Enzymes 31

Multiple Choice Questions

1. Most of the enzymes of glycolysis are: c. LD3

a. Intracellular d. LD4
b. Extracellular 10. Following myocardial infarction the first
c. Intermediate enzyme to increase is:
d. Neutral a. CKMB
2. All the following statements regarding the b. LDH
coenzymes are correct except: c. AST
a. Dialyzable and thermostable d. None of the above
b. Organic substances 11. Concerning alkaline phosphates one of the
c. NADH is the coenzyme for lactate dehydrogenase following statement is incorrect:
d. Does not bind to the enzyme a. It has more than 5 isoenzyme forms
3. One of the following factors does not affect the b. It is not present in placenta and kidney
enzyme activity: c. It elevates in bone disorder
a. pH d. It elevates in obstructive jaundice
b. Temperature 12. The enzyme which elevates in alcoholic
c. Isoenzyme concentration cirrhosis is:
d. Substrate concentration a. GGT
4. Amylase is a: b. ACP
a. Hydrolase class of enzyme c. CK
b. Lyase class of enzyme d. LDH
c. Oxidoreductase class of enzyme 13. Enzymes used to break down proteins in
d. Ligase class of enzyme biological washing powders belong to the
5. Optimum pH for alkaline phosphatase is: group:
a. 5.0 a. Lactases
b. 7.0 b. Lipases
c. 9.0 c. Proteases
d. 2.0 d. Hydrolases
6. The enzyme which inhibits xanthine oxidase 14. Enzymes act as biological:
is: a. Inhibitors
a. Malonate b. Substrates
b. Succinate c. Solvents
c. Allopurinol d. Catalysts
d. Methotrexate 15. Enzymes speed up biochemical reactions by:
7. Aminopterin or amethopterin competitively a. Increasing the activation energy of the reaction
inhibits: b. Lowering the activation energy of the reaction
a. Folatereductase c. Increasing the temperature of the reaction
b. Glutathione reductase d. Lowering the temperature of the reaction
c. Lactate dehydrogenase 16. The diagram shows a typical relationship
d. Xanthine oxidase between enzyme activity and:
8. The enzyme which elevates in prostate cancer
is:
a. Alkaline phosphatase
b. Acid phosphatase
c. Lactate dehydrogenase
d. Amylase
9. The isoenzyme form of lactate dehydrogenase
increases in skeletal muscle disease is:
a. LD5
b. LD1
 32 Essentials of Biochemistry

