Exercise on Enzyme Regulation
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Exercise on Enzyme Regulation

1. Fill in the table below that compares and contrasts types of enzyme regulation.

Table 1: Regulation of Enzyme Activity

type of regulation

proteolysis

covalent modification

allosteric regulation

Is it reversible?

     

Is another enzyme required?

     

Can it involve activation only, inhibition only, or both?

     

Can amplification occur?

     

2. Fructose-6-phosphate (fructose-6-P), ATP, fructose-2,6-bis-P, and AMP (an allosteric activator) all bind to the enzyme PFK-1 in liver cells. PFK-1 catalyzes a reaction in glycolysis:
fructose-6-phosphate + ATP → fructose-1,6- bis -phosphate + ADP.
ATP is an allosteric inhibitor of PFK-1, and both fructose-2,6-bis-P and AMP are allosteric activators of PFK-1.

   a. Fill in Table 2 to summarize the relationship of these molecules to PFK-1.

Table 2: Classifying the Molecules that Bind PFK-1

 

fructose-6-P

ATP

fructose-2,6-bis-P

AMP

Does it bind in active site (C),
allosteric site (L), or both (B)?

       

Substrate (S), modulator (M), or both (B)?

       

Conformation (R or T) stabilized?

       

Converted to product (P) or unchanged (U)?

       

b. If a molecule can have two answers (both R and T, or both P and U), explain.

3. PFK-1 has a K0.5 for each substrate and a Kdfor each modulator.
a. What is the difference between K0.5 and Kd?
b. Do both substrates have the same K0.5?
c. Do both modulators have the same Kd?

4. The molecular structures of fructose-6-P, fructose-2,6-bis-P, ATP, and AMP are shown below.
a. Identify all molecules.

                               

                                                                                                                       

                                           

                                                                                                                       

     b. Based upon their structures, are any modulators likely to share the same allosteric
         binding site?

     c. Each modulator has weak interactions with the amino acid side chains in its
         binding site. Choose one modulator, name a type of interaction it could have, and
         name an amino acid side chain with which it could have that interaction.

5. Draw a graph that shows the effect of increasing substrate concentration on enzyme activity for PFK-1. Label the x and y axes and the Vmax .

a. Add a line that shows the effect of adding a positive heterotropic modulator.

b. Add a third line that shows the effect of adding a negative heterotropic modulator.

c. Choose a substrate concentration at which the enzyme, in the absence of modulators, has half its maximal v0. What is the activity for the enzyme in the presence of modulators for the same substrate concentration?

6. This protein sequence includes
(a) side chains with which the negative modulator citrate3- interacts (p. 608);
(b) the consensus sequence recognized by protein kinase A (p. 231);
(c) a substrate for trypsin , which activates some enzymes by proteolysis (p. 100).
For each, identify the relevant part of the protein sequence and describe the change that occurs as a result of interaction with the underlined molecule. Then describe what could be done to return to the original protein sequence. The page numbers provide structure or consensus sequence information.

 

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