Question

Where do lines intersect on a Lineweaver-Burk plot showing competitive inhibition? On a Lineweaver-Burk plot showing noncompetitive inhibition?

Short answer

Lines intersect on the y-axis for competitive inhibition and at the x-axis for noncompetitive inhibition.

Step-by-step solution

Understanding Lineweaver-Burk Plot

A Lineweaver-Burk plot is a double reciprocal graph of the Michaelis-Menten equation. It plots 1/velocity (1/V) against 1/[substrate] (1/[S]). The y-intercept represents 1/Vmax and the x-intercept represents -1/Km.

Analyzing Competitive Inhibition

In competitive inhibition, the inhibitor competes with the substrate for the active site of the enzyme. This type of inhibition increases the apparent Km (Michaelis constant) but does not affect Vmax (maximum velocity). On a Lineweaver-Burk plot, lines representing different concentrations of competitive inhibitor intersect at the y-axis.

Analyzing Noncompetitive Inhibition

In noncompetitive inhibition, the inhibitor binds to a site other than the active site of the enzyme, affecting the enzyme's activity but not its binding affinity for the substrate. This type of inhibition decreases Vmax but does not affect Km. On a Lineweaver-Burk plot, lines representing different concentrations of noncompetitive inhibitor intersect at the x-axis.

Key concepts

Lineweaver-Burk Plot and Michaelis-Menten Equation

The Lineweaver-Burk plot is an essential graph used in enzyme kinetics. It transforms the Michaelis-Menten equation into a straight line, making it easier to analyze enzyme behavior. The Michaelis-Menten equation describes the rate of enzymatic reactions by relating reaction velocity (V) to substrate concentration [S]. The equation is: \[ V = \frac{V_{max} [S]}{K_m + [S]} \] In a Lineweaver-Burk plot:

• The y-axis represents the reciprocal of velocity (1/V).
• The x-axis represents the reciprocal of substrate concentration (1/[S]).
• The y-intercept gives you 1/Vmax.
• The x-intercept gives you -1/Km.

This reciprocal transformation makes it easier to distinguish between different types of enzyme inhibition by conveniently showing linear relationships.

Competitive Inhibition

In competitive inhibition, an inhibitor competes directly with the substrate for binding to the enzyme's active site. Here's how it works:

• The inhibitor and substrate can't both bind to the enzyme's active site simultaneously.
• This type of inhibition increases the apparent Km (Michaelis constant), meaning the enzyme's apparent affinity for the substrate decreases.
• However, the maximum reaction velocity (Vmax) remains unchanged because the inhibition can be overcome by increasing substrate concentration.

On a Lineweaver-Burk plot, competitive inhibitors affect the slope but not the y-intercept. Lines that represent different concentrations of a competitive inhibitor will intersect on the y-axis, illustrating that Vmax remains constant.

Noncompetitive Inhibition

Noncompetitive inhibition occurs when an inhibitor binds to an enzyme at a site other than the active site. This binding changes the enzyme's shape, reducing its ability to catalyze the reaction:

• The inhibitor does not compete with the substrate for the active site.
• This type of inhibition decreases the Vmax, indicating the maximum reaction speed is reduced.
• The Km remains unchanged, meaning the enzyme's affinity for the substrate is not affected.

On a Lineweaver-Burk plot, noncompetitive inhibitors influence the y-intercept but not the slope. Lines representing different concentrations of a noncompetitive inhibitor will intersect on the x-axis, showing that Km stays the same.