[stride23]

The environment of a particular residue in a protein can influence its pKa. What would be the
average charges on particular residues with the indicated pKa in proteins in solutions buffered at
the indicated pH?


Residue /pKa/ pH of solution
    H        5.5       7
    H        7.5       7
    C        9.0       7
    K        8.5       8
    Y        8.5       8


The pKa of the first histidine (5.5) is a pH unit lower than normal while the pKa of the second
histidine (7.5) is a pH unit higher than normal. Can you suggest some properties of the local
environments that might cause these shifts?


I don't quite understand as how the residue only has 1 pka value when it has 3 pka values (N-terminal, C-terminal and R group). How I approached this was If the pH was higher than the pKa, then the average charge for that residue would be or negative and vice versa if pH were lower than the pKa.

[Babcock_Hall]

In the context of a polypeptide, there is one one pK_a value per residue, that of the side chain.  The amino groups and carboxylate groups are tied up in carboxamide bonds that in this situation are also called peptide bonds.