The side chain of histidine has a typical pKa value in the range of 6.5-7.4. However, when analyzing the pKa values in a particu
lar protein, scientists determined that one particular His residue has an unusually low pKa value of 4.8. Which of the following statements correctly explain this anomaly? 1) The microenvironment around a residue can impact its pKa value.
2) A positively charged amino acid must be in close proximity to this residue.
3) A negatively charged amino acid must be in close proximity to this residue.
Statements 1 and 2 are correct.
Statements 1 and 4 are correct.
Statements 1, 2, and 4 are correct.
Statements 2, 3, and 4 are correct.
4) This residue must be located on the surface of the protein.
The microenvironment around a amino acid residue can impact its pKa value according with the Born effect. If there is a positively charged environment, like in the second option, the pKa value will decrease, just as it happens in the histidine residue. So the correct answers are 1 and 2.