BCH210H1 Lecture Notes - Lecture 3: Amino Acid, Glutamic Acid, Histidine

Tyrosine has a polar side chain because:
a.) It is asymmetrical
b.) It has two types of atoms that have large differences inelectronegativity
c.) It is symmetrical
d.) both a and b
e.) both b and c

2. Consider the amino acid T at pH 9.23. Its alpha carboxyl grouphas a pKa of 2.63. What is the ratio of
base to acid for this group at this pH?
a.) 1:1
b.) 45637:1
c.) 2,634,678:1
d.) 3,981,072:1
e.) 9,364,345:1

3. Which statement about pKa is not true?
a.) It represents a pH at which 50% of the titratable group is inthe acid form and 50% is in the
base form.
b.) If the pH is less than the pKa, more than 50% of the titratablegroups are unprotonated.
c.) If the pH is greater than the pKa, more than 50% of thetitratable groups are in the base
form.
d.) If the pH is less than the pKa, more than 50% of the titratablegroups are in the acid form.
e.) All of the above are true.

4. At what approximate pH will D most likely exist as azwitterion?
a.) 3.0
b.) 4.0
c.) 5.0
d.) 6.0
e.) 7.0

5. Cysteine is the only amino acid that can form covalent bondswith other amino acids (i.e. cysteine)
through its side chain.
a.) true
b.) false

6. At pH 2, will the R group of H likely be charged OR uncharged?

A histidine residue in a protein has a side chain pKa of 6.3. Due to a conformational shift, an amino acid move into the close proximity of this histidine, shifting the histidine pKa to 5.7.

if the protein has the above histidine in it was in a solution of pH 6.5, what is the change in the net charge of this histidine (show all the calculation)?

what is the likely identity of the residue that causes the pKa shift for the above histidine residue? If there is more than one possible candidate, then list them all. Explain why these candidates cause the shift in the histidine pKa.

Extra Info:

equations: Vo = {Vmax [S]} / {Km + [S]}; pH = pKa + log {[base]/[acid]}; ΔG = ΔH - TΔS

pKa of the side chain R-groups are: D (3.9), E (4.2), H (6.0)' C (8.3), Y (10.1), K (10.5), and R (12.5).

pKa of amino acid α-amino group (9.60) and α-carboxyl group (2.10).

pKa of N-terminal oligopeptide a-mino group (7.40) and C-terminal a-carboxyl group (3.6)