ch04

Chapter 4: Amino Acids

Matching

A) glutamateB) unchargedC) deprotonatedD) threeE) amino groupF) twoG) C-terminusH) lysineI) racemic mixtureJ) cysteineK) zeroL) protonatedM) isoleucineN) tryptophanO ) phenylalanineP) N-terminus

1. The net charge of the zwitterion form of Gly is ______.Ans: K

Section: 4.1.ALevel of Difficulty: EasyLearning objective: Amino Acid Structure

2. When a peptide bond is formed, an ______ reacts with a carboxylate group.Ans: E

Section: 4.1.BLevel of Difficulty: EasyLearning objective: Amino Acid Structure

3. At a pH above its pKa, the phenol group of tyrosine is ______.Ans: C

Section: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

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4. In the tripeptide Lys─Pro─Ile, there are ______ charged groups at pH 7.Ans: D

Section: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

5. At a pH below its pKa, the -amino group of lysine is ______.Ans: L

Section: 4.1.DLevel of Difficulty: EasyLearning objective: Amino Acid Structure

6. At a pH below its pKa, the -carboxylate group of Asp is ______.Ans: B or L

Section: 4.1.DLevel of Difficulty: EasyLearning objective: Amino Acid Structure

7. In the peptide Trp─Ser─Val, valine is at the ______.Ans: G

Section: 4.1.ELevel of Difficulty: EasyLearning objective: Amino Acid Structure

8. In the tripeptide Trp─Val─Phe, the N-terminal residue is ______.Ans: N


9. In the tripeptide Lys─Pro─Ile, the C-terminal residue is _________.Ans: M


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10. Biological systems usually produce a single enantiomer, whereas chemical synthesis produces a ______.Ans: I

Section: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

11. GABA is a neurotransmitter derived from ______.Ans: A

Section: 4.3.BLevel of Difficulty: EasyLearning objective: Amino Acid Derivatives

12. Glutathione is a tripeptide containing a central ______ residue.Ans: J

Section: 4.3.BLevel of Difficulty: EasyLearning objective: Amino Acid Derivatives

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Multiple Choice

13. In the early 1930’s William Rose fed laboratory rats a mixture of all 19 amino acids known to be present in proteins. He observed that his rats lost weight. This observation led to the discovery of ______ as the 20th amino acid found in proteins. A) GlyB) AlaC) TyrD) AsnE) Thr

Ans: ESection: 4.1Level of Difficulty: EasyLearning objective: Amino Acid Structure

14. What percentage of the 20 amino acids is considered to be nutritionally essential to lab animals?A) 0%B) 25%C) 50%D) 75%E) 100%

Ans: CSection: 4.1Level of Difficulty: EasyLearning objective: Amino Acid Structure

15. About half of the 20 amino acids are called essential becauseA) they are essential for the synthesis of pyrimidines.B) they are essential for the synthesis of purines.C) they are essential for nitrogen metabolism.D) our bodies need them for protein synthesis.E) our bodies cannot synthesize them and therefore they need to be present in our diet.

Ans: ESection: 4.1Level of Difficulty: EasyLearning objective: Amino Acid Structure

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16. Which amino acid does not have a primary -amino group?A) glutamineB) arginineC) lysineD) prolineE) glutamate

Ans: DSection: 4.1Level of Difficulty: EasyLearning objective: Amino Acid Structure

17. Which of the following statements is true?A) Amino acids can be derived from purines. B) Purines and pyrimidines can be derived from amino acids.C) Corn contains all 20 standard amino acids.D) Amino acids can be derived from pyrimidines.E) none of the above

Ans: BSection: 4.1Level of Difficulty: EasyLearning objective: Amino Acid Structure

18. Which of the following amino acids is the least abundant in proteins?A) VB) WC) GD) FE) A

Ans: BSection: 4.1Level of Difficulty: DifficultLearning objective: Amino Acid Structure

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19. Zwitterions areA) amino acids.B) ionic molecules that are racemic.C) ions that bear both negatively and positively charged groups.D) side chain carboxylate anions.E) delocalized ions.

