Scottish Highers Chemistry proteins: Peptide molecules can be classif

Question

Peptide molecules can be classified according to the number of amino acid units joined by peptide bonds in the molecule.

Type of peptide	Example of amino acid sequence

di-peptide	aspartic acid-phenylalanine
tri-peptide	isoleucine-proline-proline
tetra-peptide	lysine-proline-proline-arginine
penta-peptide	serine-glycine-tyrosine-alanine-leucine
poly-peptide	alanine-glycine-valine-proline-tyrosine-serine

(a) Complete the table to identify the type of peptide with the following amino acid sequence
alanine-glycine-valine-proline-tyrosine-serine
(b) Partial hydrolysis of another pentapeptide molecule gave a mixture of three smaller peptide molecules with the following amino acid sequences.

leucine-glycine-valine
isoleucine
glycine-valine

Write the amino acid sequence for the original pentapeptide molecule.

(c) Some amino acids needed to form polypeptides cannot be produced in the human body.
State the term used to describe amino acids that the body cannot make.

(d) Paper chromatography is often used to analyse the mixtures of amino acids produced when peptides are broken down.
On a chromatogram, the retention factor Rf, for a substance can be a useful method of identifying the substance.

$R_f = \frac{	ext{distance moved by the substance}}{	ext{maximum distance moved by the solvent}}$

The structure of the pentapeptide methionine enkephalin was investigated.

The chromatogram obtained is shown below.

(i) Suggest why only four spots were obtained on the chromatogram of the hydrolysed pentapeptide.

(ii) It is known that this amino acid mixture contains the amino acid methionine. The Rf value for methionine in this solvent is 0-40.
Draw a circle around the spot on the chromatogram that corresponds to methionine.

(e) Over the last decade several families of extremely stable peptide molecules have been discovered, where the peptide chain forms a ring.

(i) A simple cyclic dipeptide is shown.

Draw a structural formula for one of the amino acids that would be formed on complete hydrolysis of the above cyclic dipeptide.

(ii) Alpha-amanitin is a highly toxic cyclic peptide found in death cap mushrooms.
The lethal dose for humans is 100 mg per kg of body mass.
1 g of death cap mushrooms contains 250 mg of alpha-amanitin.
Calculate the minimum mass of death cap mushrooms that would contain the lethal dose for a 75 kg adult.

Peptide molecules can be classified according to the number of amino acid units joined by peptide bonds in the molecule - Scottish Highers Chemistry - Question 6 - 2016

Worked Solution & Example Answer:Peptide molecules can be classified according to the number of amino acid units joined by peptide bonds in the molecule - Scottish Highers Chemistry - Question 6 - 2016

Complete the table to identify the type of peptide with the following amino acid sequence

Write the amino acid sequence for the original pentapeptide molecule

State the term used to describe amino acids that the body cannot make

Suggest why only four spots were obtained on the chromatogram of the hydrolysed pentapeptide

Draw a circle around the spot on the chromatogram that corresponds to methionine

Draw a structural formula for one of the amino acids that would be formed on complete hydrolysis of the above cyclic dipeptide

Calculate the minimum mass of death cap mushrooms that would contain the lethal dose for a 75 kg adult

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