Histidine Properties According to the AAMC

[You're My Boy Blue]

Hi all,

I have noted some inconsistencies about the properties of Histidine in my Kaplan study review. I was wondering if anyone had information that the AAMC would use themselves. I learned that histidine is considered a basic amino acid in graduate biochem, Kaplan agrees and disagrees depending on the question... I also learned that histidine was aromatic, again, Kaplan agrees and disagrees depending on the question. Can anyone resolve this?

---

Members don't see this ad.

 

[theonlytycrane]

His has a 6 pi aromatic system with a pka ~ 6.0. Is there a kaplan question that's disagreeing with this?

 

[You're My Boy Blue]

His has a 6 pi aromatic system with a pka ~ 6.0. Is there a kaplan question that's disagreeing with this?

There are questions that disagree with the aromatic status of Histidine. As far as the pKa goes there's no disagreement about the value but there is disagreement about whether or not it is considered basic.

 

[sleepyhead22]

There are questions that disagree with the aromatic status of Histidine. As far as the pKa goes there's no disagreement about the value but there is disagreement about whether or not it is considered basic.

From my experience, only the hardest questions might give you a choice between two similar amino acids. In most aamc materials, a question might ask what a particular change to a certain residue would do, and the choices are all pretty different. Just know what histidine looks like, the pka, and the fact that it is often at active sites because it can be protonated or deprotonated. EK always did weird little trick questions like that, and I think it's mostly to get these things to stick in your head not because you'll actually see questions like that in the future.

 

[aldol16]

There are questions that disagree with the aromatic status of Histidine. As far as the pKa goes there's no disagreement about the value but there is disagreement about whether or not it is considered basic.

Then those questions are wrong. There is no doubt about the aromaticity of an imidazole ring. Whether it's acidic or basic, though, depends on the conditions because its pKa is so close to physiological pH. In other words, His can act as base or acid depending on the situation. In some enzymes, it must act as both acid and base at times in order to perform general acid/base catalysis.