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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: liravsc on November 17, 2021, 03:58:19 PM

Title: Why is Histidine positively charged?
Post by: liravsc on November 17, 2021, 03:58:19 PM
My question is, what tells me that Histidine is positively charged just by looking at the structure:
(https://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/L-Histidine_physiological.svg/512px-L-Histidine_physiological.svg.png)

I thought H3N+ and COO- would make it neutral, or there is a positive charge on the ring?

The problem is, I'm drawing a dipeptide structure of Hys + Gly and I have to tell the structure's charge in pH 5.5 and pH 7.5 . I have no idea how or why either! Completely lost... Help.

(https://i.ibb.co/rFfGvy0/Whats-App-Image-2021-11-17-at-17-52-46.jpg)

At least tell me I drew it right.  They're asking for it very simple, this is not worth anything I'm only studying for an exam, so I wasn't expecting to show this drawing to anyone. I'm sorry. Thank you!
Title: Re: Why is Histidine positively charged?
Post by: sjb on November 17, 2021, 04:05:06 PM
A few issues with the drawing - His-Gly is more like NC(Cc1cnc[nH]1)C(=O)NCC(O)=O (or the zwitterion where the NH2 is an ammonium ion and the carboxylic acid is the carboxylate).

Look into pKas of the side chain
Title: Re: Why is Histidine positively charged?
Post by: liravsc on November 17, 2021, 04:28:47 PM
Quote from: sjb on November 17, 2021, 04:05:06 PM
A few issues with the drawing - His-Gly is more like NC(Cc1cnc[nH]1)C(=O)NCC(O)=O (or the zwitterion where the NH2 is an ammonium ion and the carboxylic acid is the carboxylate).

Look into pKas of the side chain

Thank you! Yeah I was partially tripping when I drew that (apparently TDAH), I see my mistake now. So when looking at the table only the Hys part counts?  I've looked an the pKa for ionizing Hys is around 6, so that means that above 6 it is positively charged and below it is neutral or the opposite?
Title: Re: Why is Histidine positively charged?
Post by: Babcock_Hall on November 18, 2021, 10:04:52 AM
Quote from: liravsc on November 17, 2021, 04:28:47 PM
So when looking at the table only the Hys part counts?  I've looked an the pKa for ionizing Hys is around 6, so that means that above 6 it is positively charged and below it is neutral or the opposite?
This is the sort of situation when a qualitative understanding of the Henderson-Hasselbalch equation is useful: pH = pKa + log([Base]/[HBase]).  Or you can choose pH 5 and pH 7 and see what happens to the ratio of conjugate base to conjugate acid.  Within a protein, the pKa of a histidine residue can be quite different.