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TBR 4 Bio ?'s

Discussion in 'MCAT Study Question Q&A' started by docntrainin, Aug 11, 2011.

  1. docntrainin

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    Please please please help me out if you can (I hope I am allowed to post this here, if I am not I am sorry!) I am so confused about the following 2 questions. Also, can someone explain to me in general what we need to know about pI's and pKa's for the MCAT. THAT WOULD BE BEYOND AWESOME!! :love::love::love:

    BTW, it gives structure of ADH with this info: Vasopressin. The pKa of the N-terminus is 8.0, the phenolic hydroxyl is 10.0, and the guanidino group is 12.5. (See post below for structure given)

    139. What is the isoelectric point (pI) of ADH?
    A. 9.00
    B. 10.25
    C. 11.25
    D. 15.25

    - C is the best answer. Vasopressin has 3 dissociable hydrogens. One is on the N-terminus (pKa ≈ 8.0), one is on the phenolic hydroxyl (pKa ≈ 10.0), and the last is on the guanidino group of arginine (pKa ≈ 12.5). The isoelectric point (pI) is that pH at which the molecule (vasopressin) has no net charge and is in the zwitterion form. As shown by the structure of vasopressin in Figure 1, there is an overall +2 charge. What pKa must be exceeded before we get to a vasopressin molecule with a +1 overall charge? The first hydrogen to come off, if we titrate the molecule with a base is the one with the lowest pKa. The hydrogen that comes off first is the N-terminus hydrogen. Once this happens, there is no longer a positive charge on that nitrogen. The vasopressin molecule now has an overall charge of +1. The next hydrogen to come off belongs to the phenolic hydroxyl. Removal of that hydrogen places a -1 charge on the phenolic oxygen atom. At this point, vasopressin now has a -1 charge on the phenolic oxygen and a +1 charge on the guanidino group. The overall charge is zero, and we have found the zwitterion. The pI is simply the average of the two pKa values on either side of the zwitterionic form. --> WHY!?!

    141. What is the net charge on vasopressin, if it is being eliminated in urine that has a maximum alkaline pH of 8.0?

    Answer: 1.5. WHY!?
     
    #1 docntrainin, Aug 11, 2011
    Last edited: Aug 12, 2011
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  3. docntrainin

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    i think i understand the answer to my first question, but if anyone can please help me out on the second question and some general info on MCAT worthy pIs and pKa knowledge, I would much appreciate it!

    bump...
     
  4. inaccensa

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    So pKa is the pH when of the half of the acid has dissociated ie HA-->H+ +A-
    Now the net charge on the molecule is +2 due to the N-H3 at the ends and OH is neutral. If this molecule is placed in a media with a pH=8, it simply means, half of the NH3+-->NH2 ( you can also do this by the HH equation, which tells you the concentrations of the acid and its conjugate base is the same; pH-pKa = log (base/acid) So you are only left with .5 charge on NH3+ and the other group has a full +1 charge.
     
  5. docntrainin

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    I guess where I get confused is how you know that because it was in a pH of 8, half of the NH3+ --> NH2. What led you to this? Thank you
     
  6. inaccensa

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    That is the definition of pKa
    pH =pKa + log (base/acid)
    When pH=pKa, base =acid
    pH-pKa = log (base/acid)
    0= log (base/acid)
    alog 0 = 1
    1= base/acid
    base=acid
     
  7. docntrainin

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    So because we know that the pKa of the N terminal is 8.0, we know that the pH is equal to the pKa for that specific group which means that the [acid]=[base] and that half of the acid has gone to its base leaving only half of the charge. The other protonated part of ADH is not affected though because the pH does not equal it's pKa thus it retains its full positive charge. half + one = 1.5? :idea:
     
  8. inaccensa

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    If the pH had been lets say 12, what do you think the charge would be?
    I strongly recommend that you look through a chemistry book. since this concept is important chem and bio.
     
  9. docntrainin

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    Do you have any good sources in mind? I really do need to brush up on this stuff! Btw, if the pH was 12, wouldn't the N terminus group be deprotonated and the phenolic hydroxyl? I'm not sure about the guanidine group.
     
  10. geeyouknit

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    Hey, I found that this website was really helpful.
    http://webhost.bridgew.edu/fgorga/proteins/acidbase_peptides.htm

    Since it seems like the TBR picture varies from the picture you posted, can you tell us which groups have charges on them?

    Thanks.
     
  11. docntrainin

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  12. docntrainin

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    bump... would anyone be willing to take a look at my second question here with reference to this diagram. thanks!
     
  13. geeyouknit

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    pH of solution is 8.

    The guanidino group has pKa of 12.5. Because pH < pKa, there's too many protons in the soln already, so it won't disassociate to add more protons (le chatliers). So it keeps its positive charge ( + 1).

    The phenolic group has pKa of 10. pH < pKa. Same thing. It won't dissociate. Keeps neutral charge (0).

    The amine group has pKa of 8. Since pH and pKa are the same, there are equal amounts of acid (+1 charge) and conj base ( 0 charge). Since only half the protonated acid dissociates, there's only half a charge ( +0.5).

    Thus, you have +1.5 .
     

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