Ans: CSection: 4.1.ALevel of Difficulty: Easy Learning objective: Amino Acid Structure

20. The formation of a dipeptide from two amino acids involvesA) side-chain complementarity.B) loss of water.C) oxidation of the -carbon.D) reduction of the -carbon.E) base catalysis.

Ans: BSection: 4.1.BLevel of Difficulty: EasyLearning objective: Amino Acid Structure

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21. The diagram below shows

A) an amino acid.B) a dipeptide.C) a tripeptide.D) a tetrapeptide.E) a polypeptide.

Ans: DSection: 4.1.BLevel of Difficulty: EasyLearning objective: Amino Acid Structure

22. Which of the following amino acids has a charged polar side chain at pH 7.0?A) LeuB) AlaC) MetD) TrpE) Glu

Ans: ESection: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

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23. Which of the following tripeptides carries a net positive charge at pH 7.0?A) Ala─Thr─AsnB) Gln─Val─SerC) Arg─Glu─MetD) Pro─Ile─LeuE) Leu─Lys─Gly

Ans: ESection: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

24. Which of the following amino acids has a sulfur atom in its side chain?A) AsnB) SerC) PheD) MetE) Tyr

Ans: DSection: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

25. Which of the following amino acids does not have an ionizable side chain?

A) AspB) CysC) LysD) HisE) Asn


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26. Which of the following amino acids has an uncharged polar side chain at pH 7.0?A) ArgB) ThrC) GluD) ProE) Ile

Ans: BSection: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

For questions 27 and 28 refer to the diagram below

27. What is the three-letter symbol for the amino acid shown above?A) AlaB) AsnC) GlnD) AspE) Glu


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28. The ionization of amino acids depends on the pH and the pKas of the ionizable groups. The pK1 and pK2 for the amino acid shown above are 2.1 and 8.8, respectively. At what pH is the amino acid ionized predominantly as shown?A) pH 1.0B) pH 2.1C) pH 5.5D) pH 8.8E) pH 10.0

Ans: ASection: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

29. The pK1, pK2, and pKR for the amino acid arginine are 1.8, 9.0, and 12.5, respectively. At pH 7.0 arginine would be charged predominantly as follows:A) -carboxylate 0, -amino 0, guanidino +1, net charge +1B) -carboxylate +1, -amino 0, guanidino -1, net charge 0C) -carboxylate +1, -amino −1, guanidino −1, net charge −1D) -carboxylate −1, -amino +1, guanidino +1, net charge +1E) -carboxylate −1, -amino 0, guanidino +1, net charge 0

Ans: DSection: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

30. The pK1, pK2, and pKR for the amino acid aspartic acid are 2.0, 9.9, and 3.9, respectively. At pH 7.0, aspartic acid would be charged predominantly as follows:A) -carboxylate 0, -amino +1, -carboxylate 0, net charge +1B) -carboxylate −1, -amino +1, -carboxylate −1, net charge −1C) -carboxylate 0, -amino −1, -carboxylate 0, net charge −1D) -carboxylate +1, -amino −1, -carboxylate +1, net charge +1E) -carboxylate +1, -amino +1, -carboxylate +1, net charge +3

Ans: BSection: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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31. The pK1, pK2, and pKR for the amino acid glutamate are 2.1, 9.5, and 4.1, respectively. At pH 11.0, glutamate would be charged predominantly as follows:A) -carboxylate +1, -amino 0, -carboxylate +1, net charge +2B) -carboxylate −1, -amino +1, -carboxylate −1, net charge −1C) -carboxylate 0, -amino 0, -carboxylate 0, net charge 0D) -carboxylate +1, -amino −1, -carboxylate +1, net charge +1E) -carboxylate −1, -amino 0, -carboxylate −1, net charge −2

