If you had a histidine amino acid side chain surrounded by non-polar amino acid

Question

If you had a histidine amino acid side chain surrounded by non-polar amino acid in a protein, what would you expect to be true about the pKa value for this side chain knowing that the literature value of this side chain's pKa is 6.

If you had a cysteine side chain in a protein that was in the environment with a glutamate( COO-). what would you expect to be true about the pKa value for this side chain knowing that the literature value for this side chain's pKa is 8.5.

Would you expect the pKa to be higher, lower, or the same as the literature value.

Explanation / Answer

For this exercise you must take into account that it is the pKa (or constant of acidity) this is a measure of how easily an amino acid can accept a proton (remember that its ammonium group can capture a proton) (Although the carboxyl group can also accepting protons for this case is said to be whole or not dissociated).

For the first example, the value would be the same given that in the bibliography it says that histidine alone has a pKa of 6, so it does not change It also considers that non-polar amino acids are hydrophobic (which is why it is more difficult for their species to react in aqueous media [ie to acept or donate protons]).

For the second one, the value of the cysteine can be affected downwards, since since there is glutamate this first has a lower pKa for which it is going to be taking protons from the medium (so much that its carboxyl group will be not dissociated) besides the one of the cysteine.

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