a. pH 23. All of the following about the MCW enzyme

b. Enzyme concentration model are true except:
c. Substrate concentration a. The enzyme exists in two states only
d. Temperature b. The T-state has greater affinity for substrate
17. Chemicals (other than the substrate) that affect c. In the absence of substrate, there is little R-state
enzyme activity are called: d. This model cannot account for negative
a. Exhibitors cooperativity
b. Activators 24. Concerning the cooperativity, all are true
c. Inhibitors except:
d. Inactivators a. Positive cooperativity occurs when binding
18. Enzymes belong to which group of chemicals: of the first substrate molecule increases the
a. Proteins affinity of the other active sites for substrate
b. Polysaccharides b. Phosphofructokinase shows negative coopera-
c. Lipids tivity
d. Phospholipids c. Negative cooperativity makes enzymes
19. The diagram shows a typical relationship insensitive to small changes in [S]
between enzyme activity and: d. Hill equation is often used to describe the
degree of cooperativity quantitatively in non-
Michaelis-Menten kinetics
25. The disorder of a system is measured by its:
a. Activation energy
b. Heat of reaction
c. Entropy
d. Energy
26. The optimum pH of most human enzymes:
a. 2.0
b. 6.0
c. 8.0
a. pH d. 7.1
b. Enzyme concentration 27. The “lock and key” model of enzyme action
c. Substrate concentration illustrates that a particular enzyme molecule:
d. Temperature a. Forms a permanent enzyme-substrate complex
20. Allosteric enzymes have: b. Denatured and renatured several time
a. Single subunit c. Interacts with a specific type of substrate
b. Obey Michaelis-Menten kinetics molecule
c. Catalytic and a regulatory site d. Reacts at identical rates under all conditions
d. Affinity towards inhibitors only 28. An enzyme-substrate complex may result from
21. Concerning the allosteric enzymes, one of the the interaction of molecules of:
following statements is incorrect: a. Fructose and lipase
a. The inhibitor can bind to the enzyme at the b. Fat and amylase
same time as the enzyme’s substrate. c. Sucrose and maltase
b. Binding of inhibitor to allosteric site changes d. Protein and protease
the enzyme conformation 29. The place of the enzyme molecule into which
c. They contain many subunits and catalyze the the substrate fits is:
committed step in a pathway a. Active site
d. They do not obey Michaelis-Menten kinetics b. Coenzyme
22. To confirm the Koshland mechanism, an c. Peptide
enzyme must: d. Key part
a. Have more than one subunit 30. One of the following variables is least likely to
b. Exhibit allosteric behavior affect an enzyme’s rate of reaction is:
c. Demonstrate negative cooperativity a. Temperature
d. Have more than one binding site b. Enzyme concentration
 Enzymes 33
c. CO2 concentration a. Allosteric inhibitor
d. Hydrogen bonds b. Competitive inhibitor
31. Which of the following is characteristic of c. Noncompetitive inhibitor
enzymes? d. Active inhibitor
a. They lower the energy of activation of a 36. Inactive precursors of some enzymes that are
reaction by binding the substrate activated through hydrolysis reactions are called:
b. They raise the energy of activation of a reaction a. Allosteric enzymes
by binding the substrate b. Apoenzymes
c. They lower the amount of energy present in c. Prosthetic groups
the substrate d. Zymogens
d. They raise the number of molecules moving 37. L-amino acid dehydrogenase is an enzyme that
quickly can catalyze the oxidation of different L-amino
32. An allosteric site on an enzyme is: acids. It can not catalyze the oxidation of
a. Is similar to active site D-amino acids. Based on these characteristics
b. Nonprotein in nature this enzyme one can say that it shows:
c. Where ATP attaches and gives up its energy a. Allosteric regulation
d. Involved in feedback inhibition b. Absolute specificity over substrate
33. In noncompetitive inhibition, the allosteric c. Relative specificity over substrate
inhibitor: d. Specific inhibition
a. Attaches to the active site, preventing the 38. These enzymes have different structure but the
substrate from attaching there same catalytic function. Frequently they are
b. Attaches to the substrate, preventing it from oligomers made from different polypeptide
attaching to the active site chains. These enzymes are called:
c. Changes the pH of the environment, thus a. Isozymes
preventing enzyme-substrate complex formation b. Allosteric enzymes
d. Attaches to the enzyme at a site away from the c. Proenzymes
active site, altering the shape of the enzyme d. Zymogens
34. The minimum amount of energy needed for a 39. The model that explain that the active site is
process to occur is called the: flexible and the catalytic group of the enzyme is
a. Minimal energy theory brought into proper alignment by the substrate
b. Process energy is called:
c. Kinetic energy a. Concerted model
d. Activation energy b. Induced fit model
35. An inhibitor that changes the overall shape and c. Lock and Key model
chemistry of an enzyme is known as: d. Sigmoid model

Answers
1. a 2. d 3. c 4. a 5. c 6. c 7. a 8. b
9. a 10. a 11. c 12. a 13. c 14. d 15. b 16. d
17. c 18. a 19. a 20. c 21. d 22. c 23. d 24. b
25. d 26. c 27. c 28. d 29. a 30. c 31. a 32. d
33. d 34. d 35. c 36. d 37. b 38. a 39. b
 Chemistry of Nucleic Acid 283

Figure 18.5: Structure of tRNA

SELF-ASSESSMENT QUESTIONS

1. How nucleosides differ from nucleotides? 4. Define antimetabolites and briefly explain its role
2. List the purine and pyrimidine bases present in in medicine.
DNA and RNA. 5. List the purine and pyrimidine analogs.
3. Briefly explain the functions of nucleotides in the 6. Discuss the structural aspects of the DNA.
human body. 7. Explain the structure and role of tRNA.
8. Name the different types of RNAs.