Ans: ESection: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

32. The pK1, pK2, and pKR for the amino acid histidine are 1.8, 9.3, and 6.0, respectively. At pH 4.0 would be charged predominantly as follows:A) -carboxylate +1, -amino 0, imidazole −1, net charge 0B) -carboxylate −1, -amino +1, imidazole 0, net charge 0C) -carboxylate +1, -amino +1, imidazole −1, net charge +1D) -carboxylate −1, -amino +1, imidazole +1, net charge +1E) -carboxylate 0, -amino +1, imidazole +1, net charge +2


33. The pK1, pK2, and pKR for the amino acid cysteine are 1.9, 10.7, and 8.4, respectively. At pH 5.0, cysteine would be charged predominantly as follows:A) -carboxylate 0, -amino 0, sulfhydryl 0, net charge 0B) -carboxylate +1, -amino −1, sulfhydryl −1, net charge −1C) -carboxylate −1, -amino +1, sulfhydryl +1, net charge +1D) -carboxylate −1, -amino +1, sulfhydryl 0, net charge 0E) -carboxylate +1, -amino −1, sulfhydryl 0, net charge 0


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34. The pK1, pK2, and pKR for the amino acid lysine are 2.2, 9.1, and 10.5, respectively. At pH 1.0, lysine would be charged predominantly as follows: A) -carboxylate 0, -amino −1, -amino −1, net charge −2B) -carboxylate −1, -amino +1, -amino +1, net charge +1C) -carboxylate +1, -amino +2, -amino +2, net charge +5D) -carboxylate 0, -amino +1, -amino +1, net charge +2E) -carboxylate +2, -amino +1, -amino +1, net charge +4


Questions 35-37 are based on the diagram shown below.

35. How many amino acid residues are present in the peptide shown above?A) 1B) 2C) 3D) 4E) 5

Ans: CSection: 4.1.CLevel of Difficulty: EasyLearning objective: Amino Acid Structure

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36. What is the name of the N-terminal amino acid in the peptide shown above?A) cysteineB) methionineC) valineD) alanineE) tryptophan


37. What is the three-letter symbol for the C-terminal amino acid in the peptide shown above?A) GlyB) AspC) TyrD) MetE) Phe


38. While proteins are usually composed of linear chains of amino acids, branched chains of amino acids and internally cross-linked chains can be found in certain proteins. Polypeptide chains are most commonly linked to each other through

A) hydrogen bonds.B) glycosidic bonds.C) peptide bonds.D) disulfide bonds.E) ester linkages.


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39. The disulfide bond between two cysteine moleculesA) is a peptide bond.B) is an ionic interaction that is stable at physiological pH.C) is a covalent bond formed by oxidation.D) is a hydrogen bond between the two sulfhydryl groups.E) is a dipole-dipole interaction.


40. The two peptides shown in the diagram below are linked through

A) a hydrogen bond.B) a glycosidic bond.C) a peptide bond.D) a disulfide bond. E) an ester linkage.


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41. Which one of the representations describes the oligopeptide shown below?

A) Tyr-Ala-ThrB) Tyr-Gly-CysC) Tyr-Ala-SerD) Phe-Gly-CysE) Phe-Ala-Thr


42. The tripeptide glycylarginylglutamate contains four ionizable groups with pKas of 2.1, 4.1 9.8, and 12.5. Calculate the pI for this molecule.

A) 3.1B) 6.4C) 7.0D) 7.3E) 8.3

Ans: CSection: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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43. The tripeptide alanyllysylaspartate contains four ionizable groups with pKas of 2.0, 3.9, 9.9, and 10.5. Calculate the pI for this molecule. A) 3.0B) 6.0C) 6.2D) 10.2E) none of the above

Ans: ESection: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

44. The isoelectronic point of an amino acid is the point whereA) the pKa of the -carboxylic acid is the same as the pKa of the -amino group.B) the -carboxylic acid is protonated and the -amino group is unprotonated.C) the solubility of the amino acid is maximized.D) the -carboxylic acid and the -amino group are both half protonated.E) the amino acid carries no net electrical charge.

