Question 1

In the mechanism of chymotrypsin, which of the following amino acids found in the active site is correctly defined in terms of its role in the reaction?

Accepted Answer

A) serine: hydrogen bonds with carbonyl oxygen to withdraw electron density from the substrate 
B) histidine: deprotonates aspartic acid to allow a nucleophilic attack to occur 
C) aspartic acid: electrostatic stabilization of histidine to make a stronger base 
D) cysteine: performs a nucleophilic attack on the carbonyl carbon of the substrate 
E) none of the above 
A) serine: hydrogen bonds with carbonyl oxygen to withdraw electron density from the substrate 
B) histidine: deprotonates aspartic acid to allow a nucleophilic attack to occur 
C) aspartic acid: electrostatic stabilization of histidine to make a stronger base 
D) cysteine: performs a nucleophilic attack on the carbonyl carbon of the substrate 
E) none of the above C

Question 2

In noncompetitive inhibition, which of the following best explains how the inhibitor binds to the enzyme?

Accepted Answer

A) the inhibitor binds to the active site only after the substrate binds 
B) the inhibitor binds to a site other than the active site only before the substrate binds 
C) the inhibitor binds to a site other than the active site only after the substrate binds 
D) the inhibitor binds to a site other than the active site either before or after the substrate binds 
E) none of the above 
A) the inhibitor binds to the active site only after the substrate binds 
B) the inhibitor binds to a site other than the active site only before the substrate binds 
C) the inhibitor binds to a site other than the active site only after the substrate binds 
D) the inhibitor binds to a site other than the active site either before or after the substrate binds 
E) none of the above D

Question 3

The activity of lysozyme is greater than 50% of maximum between pH 3.8 and 6.1 with peak activity occurring around pH 5. Below 3.8 and above 6.1, the activity drops rapidly. Which of the following provides the best explanation for this?

Accepted Answer

A) two histidine residues in the active site must be protonated for enzyme activity 
B) the active site contains an aspartic acid and glutamic acid, both of which must be deprotonated 
C) the active site contains an aspartic acid that must be deprotonated and a glutamic acid that must be protonated 
D) the active site contains a histidine that must be protonated and a glutamic acid that must be deprotonated 
E) none of the above 
A) two histidine residues in the active site must be protonated for enzyme activity 
B) the active site contains an aspartic acid and glutamic acid, both of which must be deprotonated 
C) the active site contains an aspartic acid that must be deprotonated and a glutamic acid that must be protonated 
D) the active site contains a histidine that must be protonated and a glutamic acid that must be deprotonated 
E) none of the above C

Question 4

A plot of enzyme activity with and without an inhibitor present gave the following plot. What type of inhibitor is present? How does this inhibitor function? What changes are seen in Vmax and KM? Draw a line that approximates the result from addition of twice as much inhibitor to the reaction.

Accepted Answer

A competitive inhibitor is present which

Question 5

Which of the following is the most common form of reversible covalent modification for control of enzyme activity?

Accepted Answer

A) acetylation/deacetylation 
B) adenylation/deadenylation 
C) phosphorylation/dephosphorylation 
D) ADP-ribosylation/de-ADP-ribosylation 
E) none of the above 
A) acetylation/deacetylation 
B) adenylation/deadenylation 
C) phosphorylation/dephosphorylation 
D) ADP-ribosylation/de-ADP-ribosylation 
E) none of the above

Question 6

An enzymatic reaction has a Vmax of 100 $\mu$M/min. At a substrate concentration of 5 $\mu$M, the velocity is 25 $\mu$M/min. What is the KM for the reaction?

Accepted Answer

A) 5 $\mu$M 
B) 10 $\mu$M 
C) 15 $\mu$M 
D) 20 $\mu$M 
E) none of the above 
A) 5 $\mu$M 
B) 10 $\mu$M 
C) 15 $\mu$M 
D) 20 $\mu$M 
E) none of the above

Question 7

Which of the following scenarios would result in a relatively low energy of activation?

Accepted Answer

A) an enzyme binds to the substrate with greater affinity than the transition state 
B) an enzyme binds to both substrate and transition state with equal affinity 
C) an enzyme binds to the transition state with greater affinity than the substrate 
D) an enzyme binds only to the transition state 
E) none of the above 
A) an enzyme binds to the substrate with greater affinity than the transition state 
B) an enzyme binds to both substrate and transition state with equal affinity 
C) an enzyme binds to the transition state with greater affinity than the substrate 
D) an enzyme binds only to the transition state 
E) none of the above

Question 8

The steady state assumption in enzyme kinetics:

Accepted Answer

A) insures that the product of an enzymatic reaction will always be formed 
B) explains why enzymes are effective catalysts 
C) states that the formation of ES is equal to its breakdown 
D) is based upon the fact that the maximum velocity of an enzyme is very high 
E) none of the above 
A) insures that the product of an enzymatic reaction will always be formed 
B) explains why enzymes are effective catalysts 
C) states that the formation of ES is equal to its breakdown 
D) is based upon the fact that the maximum velocity of an enzyme is very high 
E) none of the above

Question 9

Given the chymotrypsin catalytic triad and the peptide substrate, draw the first tetrahedral intermediate of the chymotrypsin mechanism.

