There are 24 questions in all. Questions 1–20 are multiple choice and are each worth 3 points. Questions 21–24 are essays and problems worth a total of 40 points.

These are constants you may find helpful: R = 8.314 J/mole-K; T = 298 K

21. Analysis of an enzyme and its reversible inhibitor resulted in the Michaelis-Menton and Lineweaver-Burk charts below. Based upon the charts,
a. What type of inhibitor is this (competitive, uncompetitive, or mixed)? (2)
b. Explain how you know. (2)
c. Describe the interaction of the inhibitor (I) with the enzyme (E) and define the
     term(s) K_I and/or K_I′ if they are relevant. (6)

a. Mixed inhibitor
b. The Lineweaver-Burk chart shows intersecting lines, but not on the y axis.
      A competitive inhibitor would have intersecting lines on the y axis (same V_max ).
     An uncompetitive inhibitor would have parallel lines (V_max and K_M changed the same).
c. I binds both E and ES at a site other than the active site. Because I binds both E and ES,
      both K_I and K_I′ should be defined. K_I = ([E] [I])/[EI], and K_I′ = ([ES] [I])/[ESI].
     I can bind E at a site other than the active site; in other words, a mixed inhibitor is not
     one that inhibits both competitively and in some other way. There is only one binding
     site for a mixed inhibitor.

23. Glycogen synthase a is converted to glycogen synthase b by protein kinase A (PKA).
The kinetics of glycogen synthase are described using the term K_0.5 , rather than K_M.

a. PKA causes the K_0.5 for glycogen synthase to               increase                        

     and its activity to           decrease                       .
     Write increase, decrease, or stay the same in each blank. (2)

b. What type of regulation--covalent or allosteric--is involved; how do you know? (4)
c. What is K_0.5 and why is it more appropriate for some enzymes than K_M? (4)

        b. Covalent regulation is involved, because PKA (an enzyme) converts enzyme a to
    enzyme b, indicating a change in enzyme activity.

        c. K_0.5 is [S] at which v₀ = half V_max for an enzyme that has cooperative binding. Such an
    enzyme does not have Michaelis-Menton kinetics, so K_Mis less appropriate. It is
    possible for an enzyme to have cooperative binding and be regulated covalently or
    not be regulated at all.

22. The diagrams below represent two stages in the reaction catalyzed by chymotrypsin.
a. For one diagram, label the parts of the catalytic triad. (3) See diagram 1.
b. Label the diagrams 1 and 2 to indicate which occurs first during the reaction. (1)
c. Write the reaction catalyzed by chymotrypsin as a two-step reaction. (4)
d. Choose one diagram and describe what will happen next during the reaction. (2)

c. I tried to give a fair amount of leeway on this, as it was clear that it wasn't easy even if you had a pretty good idea what was asked. In the answer below, amine terminus refers to the amine terminus of the original protein substrate.

1. Enzyme-OH + protein (might include ) →
Enzyme   +

2. Enzyme + H₂O → Enzyme-OH +

What was absolutely required was release of the first product, leaving a modified enzyme as step 1. Then in step 2, H₂O (second substrate) is added, and the enzyme is regenerated (restored to its original form.

23. The molecule below is a membrane component that can form weak interactions with membrane proteins. (10)
a. Label each circled part of the molecule with the type of residue indicated.
     A residue may not be completely circled, but you should imagine that it is.
     Be as specific as possible (for example, write adenine rather than purine).
b. Label one hydrophilic part of the molecule with L and one hydrophobic part with B.
c. Name an amino acid whose side chain would form a weak interaction with the
    hydrophilic part of the molecule you labeled and name the type of interaction.
d. Name an amino acid whose side chain would form a hydrophobic interaction
    with the hydrophobic part of the molecule.

Answers for b-d could vary quite a bit and be correct, so I'm not going to include them.

24. Glycogen synthase a is converted to glycogen synthase b by protein kinase A (PKA).
The kinetics of glycogen synthase are described using the term K_0.5 , rather than K_M.

a. PKA causes the K_0.5 for glycogen synthase to            increase                           

     and its activity to          decrease                        .
     Write increase, decrease, or stay the same in each blank. (2)

b. What type of regulation--covalent or allosteric--is involved; how do you know? (4)
c. What is K_0.5 and why is it more appropriate for some enzymes than K_M? (4)

        b. Covalent regulation is involved, because a kinase (PKA) converts glycogen synthase
     from one form to another.

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