Ans: ESection: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

45. The side-chains of amino acids haveA) a positive charge in every situation.B) pKas that assure the solubility of every protein.C) constant pKas no matter what aqueous environment they are found in.D) different pKas in peptides as compared to the free amino acidsE) polar functional groups.

Ans: DSection: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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46. Asx refers toA) a negatively charged aspartic acid.B) a positively charged asparagine.C) a dipeptide containing both aspartic acid and asparagine.D) either aspartic acid or asparagine.E) an unnatural amino acid formed during acid hydrolysis of peptide bonds.

Ans: DSection: 4.1.ELevel of Difficulty: EasyLearning objective: Amino Acid Structure

47. The peptide AYDG has an N-terminal _________ residue.A) glycineB) glutamic acidC) glutamineD) aspartic acidE) alanine

Ans: ESection: 4.1.ELevel of Difficulty: EasyLearning objective: Amino Acid Structure

48. Which of the following tripeptides would be expected to be the most hydrophobic?A) KYGB) KYAC) GYAD) DYAE) DYG

Ans: CSection: 4.1.ELevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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49. What is the three-letter abbreviation for the peptide valylarginylisoleucine?A) Val─Arg─IleB) Val─Agn─LeuC) Vln─Arg─IleD) Val─Agn─IslE) Vln─Arg─Leu

Ans: ASection: 4.1.ELevel of Difficulty: EasyLearning objective: Amino Acid Structure

50. Ribosomes use L amino acids to synthesize proteins. These amino acids are called “L” becauseA) they are chiral.B) they turn polarized light to the left.C) they have a configuration of groups around the C that can be related to the configuration of groups around the asymmetric carbon in L-glyceraldehyde.D) they are all (S)-amino acids.E) they are all (R)-amino acids.

Ans: CSection: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

51. Ribosomes use L amino acids to synthesize proteins. These amino acids are called “L” becauseA) they are chiral.B) they turn polarized light to the left.C) they are all (S)-amino acids.D) they are all (R)-amino acids.E) none of the above

Ans: ESection: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

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52. Which of the amino acids represented below has two chiral centers?

A) AlaB) LeuC) IleD) ProE) Asn

Ans: CSection: 4.2Level of Difficulty: DifficultLearning objective: Stereochemistry

53. Which of the five amino acids shown below has 2 chiral centers?

Ans: CSection: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

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54. All the standard amino acids except ____ are optically active.A) ProB) ArgC) TrpD) GlyE) Phe

Ans: DSection: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

55. The __________ character of most standard amino acids causes solutions of amino acids to rotate the plane of polarized light.A) chiralB) zwitterionicC) polarD) hydrophobicE) none of the above

Ans: ASection: 4.2Level of Difficulty: EasyLearning objective: Stereochemistry

56. All amino acids derived from proteins have the same stereochemical conformation as.A) L-glyceraldehyde.B) (R)-alanine.C) D-glucose.D) (S)-cysteine.E) glycine.

Ans: ASection: 4.2Level of Difficulty: ModerateLearning objective: Stereochemistry

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57. Typically, modified amino acids, those with side chain modifications that are present in proteinsA) are synthesized due to genetic mutations.B) are assembled by specialized bacterial enzymes.C) are formed by modification of standard side chain residues after the protein is synthesized.D) are toxic.E) are racemic.

Ans: CSection: 4.3.ALevel of Difficulty: ModerateLearning objective: Amino Acid Derivatives

58. An amide bond between a side-chain carboxylate and an -amino group is also called a(n)A) disulfide bond.B) anhydride bond.C) ester bond.D) isopeptide bond.E) glycosidic bond.

Ans: DSection: 4.3.BLevel of Difficulty: ModerateLearning objective: Amino Acid Derivatives

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Questions 59-61 are based on the diagram below.

