Question

Peptides often have functional groups other than free amino groups at the N terminus and other than carboxyl groups at the C terminus.

(a) A tetrapeptide is hydrolyzed by heating with 6 M, and the hydrolysate is found to contain Ala, Phe, Val, and Glu. When the hydrolysate is neutralized, the odor of ammonia is detected. Explain where this ammonia might have been incorporated in the original peptide.

(b) The tripeptide thyrotropic hormone releasing factor(TRF) has the full name pyroglutamylhistidylprolinamide. The structure appears here. Explain the functional groups at the N terminus and at the C terminus.

(c)On acidic hydrolysis, an unknown pentapeptide gives glycine, alanine, valine, leucine and isoleucine. No odor of ammonia is detected when the hydrolysate is neutralized. Reaction with phenyl isothiocyanate followed by mild hydrolysis gives nophenylthiohydantoin derivative. Incubation with carboxypeptidase has no effect. Explain these findings.

Short answer

(a) Possible sources of ammonia in the hydrolysate could have been presence of amide at C-terminus instead of carboxyl or glutamic acid present as its amide, that is glutamine.

(b) The N-terminus is present as the lactam which is cyclic amide. The C-terminus is present as the amide.

(c) C-terminus is not an amide and not a free carboxyl group. There is no free amine present at the N-terminus. N-terminus must have reacted with C-terminus to produce lactam.

Step-by-step solution

Step-1. Explanation of part (a):

When the hydrolysate is neutralised, odor of ammonia is detected. This possibly due to the fact that, amide must be present at the C-terminus instead of carboxylic group or glutamic acid could have been present as its amide, that is, glutamine from which ammonia can get released.

Step-2. Explanation of part (b):

The C-terminus is present as the amide. The N-terminus is present as the lactam which is also known as cyclic amide formed by combining amino group with carboxylic group of glutamic acid side chain.

N-terminus and C-terminus in pyroglutamylhistidylprolinamide

Step-3. Explanation of part (c):

The fact that no ammonia odor is detected when hydrolysate is neutralised, implies that C-terminus is not an amide. Carboxypeptidase treatment gives no reaction showing that C-terminus is not a free carboxyl group. Treatment with phenyl isothiocyanate gives no reaction, indicating that no free amine is present at the N-terminus. The conclusion which can be drawn from these findings is that, the N-terminus must have reacted with C-terminus to produce a cyclic amide, that is, a lactam.