Dipeptide Isoelectric point

[Temperature101]

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What is the Isoelectric point of the dipeptide histidine-leucine?
A. 5.5
B. 6.1
C. 7.6
D. 9.2


Why would they consider only the Pkas of histidine to calculate the isoelectric point?

 

[cloak25]

What is the Isoelectric point of the dipeptide histidine-leucine?
A. 5.5
B. 6.1
C. 7.6
D. 9.2


Why would they consider only the Pkas of histidine to calculate the isoelectric point?

Because histidine has a charged side-chain. Leucine doesn't. pI = (pKa2 of histidine + pKa3 histidine)/2 = (6 + 9)/2 ~ 7.5ish. is it C?

 

[chiddler]

because leucine's R group is just some methyls. there is no pKa associated with its R group.

 

[MedPR]

What is the Isoelectric point of the dipeptide histidine-leucine?
A. 5.5
B. 6.1
C. 7.6
D. 9.2


Why would they consider only the Pkas of histidine to calculate the isoelectric point?

Leucine is non-polar.

 

[Temperature101]

Thanks... What if it was the dipedtide histidine- glutamic acid?

 

[MedPR]

Thanks...No wonder I suck in biochem

Memorize which amino acids are non-polar (and be good at basic gen chem) and you will have a much easier time answering AA based questions. Knowing all the nonpolars was a huge shortcut for almost all AA problems I encountered.

 

[MedPR]

Thanks... What if it was the dipedtide histidine- glutamine?

Consider both.

 

[Temperature101]

Memorize which amino acids are non-polar (and be good at basic gen chem) and you will have a much easier time answering AA based questions. Knowing all the nonpolars was a huge shortcut for almost all AA problems I encountered.

Wow..EK said we dont have to memorize these facts...I guess they are wrong.

 

[MedPR]

Wow..EK said we dont have to memorize these facts...I guess they are wrong.

Well you don't have to, but it helped me a lot.

 

[Temperature101]

Well you don't have to, but it helped me a lot.

If it helps, I guess I am gonna have to. Do you have a nmemonic?

 

[MedPR]

If it helps, I guess I am gonna have to. Do you have a nmemonic?

I used EK's mnemonic.

I saw lucy methodically probe and feel alan then val tripped glycine.

Isoleucine (I saw)
Leucine (lucy)
Methionine (methodically)
Proline (probe)
Phenylalanine (feel)
Alanine (alan)
Valine (val)
Tryptophan (tripped)
Glycine (glycine)

TBR passages have a ton of AA questions though so I ended up repeating this mnemonic so many times in my head that I just have them memorized without it.

 

[Temperature101]

I used EK's mnemonic.

I saw lucy methodically probe and feel alan then val tripped glycine.

Isoleucine (I saw)
Leucine (lucy)
Methionine (methodically)
Proline (probe)
Phenylalanine (feel)
Alanine (alan)
Valine (val)
Tryptophan (tripped)
Glycine (glycine)

TBR passages have a ton of AA questions though so I ended up repeating this mnemonic so many times in my head that I just have them memorized without it.

Thanks so much...

 

[MedPR]

Thanks so much...

Don't stress too much about it. My MCAT didn't have any AA on it.

 

[Temperature101]

Don't stress too much about it. My MCAT didn't have any AA on it.

For the amount of stuff that you know, if you dont break in the 33+, I dont think I got a chance to score the 28+ that I need. Let us know when you got your score...

 

[MedPR]

For the amount of stuff that you know, if you dont break in the 33+, I dont think I got a chance to score the 28+ that I need. Let us know when you got your score...

Thanks, I'm hoping for a mid 30 score. You know more than you think. Just keep working and don't give up 👍

 

[pm1]

What is the Isoelectric point of the dipeptide histidine-leucine?
A. 5.5
B. 6.1
C. 7.6
D. 9.2


Why would they consider only the Pkas of histidine to calculate the isoelectric point?

were the pKa values of histidine given? or do we just have to know? thanks!

 

[MedPR]

were the pKa values of histidine given? or do we just have to know? thanks!

They would have to give you at least the pKa of the sidechain and from there you can figure it out. You might know that Histidine is less basic than Lysine (pI of about 10) so the answer is probably C.