59. What is the name of this molecule?A) GABAB) thyroxineC) -carboxyglutamateD) glutathioneE) epinephrine


60. Residues 1 and 2 in the peptide shown above are connected through a

A) hydrogen bond.B) an isopeptide bond. C) an ester linkage.D) disulfide bond.E) a glycosidic bond.

Ans: BSection: 4.3.BLevel of Difficulty: ModerateLearning objective: Amino Acid Derivatives

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61. What is the three-letter symbol of the N-terminal amino acid in the structure shown above?

A) AspB) AsnC) GlnD) GluE) Lys


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Short Answer

62. Glutamine is an amino acid (pKas are 2.2, and 9.1). Draw the predominant structure of glutamine at pH 1.0.

Ans:

Section: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

63. Glutamic acid is an amino acid (pKas are 2.1, 4.1 and 9.5). Calculate the isoelectric point of glutamic acid.

Ans: At pH 1.0 glutamic acid has an electric charge of +1, at pH 3.0 the electric charge equal 0, and at pH 5.0 the electric charge equals −1. Therefore the pKas of the carboxylic acid groups surround the neutral form. pI = (2.1 + 4.1)/2 = 3.1Section: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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64. Tryptophan is an amino acid (pKas are 2.5, and 9.4).

a. How many different amino acids can be incorporated into proteins during translation? b. Give the three- and one-letter abbreviation for tryptophan. c. Draw the predominant structure of tryptophan at pH 7.0. d. Calculate the pI of Tryptophan.

Ans: a. 20b. Trp and W

c. d. pI = (2.5 + 9.4)/2 = 6.0Section: 4.1.DLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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65. In general, amino acids are chiral molecules that act as building blocks for proteins. In addition, amino acids and amino acid derivative can act as extracellular messenger molecules such as hormones or neurotransmitters.

a. How many different amino acids can be incorporated into proteins during translation?b. Do ribosomes use L or D amino acids for protein synthesis?c. Name one amino acid or amino acid derivative that can act as hormone or neurotransmitter.d. Draw the predominant structure of valine at pH 10 (pKas are 2.3 and 9.7).

Ans: a. 20b. Lc. glycine, glutamate, -aminobutyric acid (GABA), dopamine, or serotonin

d. Section: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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66. The figure below shows the predominant structure of an oligopeptide at pH 7.0.

a. How many amino acids are present in this peptide?b. Circle one peptide bond in this peptide.c. Draw a square around one alpha carbon in this peptided. The pKas of the ionizable groups in this peptide are 2.0, 4.0, and 9.0. What is the ionic charge of this molecule at pH 1.0e. What is the name of the N-terminal amino acid in this peptide?Ans: a. 5

b. c. d. The charge of this molecule at pH 1.0 is +1. At pH 1.0 all ionizable groups are protonated; the two carboxylic acid groups are neutral and the amino group is positively charged. e. aspartate (aspartic acid)Section: 4.1.CLevel of Difficulty: ModerateLearning objective: Amino Acid Structure

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67. The picture below shows the structure of -amino-butyric acid (GABA).

a. From which amino acid is GABA derived (give the name of the amino acid closest in structure to GABA).b. What is the structural difference between GABA and this amino acid.c. What is the function of GABA.

Ans: a. glutamateb. GABA is missing the -carboxylic acid groupc. GABA is a neurotransmitter (a chemical that mediates communication between neurons)Section: 4.3.BLevel of Difficulty: ModerateLearning objective: Amino Acid Derivatives

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68. The picture below shows the structure of serotonin.

a. From which amino acid is serotonin derived?b. What are the structural differences between serotonin and this amino acid?c. What is the function of serotonin? d. Which illness can be caused by reduced levels of serotonin in the brain?

Ans: a. Tryptophanb. Serotonin has a hydroxyl group attached to the 6-membered ring and it misses the -amino group. c. Serotonin is a neurotransmitterd. DepressionSection: 4.3.BLevel of Difficulty: ModerateLearning objective: Amino Acid Derivatives

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