Two possible mechanisms, shown below, were designed to model the activity of the enzyme phosphodiesterase (PDE). Scientists have discovered that both nucleotides and phospholipids are susceptible to cleavage by PDE. Which statement regarding the mechanism of PDE best explains their finding?

Question

Two possible mechanisms, shown below, were designed to model the activity of the enzyme phosphodiesterase (PDE).  Scientists have discovered that both nucleotides and phospholipids are susceptible to cleavage by PDE.  Which statement regarding the mechanism of PDE best explains their finding?

Quizplus · Accepted Answer

11ecba2d_0e33_e776_a3bf_d9daaf605e5b_MD0008_00 Enzymes are catalysts, which are agents that increase the rate of biological reactions. They are classified and named according to the reactions they catalyze and the substrates they bind. In general, enzymes bind a single substrate and facilitate the breaking or forming of a specific chemical bond. However, some enzymes can bind different substrates that contain the same types of bonds. Enzyme-substrate binding has been explained using the lock and key theory and the induced fit model. The lock and key theory suggests that substrates bind readily to enzymes because their active sites are already in the correct structural conformation. Conversely, the induced fit theory states that as a substrate binds to an enzyme, the shape of the active site changes to accommodate the substrate. Phosphodiesterase (PDE) can bind both nucleotides and phospholipids, and can cleave their phosphoester bonds. Although phospholipids and nucleotides contain the same bond, they are very different in structure. Therefore, PDE must be able to alter its structure to adapt to the shape of both substrates. This indicates that PDE binds to substrates through the induced fit theory rather than the lock and key theory. Path 1 depicts the induced fit model by showing that PDE changes its active site to a more favorable shape to accommodate substrates, which best explains the scientists' finding. (Choice A) Enzyme-substrate binding occurs through either the induced fit model (Path 1) or the lock and key theory (Path 2). The use of one mechanism generally excludes the other. (Choice B) Path 2 does not show active site changes. (Choice C) Breaking bonds requires energy. Energy is absorbed when the phosphoester bond is broken and is released when new bonds form. Educational objective: Some enzymes can bind different substrates that have the same chemical bonds or linkages. The lock and key theory suggests that substrates bind readily to enzymes because their active sites are already in the correct structural conformation. The induced fit theory states that substrate binding induces a change in the shape of the active site to accommodate the substrate.

Two Possible Mechanisms, Shown Below, Were Designed to Model the Activity