Download Bio301 Biochemistry I

Bio301 Biochemistry I
Midterm Examination 1, October 27, 2004
Question 1
Draw the following oligopeptides in their predominant ionic forms at pH 7. (Help:
chemical formulas are given in Table 1)
a) Phe-Met-Arg
b) Gln-Ile-His-Thr
Question 2
Normal human hemoglobin has an isoelectric pot of 6.87. A mutant variety of
hemoglobin, known as sickle-cell hemoglobin, has an isoelectric point of 7.09. The
titration curve of hemoglobin idicates that, in this pH range, 13 groups change
ionization states per unit change in pH. Calculate the difference in ionic charge
between molecules of normal and sickle hemoglobin.
Question 3
Ionic strength, I is defined as
Where ci is the molar concentration of the ith ionic species and Zi is its ionic charge.
At high ionic strengths the solubilities ofproteins as well as those of most other
substances, decrease. This effect is known as salting out.
You have given 1.0 M solutions of NaCl, (NH4) 2SO4 and K3PO4.In which of these
solutions would you expect that a protein be most soluble? Least soluble?
Question 4
Estimate the molecular mass of fibrinogen using the figure given below. Would your
estimate be more accurate if you determine the molecular mas from its electrophoretic
mobility on SDS-polyacrylamide gel. Discuss.
Question 5.
What would be the relative positioning of the following proteins after they had been
subjected to isoelectric focusing.
Protein
pI
Insulin
5.4
Cytochrome c 10.6
Histone
10.8
Myoglobin
7.0
Ribonuclease 7.8
anode............................................ .......................................................cathode
(+electrode)
(-electrode)
Question 6.
Treatment of a polypeptide by dithiothreitol yields two polypeptides that have the
following amino acid sequences:
1. Ala-Phe-Cys-Met-Tyr-Cys-Leu-Trp-Cys-Asn
2. 2. Val-Cys-Trp-Val-Ile-Phe-Gly-Cys-Lys
Chymotrypsin catalyzed hydrolysis of the intact polypeptide yields polypeptide
fragments with the following amino acid compositions:
3. (Ala, Phe)
4. (Asn, Cys2, Met, Tyr)
5. (Cys, Gly, Lys)
6. (Cys2, Leu, Trp2, Val)
7. (Ile, Phe, Val)
Draw and indicate the positions of the disulfide bonds in the original polypeptide.
(note: chymotrpsin cleaves bulky amino acids i.e. Phe, Tyr,Trp)
Question 7:
What structural features are responsible for the observations that -keratin fibers can
stretch to over twice their normal length, whereas silk is nearly inextensible?
Question 8:
Compare (discuss the advantages and disadvantages) of x-ray crystallography and
NMR as methods of atomic structure determinations.
Question 9:
The production of a small acidic protein, hCG (human chorionic gonadotropin) during
pregnancy is the basis of most pregnancy test kits. Describe an easy method for
detecting this protein. Explain each step of this method.
Question 10:
Would you recommend to a friend of yours, to purify a double-stranded DNA using a
buffer at pH 4? Why or why not. Explain.