Accepted Answer

The answer of Given the chymotrypsin catalytic triad and the...

Question 10

In a Lineweaver-Burke plot, what does the slope represent?

Accepted Answer

A) KM 
B) Vmax 
C) Vmax/KM 
D) KM/Vmax 
E) none of the above 
A) KM 
B) Vmax 
C) Vmax/KM 
D) KM/Vmax 
E) none of the above

Question 11

In the following enzyme reaction scheme, what sort of multi-enzyme kinetics are shown?

Accepted Answer

A) ordered substrate binding with random product release 
B) ordered substrate binding with ordered product release 
C) random substrate binding with ordered product release 
D) random substrate binding with random product release 
E) ping-pong mechanism 
A) ordered substrate binding with random product release 
B) ordered substrate binding with ordered product release 
C) random substrate binding with ordered product release 
D) random substrate binding with random product release 
E) ping-pong mechanism

Question 12

Below is a curve for the enzyme glycogen phosphorylase without any allosteric effectors present showing velocity as a function of the substrate, orthophosphate (Pi). Draw and label a curve that shows the result of addition of the allosteric activator, AMP. Draw and label a curve that shows the result of addition of the allosteric inhibitor, ATP. Glycogen phosphorylase is activated by phosphorylation. Which of these three curves most resembles what happens when glycogen phosphorylase is phosphorylated?

Accepted Answer

@#IMG-DLM& The curve with AMP

Question 13

Which of the following explains why enzymes are extremely effective catalysts?

Accepted Answer

A) an enzyme stabilizes the transition state 
B) enzymes bind very tightly to substrates 
C) enzymes release products very rapidly 
D) an enzyme can convert a normally endergonic reaction into an exergonic reaction 
E) an enzyme lowers the energy of activation only for the forward reaction 
A) an enzyme stabilizes the transition state 
B) enzymes bind very tightly to substrates 
C) enzymes release products very rapidly 
D) an enzyme can convert a normally endergonic reaction into an exergonic reaction 
E) an enzyme lowers the energy of activation only for the forward reaction

Question 14

Which of the following cofactors is used during the activation and transfer of carbon dioxide?

Accepted Answer

A) nicotinamide adenine dinucleotide (NA
D)  
B) thiamine pyrophosphate 
C) biotin 
D) coenzyme A 
E) tetrahydrofolate 
A) nicotinamide adenine dinucleotide (NA
D)  
B) thiamine pyrophosphate 
C) biotin 
D) coenzyme A 
E) tetrahydrofolate

Question 15

Both ATP and CTP are allosteric regulators of the enzyme aspartate transcarbamoylase. Which of the following correctly identifies the activities of these two regulators?

Accepted Answer

A) ATP: activator; CTP: activator 
B) ATP: activator; CTP: inhibitor 
C) ATP: inhibitor; CTP: activator 
D) ATP: inhibitor; CTP: inhibitor 
E) none of the above 
A) ATP: activator; CTP: activator 
B) ATP: activator; CTP: inhibitor 
C) ATP: inhibitor; CTP: activator 
D) ATP: inhibitor; CTP: inhibitor 
E) none of the above

Question 16

Which amino acid most commonly serves as a general acid and general base in an enzyme mechanism?

Accepted Answer

A) serine 
B) arginine 
C) aspartic acid 
D) histidine 
E) cysteine 
A) serine 
B) arginine 
C) aspartic acid 
D) histidine 
E) cysteine

Question 17

What are the expected changes in kinetics in the presence of a competitive inhibitor?

Accepted Answer

A) Vmax decreases, KM appears to decrease 
B) Vmax does not change, KM appears to decrease 
C) Vmax decreases, KM appears to increase 
D) Vmax does not change, KM appears to increase 
E) Vmax decreases, KM does not change 
A) Vmax decreases, KM appears to decrease 
B) Vmax does not change, KM appears to decrease 
C) Vmax decreases, KM appears to increase 
D) Vmax does not change, KM appears to increase 
E) Vmax decreases, KM does not change

Question 18

Since the product of the reaction catalyzed by hexokinase, glucose-6-phosphate (G6P), can act as both a competitive and uncompetitive inhibitor, what can be said about the interaction between G6P and hexokinase?

Accepted Answer

A) G6P binds only to active site of the enzyme 
B) G6P binds only to a regulatory site of the enzyme 
C) G6P binds to both the active site and a regulatory site of the enzyme 
D) G6P binds to one of the substrates, ATP, thus preventing ATP from binding to the active site 
E) none of the above 
A) G6P binds only to active site of the enzyme 
B) G6P binds only to a regulatory site of the enzyme 
C) G6P binds to both the active site and a regulatory site of the enzyme 
D) G6P binds to one of the substrates, ATP, thus preventing ATP from binding to the active site 
E) none of the above

Question 19

What term describes an inactive precursor of an enzyme such as the precursors to protease enzymes produced by the pancreas?

