Ch 4 - Problems

Problems

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1. Identify the amino acids that differ from each other by a single single methyl or methylene methylene group. Answer:

Gly and Ala; Ser and Thr; Val, Leu, and Ile; Asn and Gln; Asp and Glu. 2. The 20 standard amino acids are called α-amino acids. Certain β-amino acids are found in nature. Draw

the structure of β of β-alanine (3-amino-n-propionate). Answer: +

H3 N—CH2 —CH2 —COO –

3. Identify the hydrogen bond donor and a nd acceptor acc eptor groups in asparagine. asparagine. Answer:

Hydrogen Hydrogen bond bon d donors: α-amino group, group, amide nitrogen. Hydrogen bond acceptors: ac ceptors: α-carboxylate group, amide carbonyl. 4. Draw the dipeptide Asp-His Asp-His at pH 7.0. Answer:

5. Calculate the number of possible pentapeptides that contain one residue each of Ala, Gly, His, Lys, and

Val. Answer:

The first first residue can be b e one of five residues, the second one of the remaining four, etc. 6. Determine the net charge of the predominant form of Asp at (a) pH 1.0, (b) pH 3.0, (c) pH 6.0, and (d) pH

11.0. Answer: (a) +1; (b) 0; (c) –1; (d) –2.

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Problems


7. Calculate the p I of (a) Ala, (b) His, and (c) Glu. Answer: (a) p I = (2.35 + 9.87)/2 = 6.11 (b) p I = (6.04 + 9.33)/2 = 7.68 (c) p I = (2.10 + 4.07)/2 = 3.08 8. A sample of the amino acid tyrosine is barely soluble in water. Would a polypeptide containing only Tyr

residues, poly(Tyr), be more or less soluble, assuming the total number of Tyr groups remains constant? Answer:

The polypeptide would be even less soluble soluble than free Tyr, Tyr, because most of the amino and carboxylate ca rboxylate groups that interact with water and make Tyr Tyr at least slightly slightly soluble soluble are lost in forming the peptide pe ptide bonds in poly(Tyr). 9. Circle the chiral c hiral carbons in the following following compounds: compounds:

Answer:

10. Draw the four stereoisomers of threonine. Answer:



Problems


11. The two CαH atoms of Gly are said to be prochiral, because when one of them is replaced by another

group, Cα becomes chiral. Draw a Fischer projection of Gly and indicate indicate which H must must be replaced re placed with CH3 to yield D-Ala. Answer:

12. The bacteriall bac terially y produced produce d antibi ant ibiotic otic gramicidin gramicidin A forms channels in cell membranes that allow the free

diffusion of Na+ and K + ions, thereby killing killing the cell. ce ll. This This peptide consis co nsists ts of a sequence of D- and L-amino acids. The sequence of a segment of five amino acids in gramicidin A is R–Gly– L-Ala– D-Leu– L-Ala– D-Val–R ′. Complete the Fischer projection below by adding the correct group to each vertical bond.

Answer:

13. Describe isoleucine (as shown in Table 4-1) using the RS system.



Problems


from the following α-keto acids:

Answer: (a) Glutamate; (b) aspartate 15. Identify the amino acid residue from which the following following groups groups are synthesized: synthesized: (a)

(b)

Answer: (a) Serine ( N -acetylserine); -acetylserine); (b) lysine (5-hydroxylysine); (c) methionine methionine ( N -formylmethionine). -formylmethionine). 16. Draw the peptide pe ptide ATLDAK. ATLDAK. (a) Calculate its approximate p I. (b) What is its net charge at pH 7.0? Answer:



Problems


(b) The net charge at pH 7.0 is 0 (as drawn above). 17. The protein insulin consists consists of two polypeptides termed the A and B chains. Insulins Insulins from different

organisms have been isolated and sequenced. Human and duck insulins have the same amino acid sequence with the exception exce ption of six amino amino acid residues, as shown below. Is the p I of human insulin lower than or higher than th at of duck duc k insulin? insulin? Amino acid residue A8

A9

A10 B1

Human

Thr Ser

Ile

Duck

Glu Asn Pro

B2

Phe Val Ala

B27 Thr

Ala Ser

Answer:

At position A8, duck insulin has a Glu residue, whereas human insulin has a Thr residue. Since Glu is negatively charged at physiologi physiological cal pH and a nd Thr is neutral, human insulin insulin has ha s a hig h igher her p I than duck insulin. (The other amino acids that differ between the proteins do not affect the p I because they are uncharged.)

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Ch 4 - Problems

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