Interleukin-1β is a cytokine that upregulates the synthesis of PGE1, the final enzyme in the arachidonic acid pathway that generates prostaglandin E. Which of the following plots most likely represents the change in the kinetics of prostaglandin E production in the absence (solid line) and presence (dashed line) of interleukin-1β?

Question

Interleukin-1β is a cytokine that upregulates the synthesis of PGE1, the final enzyme in the arachidonic acid pathway that generates prostaglandin E.  Which of the following plots most likely represents the change in the kinetics of prostaglandin E production in the absence (solid line) and presence (dashed line) of interleukin-1β?

Quizplus · Accepted Answer

11ecba2d_0df7_0631_a3bf_85f4af11e7bd_MD0008_00 Lineweaver-Burke plots are used to determine Michaelis-Menten kinetic parameters. These linear plots relate the inverse of reaction velocity (1/V) on the y-axis to the inverse of substrate concentration (1/[S]) on the x-axis. This linear relationship is based on rearrangements of the Michaelis-Menten equation. On a Lineweaver-Burke plot, the x-intercept corresponds to the negative inverse of the Michaelis constant (x-intercept = −1/K_m) and the y-intercept corresponds to the inverse of the maximum velocity (y-intercept = 1/V_max). Because the y-intercept in a Lineweaver-Burke plot is 1/V_max, an increase in V_max would lower the y-intercept. The enzyme affinity for substrate (K_m) is independent of enzyme concentration, so the x-intercept (−1/K_m) would remain unchanged with varying enzyme concentration. In this example, interleukin-1β upregulates the synthesis of PGE1, which results in increased enzyme concentration. The maximum velocity (V_max) of the enzyme-catalyzed reaction, which occurs when the enzyme is saturated with substrate, is equal to the product of the catalytic rate constant (k_cat) and the concentration of enzyme in solution: 11ecba2d_0df7_0632_a3bf_135205293b38_MD0008_00 Vmax=kcat [Etot] Therefore, increasing enzyme concentration would result in an increase in the maximum velocity of the reaction and a decrease in the y-intercept. (Choice A) This plot correctly shows an increase in V_max but also demonstrates a change in K_m. An enzyme's K_m is unchanged by changes in concentration. (Choice B) This plot represents an enzyme system in the presence of a competitive inhibitor, in which V_max is not changed. (Choice C) This plot represents an enzyme in the presence of a noncompetitive inhibitor, in which K_m is unchanged and V_max decreases. Educational objective: An increase in the amount of enzyme available in a particular system will cause an increase in V_max but no change in K_m in Michaelis-Menten kinetics. Lineweaver-Burke graphs are linear plots that relate inverse reaction velocity (1/V) to inverse substrate concentration (1/[S]). An increase in enzyme concentration would cause a decrease in the y-intercept (1/V_max) and no change in the x- intercept (−1/K_m) in a Lineweaver-Burke plot.

Interleukin-1β Is a Cytokine That Upregulates the Synthesis of PGE1