If one of the H side chains was methylated in this peptide, how would that chang
ID: 316656 • Letter: I
Question
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)
Explanation / Answer
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.
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