Download Quiz:1

Quiz:1. 59-365:
1. What are the two features that are common in all amino acids in living systems?
2. Which amino acid has a non-chiral alfa carbon?
3. Can amino acids be present in neutral form in aqueous solution? Explain your
answer.
4. pK for weak acids and bases are defined as the pH at which the concentration of
the ionized and unnionized species are equal, e.g.
CH3COO- + H+
Ionized form
Acetic acid (non-ionized)
CH3COOH
pKa= 4.8
Why amino acids have more than one pK values (pK1 and pK 2).
5. Name five amino acids, which have three pK values.
6. What net charge will be on glycine at pH 2.0 and at pH 10 ? (pK1= 2.3 and pK2=
9.6 )
7. The non-polar amino acids such as iso-leucine, leucine, valine, alanine, phenyl
alanine are soluble in water but when they are present in a peptide (joined
together by amide bond), the peptide is insoluble in water. Why?
8. What will be the net charge on a poly-lysine peptide at neutral pH and at pH 11.
(pK1=2.18, pK2=8.95 and pKR= 10.5)
9. An approximate amount of proteins can be measured by UV spectroscopy.
Proteins absorb light at 280 nm. Which amino acids are responsible for this
absorption?
10. Cross-linking of two or more polypeptide chains through S-S bond (didulfide
bond) is very common structural feature of many proteins. Which amino acid is
responsible for this bond?
11. Name the functional groups present in proteins.
12. All amino acids found in mammalian system are L-amino amino acids (their
structural configuration is similar to L-glyceraldehyde). All L-amino acids have
S-configuration except one which has R- configuration in R,S system. Name this
amino acid and explain the reason.
13. Other than being the monomers for proteins, some amino acids perform other
important biological functions. Give some example of such amino acids and their
functions.
14. A biochemist was studying endonucleae activities in cytoplasmic fraction. After
he passed the cytosolic fraction on a gel filtration columns, he detected the
endonuclease activity in the very early fractions eluted from the column using a
isotonic buffer. What is your guess about the molecular weight of this enzyme?
(very high or low)
15. When the fraction containing endonuclease activity (in question 14) was
analyzed on SDS-PAGE, a protein with molecular weight of 97 kDa was
obtained which showed endonuclease activity. This molecular weight is much
smaller than what was calculated from gel filtration column. Explain this
observation and highlight the advantage of gel filtration chromatography in
studying biochemical interactions.
16. Histones are very basic proteins that interact with DNA in the nuclei to form the
structure of chromatin. What amino acids would be rendering this property to
histones and why this basic nature is important?
17. Knowing the basic nature of the histones, which purification method would you
suggest for its isolation from a soluble extract? Which elution condition would
you apply?
18. Phosphorylation of proteins (addition of inorganic phosphate group to
polypeptide at side chain of Ser or Thr or Tyr residue) plays a crucial role in
regulatory functions. For example, glycogen phosphorylase is active when it is
phosphorylates and it is inactive when it is not phosphorylated. Can you suggest
an analytical method to distinguish the phosphorylated vs unphosphorylated
forms of the same protein.
19. What is the biochemical principle of salting out method of protein fractination?
20. What are the similarities and differences in ELISA and affinity chromatography?