December 05, 2021, 09:37:44 AM
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Topic: Why is Histidine positively charged?  (Read 215 times)

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Offline liravsc

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Why is Histidine positively charged?
« 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:


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.



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!
Never waste pure thoughts on an impure protein

Offline sjb

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Re: Why is Histidine positively charged?
« Reply #1 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

Offline liravsc

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Re: Why is Histidine positively charged?
« Reply #2 on: November 17, 2021, 04:28:47 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?
Never waste pure thoughts on an impure protein

Offline Babcock_Hall

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Re: Why is Histidine positively charged?
« Reply #3 on: November 18, 2021, 10:04:52 AM »
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.
« Last Edit: November 18, 2021, 10:23:46 AM by Babcock_Hall »

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