Question

Please explain why one amino acid may have a different pK in different proteins or in different regions of the same protein.

Answer 1

pKa = -log(10) of the acid dissociation constant of a solution. Therefore it is easily affected by the pH of any solution.
Protein molecule is made up of long chains of amino acids which are linked to its neighbour amino acid change through a covalent peptide Bond that's why proteins are also known as polypeptides every type of protein have an unique sequence of a minus which is exactly the same from one molecule to the next

To understand this let take an example of titratable amino acid.
When protein folds, the titratable amino acids present in protein are transferred from a liquid (solution) type of surrounding to a 3-dimensional structure of the protein surrounding. For example, when aspartic acid which is unfolded protein is present in and surrounding where it is exposed its titratable side chain to water. Then aspartic acid will find itself situated deep inside the protein where there will be no exposure to solvent.

Therefore, aspartic acid (folded protein)will be close to other titratable groups inside protein and start nteracting with ions( permanent charges) and dipoles in the protein.
So the equilibrium constant will change it values depending upon the environment amino acid is present in and it will also depend upon the interaction of amino acid.