A polypeptide with antibiotic activity was isolated from the bacterium Bacillus

Question

A polypeptide with antibiotic activity was isolated from the bacterium Bacillus brevis. The structure of the polypeptide was determined from the following observations:

(a) aa analysis yielded equimolar amounts of leu, orn (ornithine, an unusual aa not normally found in protiens), phe, pro and val. Ornithine has an ionizable R-group-NH2 as well as the usual alpha-NH2

(b) MW measurments gave an approximate value of 1200 daltons

(c) the FDNB dervivative was dinitrophenyl-orn, in which the DN was bounded to the R-group N rather that the alpha-N

(d) partial hydrolysis of the polypeptide followed by sequencing the fragment yielded the di- and tripeptides shown below, (N terminus on left of sequence):
leu-phe, phe-pro, phe-pro-val, val-orn-leu, orn-leu, val-orn, pro-val-orn.

Deduce the aa sequence showing the order and direction of the aa's in the shorter polypeptide. show that your proposed primary structure is consistent with each of the experimental observations given above.

Explanation / Answer

One of the points in the question has been left out.
When treated with the enzyme carboxypeptidase, the peptide failed to undergo hydrolysis.
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The peptide is a cyclic one.
a. This analysis gives us the different amino acids present in the peptide.
b. The overall weight of the peptide is known but number of amino acids is not known.
*** carboxypeptidase cleaves the C terminal amino acid. This shows that the C terminal is not free.
c. Treatment with FDNB gives a derivative of ornitihine. This shows that the N terminal is ornithine. As alpha amino group has not reacted it shows that amino group of the N terminal is not free
d. As both N terminal and C terminal are not free, the peptide is cyclic.
The sequence of the peptide can be obtained by comparing the sequences of the dipeptides and tripeptides obtained.
The overlapping segments are:
Leu–Phe
Phe–Pro
Phe–Pro–Val
Pro–Val–Orn
Val–Orn
Orn–(Leu

The overlapping parts are in red

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