An enzyme was assayed in the absence (−) or presence (+) of an allosteric effector, and the following Lineweaver-Burk plot was obtained: Which statement about the allosteric effector is correct?

Question

An enzyme was assayed in the absence (−) or presence (+) of an allosteric effector, and the following Lineweaver-Burk plot was obtained:  Which statement about the allosteric effector is correct?

Quizplus · Accepted Answer

11ecba2d_0eea_6644_a3bf_9dc75843ab99_MD0008_00 Lineweaver-Burk plots display enzyme kinetic data in linear form, with the inverse of reaction velocity (1/V₀) on the y-axis and the inverse of substrate concentration (1/[S]) on the x-axis. Establishing the location of the x- and y-intercepts on these graphs facilitates the determination of the Michaelis-Menten parameters known as V_max (the maximum possible reaction rate) and K_m (the concentration of substrate at which ½ V_max is achieved). The y-intercept of the plot is 1/V_max, and the x-intercept is −1/K_m. Allosteric effectors are molecules that bind enzymes at a site other than the active site and either increase activity (activators) or decrease activity (inhibitors). Lineweaver-Burk plots generated in the presence and absence of an allosteric effector can be compared to determine how the effector alters K_m and V_max. Because the x- and y-intercepts are related to the inverse of K_m and V_max, an increase in the magnitudes of the intercepts corresponds to a decrease in V_max and K_m, and vice versa. The graph shows that in the presence (+) of the allosteric effector, the y-intercept increases, indicating a decreased V_max and making the effector an inhibitor. Different inhibitors affect K_m in different ways. In this case, the x-intercept exhibits a left shift, which corresponds to a decrease in K_m. Therefore, the allosteric effector is an inhibitor that decreases both V_max and K_m. (Choices A and B) An activator is a molecule that binds to an enzyme and increases its activity, resulting in an increased reaction rate. On a Lineweaver-Burk plot, an activator would be reflected by a decreased y-intercept. However, the plot shows an increased y-intercept in the presence of the allosteric effector, indicating that the effector is an inhibitor. (Choice D) An inhibitor that increases K_m would appear as a right shift in the x-intercept on a Lineweaver-Burk plot. Instead, the plot shows a left shift in the presence of the allosteric effector, indicating that it decreases K_m. Educational objective: Lineweaver-Burk plots are linear graphs that facilitate determination of Michaelis-Menten parameters V_max and K_m. The y-intercept is equal to 1/V_max and the x-intercept is equal to −1/K_m. An upward shift in the y-intercept corresponds to a decreased V_max, and a left shift in the x-intercept corresponds to a decreased K_m.

An Enzyme Was Assayed in the Absence (−) or Presence