USMLE Forums banner
1 - 17 of 17 Posts

·
Registered
Joined
·
1,684 Posts
Discussion Starter · #1 ·
Ok, so I came across this question while doing UW and I got it right but after reading the explanation I was just like :eek::confused::toosad::rolleyes::indifferent::))

The question shows the titration curve for an amino acid & asks us which amino acid is it. The explanation states that since there are three different pKa's the amino acid must have three titratable protons - histidine, arginine, lysine, aspartic acid, glutamic acid, cysteine & tyrosine. It then states that the pKas of histine are 1.8, 6 & 9.2 which is the most consistent with the graph shown and is the correct answer.

DO WE HAVE TO KNOW THE pKa's ?!
 

Attachments

·
Registered
Joined
·
1,357 Posts
Ok, so I came across this question while doing UW and I got it right but after reading the explanation I was just like :eek::confused::toosad::rolleyes::indifferent::))

The question shows the titration curve for an amino acid & asks us which amino acid is it. The explanation states that since there are three different pKa's the amino acid must have three titratable protons - histidine, arginine, lysine, aspartic acid, glutamic acid, cysteine & tyrosine. It then states that the pKas of histine are 1.8, 6 & 9.2 which is the most consistent with the graph shown and is the correct answer.

DO WE HAVE TO KNOW THE pKa's ?!
hahahahahahahaha....this question sucks! :rolleyes:
 

·
Registered
Joined
·
395 Posts
Ok, so I came across this question while doing UW and I got it right but after reading the explanation I was just like :eek::confused::toosad::rolleyes::indifferent::))

The question shows the titration curve for an amino acid & asks us which amino acid is it. The explanation states that since there are three different pKa's the amino acid must have three titratable protons - histidine, arginine, lysine, aspartic acid, glutamic acid, cysteine & tyrosine. It then states that the pKas of histine are 1.8, 6 & 9.2 which is the most consistent with the graph shown and is the correct answer.

DO WE HAVE TO KNOW THE pKa's ?!
yup u are right histidine is one of positively charged amino acid with three pka's and histine is required for N.O synthesis by nitric oxide synthase ... :rolleyes:
 

·
Registered
Joined
·
23 Posts
I dont think so.

I've heard on the actual exam they are always blah questions which no normal person is able to answer.

They may be part of the experimental pool , they may be suckers to make you panic, throw you off track .. make you confused.

Ignore them and move on. This seems like one in a million questions.

I doubt if i would lose sleep over this blah ,and no one else should either.
 

·
Registered
Joined
·
129 Posts
yup... the Q looks pretty absurd... but, we dont need to know the pKa's to answer this Q.... i think the take home point is-just know the amino acids with 3 titrable protons and here's how i see it ....

the basic (HLA) ones- Histidine, Lysine, Arginine
the acidic ones - aspartate, glutamate
just 2 more... cysteine and tyrosine....
"acidic basic CT" :)
 

·
Registered
Joined
·
369 Posts
Hahahaha. Just had this question and had to google it. :eek: Your responses made me feel better!
Just cram that question Guys !!! ;D when i saw that at my time, i was like, what IS This?? just memorize the concept , just in case you get a similar question like this in exam.. if
 
  • Like
Reactions: phlebgirl

·
Registered
Joined
·
100 Posts
Ok, so I came across this question while doing UW and I got it right but after reading the explanation I was just like :eek::confused::toosad::rolleyes::indifferent::))

The question shows the titration curve for an amino acid & asks us which amino acid is it. The explanation states that since there are three different pKa's the amino acid must have three titratable protons - histidine, arginine, lysine, aspartic acid, glutamic acid, cysteine & tyrosine. It then states that the pKas of histine are 1.8, 6 & 9.2 which is the most consistent with the graph shown and is the correct answer.

DO WE HAVE TO KNOW THE pKa's ?!
LOL I remember this question well, I also thought the same as you when reading it the first time, but then when you read the explanation you see that you can deduct this if you know which aa have different pKa's and are titratable, so no need to know the exact pKa's there.
 

·
Registered
Joined
·
36 Posts
Awful question

I had this question yesterday and after reading the explanation I was like "WTF are you serious?" like my test is around the corner and this type of question serves only one purpose: to freak you out. I'm glad I'm not the only one who thought it was crazy.
 

·
Registered
Joined
·
119 Posts
sHi guys,

If you get this question, knowing acidic and basic amino acids helps... but I got it wrong. So I learned

pH - concentration of ions
pKa - strength of acid
Titratable proton - Proton that can be lost when an acid and base collide with each other

Higher the pka, the stronger the acid (lower pH)
Lower the pKa, the weaker the acid (higher pH)

Only histidine is listed from one of the 7 AA that have titratable protons. Remember that Histidine has a net zero charge at body pH but it has 3 POTENTIALLY titratable protons. Should one of these protons dissociate, it give histidine an ionic charge and it spills out into the blood due to ionic concentrations gradients. In addition, those H protons that dissociate wreak havoc on body systems and induce the clinical manifestations that are seen in histidinemia (HISTIDASE deficiency in sub asian pop causes buildup of AA in blood) . The graph shows gradual addition of a base which causes titration of the proteins at different pKA values (values of pka are not important but the concept is as this concept can be used in many acid base balance disturbances scenarios)
 
1 - 17 of 17 Posts
Top