TABLE 4-1 The 20 Common Amino Acids pKa values Name Abbreviation Symbol Mr* pKCOOH pKNH3+ pKR Alanine Ala A 89 2.34 9.69 β Arginine Arg R 174 2.17 9.04 12.48 Asparagine Asn N 132 2.02 8.80 β Aspartate Asp D 133 1.88 9.60 3.65 Cysteine Cys C 121 1.96 10.28 8.18 Glutamine Gln Q 146 2.17 9.13 β Glutamate Glu E 147 2.19 9.67 4.25 Glycine Gly G 75 2.34 9.60 β Histidine His H 155 1.82 9.17 6.00 Isoleucine Ile I 131 2.36 9.68 β Leucine Leu L 131 2.36 9.60 β Lysine Lys K 146 2.18 8.95 10.53 Methionine Met M 149 2.28 9.21 β Phenylalanine Phe F 165 1.83 9.13 β Proline Pro P 115 1.99 10.96 β Serine Ser S 105 2.21 9.15 β Threonine Thr T 119 2.11 9.62 β Tryptophan Trp W 204 2.38 9.39 β Tyrosine Tyr Y 181 2.20 9.11 10.07 Valine Val V 117 2.32 9.62 β *Mr values reflect the structures shown in Figure 4-3. The elements of water (Mr 18) are deleted during peptide bond formation, when the amino acid is incorporated into a polypeptide. At pH 7, a peptide with the sequence G-M-G-V-S-D-P-D-A has a net charge of: A. -3 B. -2 C. -1 D. +1 E. +2
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Step 1: Calculate the net charge for each residue in the peptide sequence: - G: 0 - M: 0 - G: 0 - W: 0 - E: 0 - S: 0 - P: 0 - A: -1 Show moreβ¦
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QUESTION 1 (20 MARKS) a) The figure below shows the structure of the amino acid Aspartate. Redraw this amino acid showing the Amino and Carboxyl groups in their predominant forms at pH 5 and at pH 11, given that pK1 (COOH) is 1.88, pK2 (NH3+) = 9.60 and pKR = 3.65. (label each structure with the relevant pH) b) Now draw aspartate and a different amino acid linked via a peptide bond. Label the peptide bond, the N-terminus and the C-terminus of your peptide. c) Name ONE example of each type of amino acid listed below: [1 mark each] Aliphatic Basic Aromatic Smallest Amide d) For each of the amino acids listed above name ONE type of bond/interaction in which the R-GROUP participates, contributing to the stability of the native three-dimensional structure of proteins.
Sri K.
1. (4 pts) For each amino acid listed below, write the letter of the corresponding chemical properties. Your answer should consider only the properties at pH near 7.0 (physiological pH). A. Non-Polar B. Polar, uncharged C. Polar, positively charged D. Polar, negatively charged Alanine Threonine Glutamate Tryptophan Asparagine Lysine Phenylalanine Arginine 2. Which of the following amino acids is a likely site of phosphorylation due to the presence of a hydroxyl group in the side chain? (Circle or list all correct choices.) A. serine B. threonine C. tyrosine D. cysteine E. methionine 3. This amino acid often disrupts the structures of alpha-helices because the side chain (R group) links back to the alpha carbon of the amino acid, causing a kink in the protein backbone. A. Met B. Pro C. Gly D. His E. Ile The structure shown below is leu-enkephalin, a peptide neurotransmitter that is found in the brains of many animals, where it has opioid-like actions in pain responses. 4. Which amino acid is found at the N-terminus of this peptide? A. glycine B. valine C. tyrosine D. leucine E. phenylalanine 5. How many amino acids are found in this peptide? A. 2 B. 3 C. 4 D. 5 E. 6 BONUS (2 pts): Write out the primary sequence of this peptide. Abbreviations are acceptable.
Histidine is one of the 20 amino acids found in proteins. Shown here is a fully protonated histidine molecule where the numbers denote the $\mathrm{p} K_{\mathrm{a}}$ values of the acidic groups. (a) Show stepwise ionization of histidine in solution. (Hint: The $\mathrm{H}^{+}$ ion will first come off from the strongest acid group followed by the next strongest acid group and so on.) (b) A dipolar ion is one in which the species has an equal number of positive and negative charges. Identify the dipolar ion in (a). (c) The pH at which the dipolar ion predominates is called the isoelectric point, denoted by pI. The isoelectric point is the average of the $\mathrm{p} K_{\mathrm{a}}$ values leading to and following the formation of the dipolar ion. Calculate the pl of histidine. (d) The histidine group plays an important role in buffering blood (see Chemistry in Action on p. 732 ). Which conjugate acidbase pair shown in (a) is responsible for this action?
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