2. From Amino Acids to Peptides Consider a polypeptide with the following
sequence:
KPYEVPAMIPES
Assume the pKas of the N- and C-termini are 7.9 and 3.8, respectively, and use the pKa values on Table 1 (Table 1) for R-groups. Estimate the net charge on this molecule at pH 7 and pH 12.
A. 0 (pH 7), -3 (pH 12)
B. 0 (pH 7), -4 (pH 12)
C. -1 (pH 7), 0 (pH 12)
D. -1 (pH 7), -4 (pH 12)
E. -4 (pH 7), -1 (pH 12)
Table 1. Amino acid pKa values
Amino Acid α-carboxylic acid α-amino Side chain
Alanine 2.35 9.87
Arginine 2.01 9.04 12.48
Asparagine 2.02 8.80
Aspartic Acid 2.10 9.82 3.86
Cysteine 2.05 10.25 8.00
Glutamic Acid 2.10 9.47 4.07
Glutamine 2.17 9.13
Glycine 2.35 9.78
Histidine 1.77 9.18 6.10
Isoleucine 2.32 9.76
Leucine 2.33 9.74
Lysine 2.18 8.95 10.53
Methionine 2.28 9.21
Phenylalanine 2.58 9.24
Proline 2.00 10.60
Serine 2.21 9.15
Threonine 2.09 9.10
Tryptophan 2.38 9.39
Tyrosine 2.20 9.11 10.07
Valine 2.29 9.72