general pka question about amino acids

[bartzx3]

From the TPR, the pka of histidine's side chain is ~6.0. The pH of our bodies is 7.4 (physiological pH). Kaplan says that at physiological pH, His will be protonated. And that's the reason why His is positively charged.

I don't understand this part. Since histidine's pKa < physiological pH. Then when His is added in our bodies, wouldn't it get deprotonated so it can reduce the pH of our bodies?

I know I am missing something here.

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[chemist16]

The pKa tells you how acidic the proton is and the pH tells you how acidic the solution is. So in this case, the proton is more acidic than the solution, so it will act as an acid and exist in the deprotonated form. So you're right in that most histidine should be deprotonated at physiological pH. However, the pKa just tells you when exactly half the acid is protonated/deprotonated. So even at physiological pH, some of the histidine will still be protonated. But it does seem odd that Kaplan only mentions that it will be protonated at physiological pH.