Question

1. PT1. Which of the following does not describe a mechanism that cells use to regulate enzyme activity? Explain. (5 points)
   1. Cells control enzyme activity by the binding of small molecules
   2. Cells control enzyme activity by phosphorylation and dephosphorylation
   3. Cells control the diffusion rates of substrate to enzymes
   4. Cells control the rate of enzyme degradation
   5. Cells control the rate of enzyme synthesis
   6. Cells control the targeting of enzymes to specific organelles

2. PT2. Which of the following properties of an enzyme is responsible for its saturation behavior? What would the graph look like? (5 points)
   1. The enzyme does not change the overall equilibrium constant for a reaction
   2. The enzyme lowers the activation energy of a chemical reaction
   3. The enzyme is not consumed by the reaction
   4. The enzyme has a fixed number of active sites for substrate binding
   5. The product of the enzyme reaction usually inhibits the enzyme activity

Answer 1

PT1.

c. Cells control the diffusion rates of substrate to enzymes.

Most enzymes are cytosolic and therefore cells cannot control the rate of diffusion of substrate to the enzyme. The other methods mentioned are often employed by cells to regulate enzyme activity.

PT2

d. The enzyme has a fixed number of active sites for substrate binding

Since there are a fixed number of enzyme molecules in solution, and the binding of the Enzyme to the substrate depends on the Kd of the Enzyme-Substrate pair, along with Substrate concentration, there are only a limited number of Substrate binding sites available.

When Enzyme-Substrate concentration reaches saturation, the E-S versus Time graph plateaus and the concentration of the E-S complex does not increase with time.