Consider the peptide: NH2-GAPAGPAGTGKTETTKDLAKSMALLCWFNCS-COOH Table I. pK values of some amino acids amino acid | pK of side chain or terminal group Alanine (Ala or A) | uncharged Arginine (Arg or R) | 12.5 Asparagine (Asn or N) | uncharged Aspartic acid (Asp or D) | 3.9 Cysteine (Cys or C) | 8.3 Glutamic acid (Glu or E) | 4.3 Glycine (Gly or G) | uncharged Histidine (His or H) | 6.0 Leucine (Leu or L) | uncharged Lysine (Lys or K) | 10.8 Phenylalanine (Phe or F) | uncharged Serine (Ser or S) | uncharged Tryptophan (Trp or W) | uncharged Tyrosine (Tyr or Y) | 10.9 Valine (Val or V) | uncharged peptide N terminus | 7.8 peptide C terminus | 3.6 Using Table I above, determine the net charge of the peptide at pH6. 1 Determine the net charge of the peptide at pH7. 1 Determine the net charge of the peptide at pH9. -2
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Step 1: Calculate the net charge of the peptide at pH 6 by considering the pKa values of the amino acids and the peptide N and C terminus. Show more…
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Isoelectric focusing can separate peptides based on their relative contents of acidic and basic residues. When voltage is applied to an electrophoretic gel with a pH gradient, each peptide migrates until it reaches a point where it has no net charge. The pH at this point is the isoelectric point (pI) for that peptide. One can estimate the pI fairly accurately (within 0.1 or 0.2 pH units) using the pK values of all the proton dissociable groups in the peptide. Consider the hexapeptide Leu-His-Glu-Lys-Met-Asp. Amino acid pK: Arg 12.5, Asp 3.7, Cys 8.2, Glu 4.3, His 6.0, Lys 10.5, Tyr 10.5, *peptide-NH3+ 8.0, *peptide-COOH 3.4. *Amino and carboxy terminal values differ from the alpha amino and carboxy values of a single amino acid. Step 1: Determine the total positive charge on the hexapeptide when all acidic and basic groups are fully protonated. Enter your answer without the sign. charge: Step 2: Determine the pK value of the group that loses a proton to form the zwitterionic species with a net charge of zero. pK = Step 3: Determine the pK value of the group that gains a proton to form the zwitterionic species with a net charge of zero. pK = Step 4: Estimate the pI value of the hexapeptide. pI =
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.
The amino acid residue that would LEAST likely be buried in the interior of a water-soluble globular protein is: lysine. tyrosine phenylalanine. methionine isoleucine What would be the overall charge of the peptide Asp-Ala-Arg-His at pH 4? 3 0 2 –1 1 Citric acid is an important intermediate in glucose metabolism and is synthesized in the mitochondrial matrix. The pKa values for each of the three carboxylic groups of the citric acid are 3.1, 4.8, and 6.4. What is the charge on a citrate molecule formed in the mitochondrial matrix where the pH is 5.8? –3 +2 –2 +1 +3 Which amino acids would be most soluble in a nonpolar solvent such as toluene? threonine Leucine histidine glutamine glycine Which statements about the polypeptide Ser-Tyr-Gly-Phe-Met-Gly are CORRECT? Select all that apply. Serine is the N-terminal residue. Glycine is the C-terminal residue. Glycine is the N-terminal residue. Methionine is the N-terminal residue. Serine is the C-terminal residue
Keemin L.
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