Meredith92

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Histidine's side chain has a pKa of 6.05... doesnt that make it an acidic side chain? Why is it considered basic?
Thanks!
 

rjosh33

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Histidine's side chain has a pKa of 6.05... doesnt that make it an acidic side chain? Why is it considered basic?
Thanks!
Good question. I had to brush off the dust on my Biochem book for this one. It sounds like the two nitrogens that are part of the imidazole ring on histidine's side chain give the amino acid both acidic and basic properties. The basic nitrogen (with the free electrons) can abstract a proton from a donor group, only to have it shuttled and relayed to another group via its more acidic nitrogen.

It's this amphoteric property that makes it an important AA in the active site of enzymes, and explains why it's considered a basic amino acid -- even though its pKa would suggest otherwise.
 
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Meredith92

Meredith92

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thanks for that explanation! So we should still think of it as deprotonated at pH's past 6.05?
 

rjosh33

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thanks for that explanation! So we should still think of it as deprotonated at pH's past 6.05?
Yeah, for the most part. Some small fraction will still remain protonated (remember the Henderson-Hasselbach equation), but you should think of histidine as being basic under physiologic conditions.

If you're using Examkrackers they kind of beat that point into your head (basic vs. acidic aa's), but the MCAT probably isn't too likely to directly test such trivia.
 
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Meredith92

Meredith92

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Sorry I'm still confused! How can we think of it as basic at physiological pH if it is in its deprotonated form since the physiological pH is greater than the pKa??
 

Bru

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Well at some level it isn't going to acidic because it doesn't have a proton to donate. There are also factors which can raise the pKa like neighboring side chains. Still regardless it will be slightly basic/protonated due to the not vast difference in pH to pKa.