Accepted Answer

A) allosteric enzyme 
B) zymogen 
C) isozyme 
D) ribozyme 
E) hydrolase 
A) allosteric enzyme 
B) zymogen 
C) isozyme 
D) ribozyme 
E) hydrolase

Question 20

What must be true if KM is truly a measure of the affinity of enzyme and substrate?

Accepted Answer

A) kcat must be larger than KM 
B) kcat must be smaller than KM 
C) kcat must be about equal to k1 
D) kcat must be much smaller than k-1 
E) kcat must be much larger than k-1 
A) kcat must be larger than KM 
B) kcat must be smaller than KM 
C) kcat must be about equal to k1 
D) kcat must be much smaller than k-1 
E) kcat must be much larger than k-1

Exam 11: Enzymes: Biological Catalysts

In the mechanism of chymotrypsin, which of the following amino acids found in the active site is correctly defined in terms of its role in the reaction?

In noncompetitive inhibition, which of the following best explains how the inhibitor binds to the enzyme?

The activity of lysozyme is greater than 50% of maximum between pH 3.8 and 6.1 with peak activity occurring around pH 5. Below 3.8 and above 6.1, the activity drops rapidly. Which of the following provides the best explanation for this?

A plot of enzyme activity with and without an inhibitor present gave the following plot. What type of inhibitor is present? How does this inhibitor function? What changes are seen in Vmax and KM? Draw a line that approximates the result from addition of twice as much inhibitor to the reaction.

Which of the following is the most common form of reversible covalent modification for control of enzyme activity?

An enzymatic reaction has a Vmax of 100 μ\muμ M/min. At a substrate concentration of 5 μ\muμ M, the velocity is 25 μ\muμ M/min. What is the KM for the reaction?

Which of the following scenarios would result in a relatively low energy of activation?

The steady state assumption in enzyme kinetics:

Given the chymotrypsin catalytic triad and the peptide substrate, draw the first tetrahedral intermediate of the chymotrypsin mechanism.

In a Lineweaver-Burke plot, what does the slope represent?

In the following enzyme reaction scheme, what sort of multi-enzyme kinetics are shown?

Which of the following explains why enzymes are extremely effective catalysts?

Which of the following cofactors is used during the activation and transfer of carbon dioxide?

Both ATP and CTP are allosteric regulators of the enzyme aspartate transcarbamoylase. Which of the following correctly identifies the activities of these two regulators?

Which amino acid most commonly serves as a general acid and general base in an enzyme mechanism?

What are the expected changes in kinetics in the presence of a competitive inhibitor?

Since the product of the reaction catalyzed by hexokinase, glucose-6-phosphate (G6P), can act as both a competitive and uncompetitive inhibitor, what can be said about the interaction between G6P and hexokinase?

What term describes an inactive precursor of an enzyme such as the precursors to protease enzymes produced by the pancreas?

What must be true if KM is truly a measure of the affinity of enzyme and substrate?

The Scope of Biochemistry

The Matrix of Life: Weak Interactions in an Aqueous Environment

The Energetics of Life

Nucleic Acids

Introduction to Proteins: the Primary Level of Protein Structure

The Three-Dimensional Structure of Proteins

Protein Function and Evolution

Contractile Proteins and Molecular Motors

Carbohydrates: Sugars, Saccharides, Glycans

Lipids, Membranes and Cellular Transport

Chemical Logic of Metabolism

Carbohydrate Metabolism: Glycolysis, Gluconeogenesis, Glycogen Metabolism, and the Pentose Phosphate Pathway

Citric Acid Cycle and Glyoxylate Cycle

Electron Transport, Oxidative Phosphorylation, and Oxygen Metabolism

Photosynthesis

Lipid Metabolism I: Fatty Acids, Triacylglycerols, and Lipoproteins

Interorgan and Intracellular Coordination of Energy Metabolism in Vertebrates

Lipid Metabolism Ii: Membrane Lipids, Steroids, Isoprenoids, and Eicosanoids

Metabolism of Nitrogenous Compounds I: Principles of Biosynthesis, Utilization, and Turnover

Metabolism of Nitogenous Compounds II: Amino Acids, Porphyrins, and Neurotransmitters

Nucleotide Metabolism

Mechanisms of Signal Transduction

Genes, Genomes and Chromosomes

DNA Replication

DNA Restructuring: Repair, Recombination, Rearrangement, Amplification

Information Readout: Transcription and Post-Transcriptional Processing

Information Decoding: Translation and Post-Translational Protein Processing

Regulation of Gene Expression

Filters

A plot of enzyme activity with and without an inhibitor present gave the following plot. What type of inhibitor is present? How does this inhibitor function? What changes are seen in V_max and K_M? Draw a line that approximates the result from addition of twice as much inhibitor to the reaction.

An enzymatic reaction has a V_max of 100 $\mu$ M/min. At a substrate concentration of 5 $\mu$ M, the velocity is 25 $\mu$ M/min. What is the K_M for the reaction?

What must be true if K_M is truly a measure of the affinity of enzyme and substrate?