1. The following mini-protein is dissolved in a buffer at pH = 9. What is its NE
ID: 12679 • Letter: 1
Question
1. The following mini-protein is dissolved in a buffer at pH = 9. What is its NET charge at this pH? YOU NEED TO SHOW YOUR WORK FOR LEU, GLU AND LYS
(the structures and pKa values are in the ppts)
N terminus leu – glu – tyr – gly – asp – lys C terminus
pKa chart
leucine= -carboxylic acid: 2.33; -amino: 9.74
glutamic acid= -carboxylic acid: 2.10;-amino:9.47; side chain: 4.07
tyrosine= -carboxylic acid: 2.20; -amino:9.11; side chain: 10.07
aspartic acid= -carboxylic acid: 2.10; -amino:9.82; side chain:3.86
lysine= -carboxylic acid:2.18; -amino:8.95; side chain: 10.53
Explanation / Answer
The net charge of the peptide can be calculated in this way: N'-NH3+ Leu-Glu-Tyr-Gly-Asp-Lys-COOH-C' The amino acids of Glu, Asp, Lys have side chains which also play an important role in the charge of the peptide. As the pKa value of the amino terminal of leucine is 9.74, the proton will not dissociate at pH 9. Hence it will have +1 charge. As the pKa value of the side group of Glutamic acid is 4.07, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of aspartic acid is 3.86, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of lysine is 10.53, the proton will not dissociate at pH 9. Hence it will have +1 charge. As the pKa value of the carboxy terminal of lysine is 2.18, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of Glutamic acid is 4.07, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of aspartic acid is 3.86, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of lysine is 10.53, the proton will not dissociate at pH 9. Hence it will have +1 charge. As the pKa value of the carboxy terminal of lysine is 2.18, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of aspartic acid is 3.86, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of lysine is 10.53, the proton will not dissociate at pH 9. Hence it will have +1 charge. As the pKa value of the carboxy terminal of lysine is 2.18, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the side group of lysine is 10.53, the proton will not dissociate at pH 9. Hence it will have +1 charge. As the pKa value of the carboxy terminal of lysine is 2.18, the proton will be dissociated at pH 9. Hence it will have -1 charge. As the pKa value of the carboxy terminal of lysine is 2.18, the proton will be dissociated at pH 9. Hence it will have -1 charge. Hence the overall charge of the peptide is -1 at pH 9.Related Questions
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