Multiple choice questions

1. In a double stranded molecule of DNA, the ratio 2. Which of the following nucleotide bases is not
of purines :pyrimidines is: found in RNA?
a. Variable a. Thymine
b. Determined by the base sequence in RNA b. Adenine
c. Genetically determined c. Uracil
d. Always 1:1 d. Guanine
284 Essentials of Biochemistry

3. Which of the following molecules does not form c. 5’ GCTAGC 3’

part of DNA? d. 5’ GATCGA 3
a. Pyrimidine 10. The differences between RNA and DNA in the
b. Deoxyribose human cell include the following:
c. Amino acid a. RNA is a double helix
d. Phosphate b. RNA contains the sugar ribose and DNA
4. The transcription of DNA to a molecule of contains deoxyribose
messenger RNA occurs: c. DNA contains the base uracil. RNA does not
a. On the ribosomes d. There is an RNA polymerase but no DNA
b. In the nucleus polymerase.
c. Only during cell division 11. Purine and pyrimidine bases are bound by
d. When amino acids are made available by hydrogen bonds in the DNA double helix. There
transfer RNA are two typical base pair combinations. They
5. The process of translation requires the presence include:
of: a. Guanine to cytosine
a. mRNA, tRNA and ribosomes b. Adenine to guanine
b. mRNA, ribosomes and RNA polymerase c. Adenine to thymine
c. DNA, mRNA and RNA polymerase d. a and d
d. Free nucleotide bases, amino acids and 12. In the DNA double helix shown the bases are
ribosomes paired by dashed lines which represent what
6. Codons are composed of: kind of bonds?
a. Triplet sequences of nucleotide bases in mRNA a. Covalent bonds
b. Triplet sequences of nucleotide bases in DNA b. Ionic bonds
c. Triplet sequences of amino acids in c. Vander Waal’s bond
polypeptide chains d. Hydrogen bonds
d. Triplet sequences of deoxyribose sugars in 13. In DNA base pairing, the purine and pyrimidine
DNA bases pair up. The following bases pair up with
7. Which of the following best describes the each other in the DNA molecule:
relationships between proteins, DNA, RNA and a. Adenine to thymine, guanine to uracil.
genes? b. Adenine to guanine, thymine to cytosine.
a. DNA → genes → RNA → proteins c. Adenine to guanine, cytosine to uracil.
b. RNA → DNA → genes → proteins d. Adenine to thymine, guanine to cytosine.
c. Proteins → RNA → DNA → genes 14. The following is true regarding ribosomes:
d. Genes → RNA → DNA → proteins a. They contain no protein
8. The base thymine is always paired with: b. They contain no RNA
a. Adenine c. They have two tRNA binding sites
b. Guanine d. They are found only on rough endoplasmic
c. Cytosine reticulum
d. Thymine 15. Common lesions found in DNA after exposure
9. The sequence of one strand of DNA is 5’ TCGATC to UV light are
3’. The sequence of the complementary strand a. Pyrimidine dimers
would be: b. Double strand breaks
a. 5’ AGCTAG 3’ c. Base deletions
b. 5’ TCGATC 3’ d. Purine dimers.

Answers
1. d 2. a 3. c 4. b 5. a 6. a 7. a 8. a
9. d 10. b 11. d 12. d 13. d 14. a 15. a
 Nucleic Acid Metabolism 315

