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If a protonated His residue acts as the proton donor in an acid-catalyzed enzymatic reaction, what would happen to the enzyme’s activity if pH increased to histidine’s pKa value? What if pH increased 1 pH unit above its pKa value?

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Jean Keeling
Jean KeelingLv2
28 Sep 2019

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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
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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 cysteine found in the interior of a protein can act as a base in a chemical reaction. Since the pH for living organisms is held constant near 7.3, what would have to happen to the pKa of the side chain (as a result of microenvironmental effects) to optimize cysteine’s ability to act as a base. Explain your answer.

Using site specific mutagenesis to change residues in the substrate binding cleft of PKA (not residues involved in catalytic roles), how would you alter PKA’s substrate specifity? By this, I mean how would you specifically make mutations in the PKA enzyme amino acid sequence (not the substrate sequence) that would alter amino acid preferences for particular residues on a substrate peptide that you might want to design for this enzyme. Please think carefully about what this means. It does not mean changing ATP binding or altering the active site base or Mg binding site, as such mutations would only decrease overall catalytic activity of the phosphotransfer reaction. Please provide three detailed examples of how you could try to modify the substrate specificity of PKA (assuming that protein sequences exist in the cell that would match the newly designed substrate specificity of the enzyme).

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