Chem 336 - Spring 2002
Organic Chemistry III
Dr. Carl C. Wamser
Carey, pages 1106 - 1108:
Problems 27.24 - 40
1. Write the structure of the tripeptide alanylprolyllysine, as it would appear at pH 7.
After acid hydrolysis into individual amino acids, show the products as they would appear at pH 1.
2. Show how you might start with a dipeptide, isoleucylglutamine, and specifically add a methionine to the N-terminus.
Repeat for the C-terminus.
3. An unknown hexapeptide is determined by complete hydrolysis to have the amino acid composition Ala (2), Lys, Glu, Phe, Cys.
Edman degradation releases Ala followed by Cys.
Chymotrypsin hydrolysis gives two tripeptides, both of which have Ala at their N-termini.
Trypsin hydrolysis gives a pentapeptide plus Glu.
Identify the amino acid sequence of the hexapeptide.
Ala-Cys-Phe-Ala-Lys-Glu
4. Sickle cell anemia is a genetic defect in the structure of hemoglobin, in which just one amino acid is altered. One glutamic acid is replaced by a valine. Describe why this change could be so serious for the protein tertiary structure. Suggest a possible code in the DNA that might have been altered in order to read Val instead of Glu.
The Glu residue in hemoglobin carries a negative charge and may be important for interacting with a positive charge somewhere or just for making that part of the molecule water-soluble.
A change to Val makes that residue nonpolar. It may be that the Val residue avoids water or the other polar environment where Glu was normally found, and this causes the drastic conformational change in tertiary structure that leads to sickle-shaped red cells.
The m-RNA codons for Glu are GAA and GAG.
The m-RNA codons for Val are GUX (where X can be any of the four bases).
The most likely error is that a GUA or GUG was accidentally read instead of GAA or GAG. In both cases, U appears in the second base where A should have been.
5. Some codons can be deduced from simple homopolynucleic acids, e.g., poly-A.
Explain. Identify the four amino acid codes that can be deduced this way.
Poly-A would produce only Lys.
Poly-C would produce only Pro.
Poly-G would produce only Gly.
Poly-U would produce only Phe.