Multiple Choice Questions

1. Precursors for de novo purine synthesis include 8. The two nitrogen atoms of pyrimidine
all of the following: nucleotides are contributed by:
a. Ribose 5-phosphate, CO2 and NH3 a. Glutamine and carbamoyl-phosphate
b. Ribose 5-phosphate, ADP and NH3 b. Asparate and carbamoyl-phosphate
c. Ribulose 5-phosphate, CO2 and ADP c. Glutamate and NH3
d. Ribulose 5-phosphate, CO2 and NH3 d. Glutamine and NH3
2. Orotic acidurias can develop when the drug 9. Phosphoribosyl pyrophosphate (PRPP)
allopurinol is administered because: glutamyl amidotransferase, the first enzyme
a. It causes an overproduction of carbamoyl uniquely committed to purine synthesis, is
phosphate inhibited by:
b. Products of its breakdown inhibit OMP a. AMP
decarboxylase b. IMP
c. It allosterically inhibits orotate phosphori- c. XMP
bosyltransferase d. CMP
d. Products of its breakdown inhibit PRPP 10. In man a major catabolic product of pyrimidine
synthase is:
3. One of the end products of pyrimidine a. Urea
degradation is: b. Uric acid
a. β-valine c. Guanine
b. α-alanine d. β-alanine
c. γ-aminoisobutyrate 11. Orotic aciduria type II reflects a deficiency of
d. Carbon dioxide the enzyme:
4. Defects in one of the following enzymes can a. Orotate phosphoribosyl transferase
result in Lesch-Nyhan syndrome: b. Orotidylate decarboxylase
a. Hypoxanthine-guanine phosphoribosyltrans- c. Dihydroorotase
ferase d. Dihydroorotate dehydrogenase
b. Adenosine deaminase 12. In the synthesis of purine nucleotides, the N
c. Adenine phosphoribosyltransferase atom at position 7 and the carbon atoms at
d. Xanthine oxidase positions 4 and 5 are contributed by:
5. The major determinant of the overall rate of a. Glutamine
de novo synthesis of purines is the concentration b. Alanine
of: c. Glycine
a. 5-phosphoribosyl 1-pyrophosphate d. Aspartate
b. Glycinamide ribosyl 5-phosphate 13. Some anticancer drugs act by blocking the
c. 5-phospho β-D-ribosylamine action of the following enzyme:
d. Formylglycinamide ribosyl 5-phosphate a. Orotidylate decarboxylase
6. Orotic aciduria type I reflects a deficiency of the b. Dihydroorotate dehydrogenase
enzyme: c. Dihydrofolate reductase
a. Dihydroorotate dehydrogenase d. Xanthine oxidase
b. Dihydroorotase 14. In de novo pyrimidine synthesis UMP is formed
c. Orotate phosphoribosyl transferase and by the catalytic action of:
orotidylate decarboxylase a. Orotidylate decarboxylase
d. Carbamoyl phosphate synthetase b. Orotate phosphoribosyltransferase
7. All of the following enzymes are unique to c. Aspartate transcarbamoylase
purine nucleotide synthesis, except: d. Dihydroorotase dehydrogenase
a. PRPP synthetase 15. Precursors of pyrimidine de novo synthesis
b. PRPP glutamyl amido transferase include:
c. Adenylosuccinate synthetase a. Aspartame, glutamate and carbonate
d. IMP dehydrogenase b. Aspartate, glutamine and carbonate
 316 Essentials of Biochemistry

c. Aspartame, histamine and carbamoylate 18. The enzyme glutamine-PRPP amidotransferase

d. Aspartate, histamine and carbamoylate is allosterically inhibited by:
16. Gout can be treated by the actions of allopurinol a. AMP, GMP and UMP
on: b. AMP, GMP and IMP
a. Hypoxanthine-guanine phosphoribosyltrans- c. IMP and UMP only
ferase d. AMP and GMP only
b. Dihydroorotase 19. Lesch-Nyhan syndrome can result from a
c. PRPP synthase deficiency in hypoxanthine-guanosine ribosyl
d. Xanthine oxidase transferase (HGPRTase) and a resultant buildup in:
17. Orotic acidurias can develop when the drug a. Urea
allopurinol is administered because: b. Adenosine
a. It allosterically inhibits orotate phosphoribo- c. Guanosine
syltransferase d. Uric Acid
b. It causes an overproduction of carbamoyl 20. Which of the following compounds is a general
phosphate inhibitor of deoxyribonucleotide biosynthesis?
c. Products of its breakdown inhibit OMP a. dATP
decarboxylase b. ATP
d. Products of its breakdown inhibit PRPP c. dTTP
synthase d. UDP

Answers
1. a 2. b 3. d 4. a 5. a 6. c 7. a 8. b
9. a 10. d 11. b 12. c 13. c 14. a 15. b 16. d
17. c 18. b 19. d 20. a