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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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 p
Ka 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.