Homework SourseIf one of the H side chains was methylated in this peptide h
If one of the H side chains was methylated in this peptide, how would that change the classification of that H amino acid? How would this methylation change change the overall pI of the protein (would it increase or decrease relative to the pI you concluded in 2a)?
I determined the pI in 2a to be pI = 10
Assuming that the
N-terminal pka is 9~10
C-terminal pka is 2
Acidic side chain pka is 4
Basic side chain pka is 11
And His side chain pka is 6
If one of the H side chains was methylated in this peptide, how would that change the classification of that H amino acid? How would this methylation change change the overall pI of the protein (would it increase or decrease relative to the pI you concluded in 2a)?
I determined the pI in 2a to be pI = 10
Assuming that the
N-terminal pka is 9~10
C-terminal pka is 2
Acidic side chain pka is 4
Basic side chain pka is 11
And His side chain pka is 6
2c. If one of the H side chains was methylated in this peptide, how would that change the classification of that H amino acid? How would this methylation change the overall pl of the protein (would it increase or decrease relative to the pl you concluded in 2a)? (10 points) Solution
If the side chain does not have an ionizable group, then the pI is simply the average of the -NH3 and -COOH pKa values.
If the side chain has an ionizable group then all three pKa values must be considered.
• If the side chain is acidic (asp and glu), then average the side chain pKa with the -COOH pKa
• If the side chain is basic (his, arg, and lys), then average the side chain pKa with the -NH3 pKa.
• For other ionizable groups (tyr and cys), determine which is the middle pKa and average it with the -COOH pKa.
Peptides:
A general rule of thumb that works for peptides (also works for amino acids):
Find the approximate pH where the net charge would be zero, and average the closest pKa values
on either side of this value.
