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What is meant by the Pka value of a side chain in an amino acid?

  1. Oct 20, 2009 #1
    I know how to get the answer for this question using hesselback equation but I don't understand the question. I have some questions about the questiuon.

    1. The PKa value of the sulphydryl (-SH) group of cysteine is 8.33. Calculate the fraction of anion to free sulfhydryl group at PH 7.O.

    So is this Pka value referring to

    SH + H20 <---> H+ + S- reaction

    Why is this reaction significant. How does getting the PH of the equlibrium constant of this reaction provide any meaningful value.

    Also in this amino acid there is only one SH group(meaning two atoms) so how can an equilibrium reaction occur.

    My question is related to PKa values. I don't understand how getting the PH of the equilibrium constant can be meaningful and that two atom problem.

    Thank you so much
  2. jcsd
  3. Oct 27, 2009 #2


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    Since you already know that your sulfhydryl side chain can deprotonate, you are halfway there. Sulfhydryl deprotonates just as an ammonium ion might or a carboxylic functionality might. The characteristic pH at which exactly half of them are deprotonated is a useful way to characterize them (pKa) just as it is useful to describe the strength of an acid using pKa. The fraction of these side chain groups that are charged often determines the tertiary structure and solubility of proteins or peptides that contain them. So it is very meaningful to know what fraction of the ionizable side chains are present in a charged state and what fraction is present in an uncharged state. Their pKa is useful in that determination.

    If your question relates to how to employ the Henderson-Hasselbalch equation to answer your question, you need to show us a bit more work.

    Start by writing down the equation and using what you know in that equation.....
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