USMLE Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
316 Posts
Discussion Starter · #1 ·
guys i dint understand this question:

A specially-modified peptide (it has a methyl cap) is ordered from a vendor. The peptide has the sequence (NH3+)-MEDGRGKQGGKA-(COCH3). What is the charge of the following modified peptide at pH 7.0:

A. -2
B. -1
C. 0
D. +1
E. +2
 

·
Registered
Joined
·
487 Posts
here the drug is weak basic....due to NH3+ ....
n basic drugs hv pKa > 7....
so pH - pKa must be negative value....either -1 or -2....
drug here is fully in ionised form...means pH -pKa must be -2...:eek:

let me know the correct ans...
 

·
Registered
Joined
·
73 Posts
Choice E.

Lysine/K (pKa = 10.53) and Arginine/R (pKa = 12.48) are positively charged at pH 7.0. Aspartate/D (pKa = 3.65) and Glutamate/E (pKa = 4.25) are negatively charged at pH 7.0. Also, don't forget to add +1 for the N-terminus and -1 for the C-terminus. Here, the C-terminus is COCH3 instead of COO-, thus you do not count the -1. In total, that's +1,-1,-1,+1,+1,+1 = +2

Note: Histidine/H (pKa = 6.00) will also contain a significant fraction in the charged form. This exact amount can be determined using the Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])). If included in the question stem, the pH will usually be farther away from 6.0.

It's from wikitestprep. I hope that such questions wont be on exam
 

·
Registered
Joined
·
316 Posts
Discussion Starter · #5 ·
Lysine/K (pKa = 10.53) and Arginine/R (pKa = 12.48) are positively charged at pH 7.0. Aspartate/D (pKa = 3.65) and Glutamate/E (pKa = 4.25) are negatively charged at pH 7.0. Also, don't forget to add +1 for the N-terminus and -1 for the C-terminus. Here, the C-terminus is COCH3 instead of COO-, thus you do not count the -1. In total, that's +1,-1,-1,+1,+1,+1 = +2

Note: Histidine/H (pKa = 6.00) will also contain a significant fraction in the charged form. This exact amount can be determined using the Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])). If included in the question stem, the pH will usually be farther away from 6.0.

It's from wikitestprep. I hope that such questions wont be on exam
man u have copy&pasted the xact xplanation given in the wikitestprep site... i dint understand their xplanation plzz explain more.. m totally blank at this question...

emmm... u cant challenge usmle if u hav such weird question on ur test day....:p and usmle is putting weird questions in the test more often these days....
 

·
Registered
Joined
·
73 Posts
man u have copy&pasted the xact xplanation given in the wikitestprep site... i dint understand their xplanation plzz explain more.. m totally blank at this question...

emmm... u cant challenge usmle if u hav such weird question on ur test day....:p and usmle is putting weird questions in the test more often these days....
I am not biochemist and can't explain more than explanation from wiki. Which I don't understand either to solve similar stem.

If you have such weird question you have 3 options:
1. try to solve by recollecting which letter assigned to exact AA and calculate charge. Total time spent for this will be more than 1-2-3 minutes
2. try to eliminate "incorrect" answer by guessing. Again time consuming, because resemble option 1.
3. third option in such situation I prefer more. If you see absolutely unknown question, put your favorite letter, mark this question and move on. Spare time for other questions. In the end of block if you have enough time, try to solve it.
Last option is advice from book Kaplan Qbank: " If you don't know the answer, admit it, make your best guess, and move on to the next question"

For sure we'll challenge weird questions these days, but it doesn't mean that we have to solve them wasting valuable time. They putting such questions to slow you and make you anxious.

I hope this will help you :D
 

·
Registered
Joined
·
82 Posts
Here you go

(NH3+)-MEDGRGKQGGKA-(COCH3)

These letters MED etc are symbols for AAs.

Amino Acid Symbols

M=methionine
D= aspartate
(acidic -1)
E=
glutamic acid (acidic -1)
G=
glycine
K=
lysine (basic +1)
R=
arginine (basic +1)
Q=glutamine,


So, we have to know what amino acid is acidic with -1 or basic with +1.
Also add +1 for NH3+ end ,we cant add -1 for carboxyl group b/c it is methylated.

So, we will end up with having 3 basic AAs (one Arginine & two lysine) and 2 Acidic (Aspartate & Glutamic Acid) and additional +1 for the (NH3+),

this way we will get +1,-1,-1,+1+1+1= +2

I hope this solves it ........

I wouldn't take any credit for this solution. I found the main concept while searching the internet...:eek:

Always remember "Google" is your best friend.
 

·
Registered
Joined
·
316 Posts
Discussion Starter · #9 ·
awesum!!! thanks man!!
 

·
Registered
Joined
·
209 Posts
@doc_study thanks

but plzzz change +1 and _1 when u are mentioning the amino acids and there symbols u mentioned them reverse

acidic is -1 and basic is +1
 

·
Registered
Joined
·
82 Posts
@doc_study thanks

but plzzz change +1 and _1 when u are mentioning the amino acids and there symbols u mentioned them reverse

acidic is -1 and basic is +1
@ecgram thanks for pointing out. My apologies.
I have updated the previous post.
 
1 - 11 of 11 Posts
Top