2013 paper 2 qn 5bi
i dont understand how to get the primary structure of
cys-tyr-ile-gln-asn-cys-pro-leu-gly
why cannot(or can?) the other way round ,
gly-leu-pro-cys-asn-gln-ile-tyr-cys ?
i understand by convention, N terminal is on the left, C
terminal is on the right, but i can't tell/dk how use this
convention here
part bii)
both phenol(at tyr there) and amine (at cys there) can be
deprotonated and protonated at pH 7?
The trickiness of this question (other than the disulfide bond /
linkage / bridge), is that the last amino acid residue gly's COOH
terminus has been acylated (ie. biochemically catalyzed
nucleophilic acyl substitution, addition-elimination, condensation)
with NH3 to generate a terminal amide, which Cambridge deliberately
employed to confuse students as to whether or not to begin with Gly
or Cys as the N terminus.
This is known as post-translational
modification of proteins (H2 Biology). Since NH3 isn't an amino
acid, the C terminus remains as Gly, and hence the only correct
acceptable answer is to begin with the correct N terminus, ie.
Cys.
Yes, Cambridge will accept either phenol or amine as the answer.
Don't be confused : phenol and amine exist in equilibrium with
phenoxide and ammonium respectively, it doesn't mean at pH 7,
phenol is completely deprotonated (it's not), or that amine is
completely protonated (it's not). That's why Ka and Kb values
exist, to describe the position of equilibrium and extent of
hydrolysis for acidic and basic groups.