How can you distinguish between competitive and non-competitive inhibition using graphs at specified substrate concentrations?

Sourav PanNovember 9, 2024

Answered

To distinguish between competitive and non-competitive inhibition using graphs at specified substrate concentrations, you can analyze how each type of inhibitor affects the reaction rate as substrate concentration varies. The following points outline the key differences and how to visualize them through graphical representation.

Competitive Inhibition

Characteristics

• Binding: Competitive inhibitors bind to the active site of the enzyme, competing directly with the substrate.
• Effect on Vmax: The maximum reaction rate (Vmax) remains unchanged because, at sufficiently high substrate concentrations, the effect of the inhibitor can be overcome.
• Effect on Km: The Michaelis constant (Km) increases because a higher concentration of substrate is required to reach half of Vmax due to competition with the inhibitor.

Graphical Representation

1. Michaelis-Menten Plot:
   • A plot of reaction velocity (v) versus substrate concentration $[S]$ will show that the curve for the inhibited reaction approaches Vmax but does not reach it at lower substrate concentrations. As $[S]$ increases, the reaction rate will eventually reach Vmax.
   • At low $[S]$, the presence of a competitive inhibitor significantly reduces the reaction rate.
2. Lineweaver-Burk Plot:
   • This double reciprocal plot (1/v vs. 1/[S]) will yield straight lines that intersect on the y-axis. The slope will increase due to a higher Km, while Vmax remains constant.
   • The x-intercept (representing $-1/Km$) shifts left (increases), indicating an increased Km.

Non-Competitive Inhibition

Characteristics

• Binding: Non-competitive inhibitors bind to an allosteric site on the enzyme, which can occur whether or not the substrate is bound.
• Effect on Vmax: Vmax decreases because non-competitive inhibitors reduce the overall amount of functional enzyme available for catalysis, regardless of substrate concentration.
• Effect on Km: Km remains unchanged because the affinity of the enzyme for its substrate does not change; however, less enzyme is available for catalysis.

Graphical Representation

1. Michaelis-Menten Plot:
   • The curve will show a decrease in Vmax compared to the uninhibited reaction. Even at high $[S]$, the reaction rate does not reach Vmax due to the presence of the non-competitive inhibitor.
   • The shape of the curve remains hyperbolic, but it levels off at a lower maximum velocity.
2. Lineweaver-Burk Plot:
   • In this plot, straight lines will intersect on the x-axis (indicating no change in Km). However, there will be an increase in y-intercept (representing $1/Vmax$), indicating a lower Vmax.
   • The slope will increase due to a decreased Vmax while maintaining a constant Km.

Summary of Graphical Differences