4a. If you had a cysteine side chain in a protein such that it was in the enviro

Question

4a. If you had a cysteine side chain in a protein such that it was in the environment shown below, what would you expect to be true about the pK, value for this side chain knowing that the literature value for this side chain's pKa value is 8.5. Would you expect the pK, to be higher, lower or the same as the literature value? Explain in 2 sentences of less your reasoning for the pKa of this cysteine side chain. NOTE: the thick black line here represents the peptide backbone and not all amino acids present in this protein are shown. Only the cysteine and another amino acid side chain are shown. 4b. If you had a histidine amino acid side chain surrounded by non-polar amino acids in a protein, 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 value is 6. Would you expect the pK, to be higher, lower or the same as the literature value? Explain in 2 sentences of less your reasoning for the pKa of the histidine side chain.

Explanation / Answer

Biochemistry

4a. With cysteine side chain having -SH functionality in an carboxylate -COO-exnvironment, we would expect the pKa value for this side chain to be lower than the literature value. This is due to the fact that the H of the -SH functionality is now bonded to the O- of the carboxylate which makes it easily ionizable. Lower pKa means stronger acid.

4b. If histidine amino acid side chain (basic) is surrounded by a non-polar amino acid in a protein, the resulting pKa value would be expected to be same as the literature value. No polarization of the histidine side chain would be expected in such an case and therefore, the pKa value for the histidine side chain remains unchanged.

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