Next Step End of Chapter 5 Biochem Test Question 20

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jeffbassett87
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Next Step End of Chapter 5 Biochem Test Question 20

Post by jeffbassett87 » Tue Feb 11, 2020 7:38 pm

I'm confused how to know where each C-terminus is. What exactly do the aromatic AA have to do with the C-terminus?
NS_Tutor_Mathias
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Re: Next Step End of Chapter 5 Biochem Test Question 20

Post by NS_Tutor_Mathias » Wed Feb 12, 2020 1:56 am

Terminus means "end". The C-terminus of a peptide is the end with a free carboxylic acid group. So there is only one in this case, and it doesn't matter too much that it is on serine (since we only care about the C-terminus, not any of the functional groups).

This question is solely about esterification, in this case conventional acid-catalyzed Fischer esterification. You are already provided with a carboxylic acid group, and with an alcohol (ethanol) and some catalytic amount of hydrogen ions (from sulfuric acid) we are all set.

Note that if acidic residues were present, this reaction would happen at those sites too, not just at the C-terminus. However, the only carboxylic acid group here is the C-terminus and the question is being very kind and specifically telling us that this is the site we care about.

Written out, the peptide would look like this:
NH2-Ala-Phe-Ser-COOH
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