mg2+
Name the cofactor of the enzyme pyruvate kinase in the conversion of PEP to pyruvate.
Pyruvate kinase
What cofactor assists carbonic anhydrase in the formation of carbonic acid from water and carbon dioxide?
Zn+
These are enzymes that require a metal in their composition.
Metalloenzymes
The molecule acted upon by the enzyme to form a product is called ___________.
Substrate
The enzyme that catalyses the rate-limitin or committed step of a metabolic pathway is called ___________.
Regulatory enzyme
What is the regulatory enzyme in the de novo synthesis of fatty acids?
Acetyl CoA carboxylase
What is the regulatory enzyme in the pentose phosphate pathway?
Glucose-6-phosphate dehydrogenase
Fatty acid oxidation, decarboxylation of pyruvate and the TCA cycle occurs in what organelle?
Mitochondria
Glycolysis, HMP pathway, fatty acid synthesis occurs in which part of the cell?
Cytosol
All of the following are true about isoenzymes except:
a. Isoenzymes are different structural forms of an enzyme that catalyse the same chemical reactions.
b. Different isoenzymes are expressed in specific tissues of the body.
c. Isoenzymes exhibit the same degree of efficiency.
d. Isoenzymes act on the same substrate and product the same product.
c. Isoenzymes exhibit the same degree of efficiency. (Isoenzymes have differing degrees of efficiency.)
What enzyme catalyses the reversible conversion of pyruvate to lactate?
Lactate dehydrogenase (LDH)
Which of the following is incorrect?
a. LDH is composed of a tetramer with 2 subunits.
b. Five distinct LDH isoenzymes exist.
c. An increase of isoenzyme M4 in the blood is used as a marker for heart attack.
d. The LDH tetramer is composed of the M and H subunits.
c. An increase of isoenzyme M4 in the blood is used as a marker for heart attack. (H4 is used as a marker for tissue damage in heart attack.)
This class of enzymes catalyse the transfer of electrons and hydrogens atoms from donors.
Oxidoreductases
_________________ transfer C-,N- or P-containing functional groups from donors to acceptors.
Transferases
It is a type of transferase that transfers the functional group phosphate from ATP to an acceptor.
Kinase
What type of transferase transfers a carbohydrate residue from a donor to an acceptor?
Glycosyltransferase
______________ catalyze cleavage of chemical bonds by the addition of water, producing two products.
Hydrolases
This class of enzymes cleave C-C, C-O, and C-N bonds by means other than hydrolysis or oxidation.
Lyases
This physiologically significant lyase catalyses the formation of dopamine from its precursor.
DOPA decarboxylase
This type of lyase catalyses a physiologically important reaction that favours the formation of a C-C bond.
Synthase
What class of enzymes transfer functional groups or double bonds within the same molecule?
Isomerases
_____________ catalyses the joining of substrates in the presence of ATP.
Ligases
True or False: Enzymes change the equilibrium of the chemical reaction.
False. Enzymes do not alter the equilibrium of a chemical reaction.
Which of the following statements is incorrect?
a. The difference in the change in free energy between the uncatalysed and catalysed reaction is zero.
b. An enzyme hastens a reaction by lowering the activation energy.
c. The transition state is a low-energy intermediate that is stabilised by an enzyme’s active site.
d. An enzyme accelerates the rate with which equilibrium is reached.
c. The transition state is a low-energy intermediate that is stabilised by an enzyme’s active site. (Transition states are high-energy intermediates and this is why there is a need for stabilisation.)
How does an enzyme catalyse a reaction?
a. By stabilising the transition-state
b. By providing catalytic groups that enhance the probability that the transition state is formed
c. By providing an alternate reaction pathway for the substrate
d. All of the above
d. All of the above
Expressed as micromoles of product formed per minute, this refers to the number of substrate molecules converted to product per unit time.
Maximal velocity
Which of the following statements is false?
a. Most enzymes exhibit Michaelis-Menten kinetics.
b. In Michaelis-Menten kinetics, the plot of the initial reaction velocity against substrate concentration is hyperbolic.
c. Allosteric enzymes do not follow Michaelis-Menten kinetics; they exhibit a sigmoidal curve.
d. The reaction velocity decreases with an increase in temperature until a peak velocity is reached.
d. The reaction velocity decreases with an increase in temperature. (Reaction velocity is directly proportional to an increase in temperature in normal conditions.)
Further elevation of temperature beyond the peak velocity results in a decrease in velocity due to what temperature-induced process
Denaturation
How does the pH affect enzyme activity?
a. pH determines which functional groups are ionized or unionised
b. Extremes of pH may lead to enzyme denaturation
c. pH determines protonation and deprotonation of enzyme components
d. All of the above.
d. All of the above.
