Step I Uworld QID 2121

[indsha008]

Hi guys ...

Please can someone explain me the explanation given in UW of this question. It seems so random. Why does bump in curves reflect AA's pKas???

Thanks a lot.

 

[septoplasty]

Yea, I remember this question out of the 2086.. it was so odd.

I wouldn't mind an explanation either (I just skipped it cause it sounded stupid)

 

[qmcat]

i already took step 1, and my uworld subscription expired, but i'll give it a shot at explaining it from memory.

each AA has certain pKa's for when hydrogen ions come on or off the AA. so i think that each bump in the graph represents a hydrogen coming on/off. if you're in the basic pH range then hydrogen ions are more likely to come off the AA. if you're in the acidic pH range, there's going to be more H+ around in the environment, so the AA is more likely to be protonated.

anyway, i'm not sure if that's 100% correct... i already forgot a lot since i took the exam, but hope it helps somewhat.

 

[Tig2575]

i already took step 1, and my uworld subscription expired, but i'll give it a shot at explaining it from memory.

each AA has certain pKa's for when hydrogen ions come on or off the AA. so i think that each bump in the graph represents a hydrogen coming on/off. if you're in the basic pH range then hydrogen ions are more likely to come off the AA. if you're in the acidic pH range, there's going to be more H+ around in the environment, so the AA is more likely to be protonated.

anyway, i'm not sure if that's 100% correct... i already forgot a lot since i took the exam, but hope it helps somewhat.

This is accurate. As a rule of thumb, only the 3 basic and 2 acidic amino acids have 3 pKa's, while the others have 2. The "middle" pKa of both aspartate and glutamate (the acidic acids) are far at the acidic end, while the middle pKa of both lysine and arginine (the strong basic acids) are far at the basic end. The middle pKa of histidine (which is frequently neutral at blood pH) is located in the middle of the spectrum, which is how they expected you to answer that question.

 

[indsha008]

see below !

 

[indsha008]

i already took step 1, and my uworld subscription expired, but i'll give it a shot at explaining it from memory.

each AA has certain pKa's for when hydrogen ions come on or off the AA. so i think that each bump in the graph represents a hydrogen coming on/off. if you're in the basic pH range then hydrogen ions are more likely to come off the AA. if you're in the acidic pH range, there's going to be more H+ around in the environment, so the AA is more likely to be protonated.

anyway, i'm not sure if that's 100% correct... i already forgot a lot since i took the exam, but hope it helps somewhat.

This is accurate. As a rule of thumb, only the 3 basic and 2 acidic amino acids have 3 pKa's, while the others have 2. The "middle" pKa of both aspartate and glutamate (the acidic acids) are far at the acidic end, while the middle pKa of both lysine and arginine (the strong basic acids) are far at the basic end. The middle pKa of histidine (which is frequently neutral at blood pH) is located in the middle of the spectrum, which is how they expected you to answer that question.

Thanks a lot guys .. appreciate it.