The formation of ES complex refers to what step of enzyme-catalysed reactions?
a. Catalytic step
b. Termination step
c. Substrate-binding step
d. None of the above
c. Substrate-binding step
Which of the following are assumptions made by the Michaelis-Menten rate equation?
a. Rate of ES formation is equal to its degradation
b. [S] > [E]
c. The concentration of products at the start is very small; rate of back reaction from P to S can be ignored.
d. All of the above.
d. All of the above.
It is the substrate concentration at which half of the active sites of the enzyme are filled up.
Michaelis constant (Km)
It is an inverse measure of the affinity of the substrate for the enzyme.
Michaelis constant (Km)
Which of the following is true?
a. A low Km means that a low concentration of substrate is needed to half-saturate the enzyme
b. A numerically large Km reflects a low affinity of enzyme for substrate,
c. When [S] is much greater than Km, the velocity is constant and equal to vmax.
d. When [S]
d. When [S]
What is not an application of the Lineweaver-Burke double reciprocal plot?
a. It allows us to predict when vmax has been achieved.
b. It can be used to calculate Km and vmax.
c. It can be used to determine the mechanism of action of enzyme inhibitors.
d. None of the above.
d. None of the above.
Any substance that can diminish the velocity of an enzyme-catalysed reaction is called a/an __________.
Inhibitor
Which of the following statements is incorrect?
a. Irreversible inhibitors bind to enzyme through covalent bonds.
b. Competitive inhibition is an irreversible inhibition.
c. Reversible inhibitors bind through non covalent interactions.
d. None of the above.
b. Competitive inhibition is an irreversible inhibition.
What type of inhibition occurs when the inhibitor binds reversibly to the active or catalytic site?
Competitive inhibition
Which of the following statements is correct?
a. The effect of a competitive inhibitor can be reversed by increasing [S].
b. A competitive inhibitor increases the apparent Km for a given substrate.
c. In the presence of a competitive inhibitor, more substrate is needed to achieve vmax.
d. All of the above.
d. All of the above.
Which of the following does not describe noncompetitive inhibition?
a. In noncompetitive inhibition, the inhibitor and substrate bind at different sites on the enzyme.
b. The noncompetitive inhibitor can bind either free enzyme or the ES complex.
c. Noncompetitive inhibition can be overcome by increasing substrate concentration.
d. Noncompetitive inhibition decreases the vmax of a reaction.
c. Noncompetitive inhibition can be overcome by increasing substrate concentration.
What happens to Km during noncompetitive inhibition?
a. Doubled
b. Halved
c. Stays the same
d. Becomes zero
c. Stays the same
True or False: Reversible inhibition can still happen even if the inhibitor binds covalently to the enzyme.
True
In this type of inhibition, both vmax and Km decrease.
Uncompetitive inhibition
Which antibiotic drug contains a beta-lactam ring and inhibits the activity of the bacterial enzyme glycopeptide transpeptidase?
Penicillin
In this type of enzyme regulation, the latter products inhibit a previous step in the same pathway.
Feedback inhibition
What type of enzyme regulation occurs when an effector binds to an allosteric site, initiating a conformational change in the regulatory enzyme?
Allosteric modification
When the substrate itself serves as an effector in allosteric modification, it is said to be _______________.
Homotropic
When the substrate is different from the substrate in allosteric modification, it is called ______________.
Heterotropic
Protein phosphorylation (i.e. addition or removal of phosphate groups) is an example of what type of enzyme regulation?
Covalent modification
True or False: Phosphorylation of enzymes always leads to activation.
False
An increase in enzyme synthesis is called __________, while a decrease in enzyme synthesis is ___________.
Induction, repression
Which of the following statements is true?
a. Enzymes subject to regulation of synthesis are those that are needed at only one stage of development or under selected physiologic conditions.
b. Elevated levels of glucagon induces the synthesis of key enzymes for glucose uptake.
c. Enzymes that are not in constant use are usually not regulated by altering the rate of enzyme synthesis.
d. A and C
d. A and C
Which of the following factors affect enzyme activity?
a. Temperature and pH
b. Substrate concentration
c. Co-factors
d. All of the above
d. All of the above
Which of the following statements is true?
a. Enzyme activity increases infinitely with every increase in temperature.
b. When exposed to pH levels immediately after its optimum, the enzyme automatically loses its function.
c. Pepsin favors acidic environments, while alkaline phosphatase favors basic ones.
d. Chlorides, bromides, and iodides decrease the rate of enzyme-catalysed reactions.
c. Pepsin favors acidic environments, while alkaline phosphatase favors basic ones.
