Question

Match the term with the description or compound. (a) Competitive inhibition_____ (b) Uncompetitive inhibition_____ (c) Noncompetitive inhibition_____ 1\. Inhibitor and substrate can bind simultaneously 2\. \(V_{\text {max }}\) remains the same but the \(K_{\mathrm{M}}^{\text {app }}\) increases 3\. Sulfanilamide 4\. Binds to the enzymesubstrate complex only 5\. Lowers \(V_{\max }\) and \(K_{M}^{\text {app }}\) 6\. Roundup 7\. \(K_{M}\) remains unchanged but \(V_{\text {max }}\) is lower 8\. Doxycycline 9\. Inhibitor binds at the active site

Short answer

(a) Competitive inhibition: 2, 3, 9; (b) Uncompetitive inhibition: 4, 5; (c) Noncompetitive inhibition: 1, 7, 6, 8.

Step-by-step solution

Identify Competitive Inhibition

Competitive inhibitors typically bind to the enzyme's active site, preventing the substrate from binding. This type of inhibition can be recognized by a key characteristic: the apparent increase in the Michaelis constant \( K_M^{\text{app}} \), while the maximum rate \( V_{\text{max}} \) remains unchanged. Match competitive inhibition with descriptions that illustrate these points.

Find Descriptions for Competitive Inhibition

The descriptors that fit competitive inhibition include:- \( V_{\text{max}} \) remains the same but the \( K_{M}^{\text{app}} \) increases (Descriptor 2).- Sulfanilamide is a competitive inhibitor (Descriptor 3).- Inhibitor binds at the active site (Descriptor 9).Thus, match competitive inhibition with descriptions 2, 3 and 9.

Identify Uncompetitive Inhibition

Uncompetitive inhibitors bind only to the enzyme-substrate complex, preventing the reaction from completing, thereby lowering both \( V_{\max} \) and the apparent \( K_M \). Identify descriptions that depict these effects on kinetic parameters.

Find Descriptions for Uncompetitive Inhibition

The descriptors matching uncompetitive inhibition include:- Lowers \( V_{\max} \) and \( K_{M}^{\text{app}} \) (Descriptor 5).- Binds to the enzyme-substrate complex only (Descriptor 4).Thus, match uncompetitive inhibition with descriptions 4 and 5.

Identify Noncompetitive Inhibition

Noncompetitive inhibitors bind to an enzyme regardless of whether the substrate is present, leading to a reduction in \( V_{\max} \) without changing \( K_M \). Locate descriptions that exemplify these characteristics.

Find Descriptions for Noncompetitive Inhibition

The descriptors for noncompetitive inhibition include:- \( K_{M} \) remains unchanged but \( V_{\text{max}} \) is lower (Descriptor 7).- Inhibitor and substrate can bind simultaneously (Descriptor 1).Thus, match noncompetitive inhibition with descriptions 1 and 7.

Match Remaining Compounds

With the primary three types of inhibition identified with descriptors, the compounds that align with these inhibitions include: - Doxycycline as a competitive inhibitor (Descriptor 8). - Roundup as a noncompetitive inhibitor (Descriptor 6). Match these compounds to their appropriate inhibition processes as described in previous steps.

Key concepts

Competitive Inhibition

Competitive inhibition occurs when an inhibitor molecule competes with the substrate molecule for binding to the active site of the enzyme. This means that the inhibitor and the substrate cannot bind to the enzyme at the same time.
When an inhibitor occupies the active site, it blocks the substrate from entering, halting the enzyme's activity and reducing the rate of reaction. But once the inhibitor is removed or diminished, the enzyme activity can be restored by increasing the substrate concentration.Key Characteristics:

• The apparent Michaelis constant (\( K_M^{\text{app}} \)) increases.
• The maximum rate (\( V_{\text{max}} \)) remains unchanged.
• Inhibitor typically binds at the enzyme's active site.

An example of a competitive inhibitor is sulfanilamide, which competes with para-aminobenzoic acid (PABA) in bacterial cells. By resembling the substrate's structure, competitive inhibitors effectively "trick" the enzyme into binding them instead of the true substrate.

Uncompetitive Inhibition

Uncompetitive inhibition is a unique form of inhibition where the inhibitor binds only to the enzyme-substrate complex. This essentially "locks" the substrate in place and prevents the enzyme from converting the substrate into products.
This type of inhibition is especially fascinating because the inhibitor does not bind to the free enzyme, only the complex. As the concentration of substrate increases, more enzyme-substrate complexes form, making more binding sites available for the inhibitor.Key Characteristics:

• Both the maximum rate (\( V_{\max} \)) and the apparent \( K_M \) (\( K_{M}^{\text{app}} \)) are lowered.
• Inhibitor binds exclusively to the enzyme-substrate complex, not to the free enzyme.

As a result, increasing the substrate concentration does not overcome uncompetitive inhibition. Instead, both the maximum rate and affinity for the substrate are decreased. This kind of inhibition is important in drug design, for example, in treatments that target certain receptors.

Noncompetitive Inhibition

Noncompetitive inhibition takes place when an inhibitor binds to an enzyme at a location other than the active site. This means that both inhibitor and substrate can bind simultaneously.
This type of inhibition affects the enzyme's function but not its binding affinity for the substrate. As a result, noncompetitive inhibitors reduce the efficiency of the enzyme by lowering the maximum rate of reaction without affecting the substrate concentration needed to reach it.Key Characteristics:

• \( K_M \) remains unchanged.
• \( V_{\text{max}} \) is lower.
• Inhibitor can bind whether or not the substrate is present.

An example of a noncompetitive inhibitor is the herbicide Roundup, which inhibits key enzymes in plants. Noncompetitive inhibitors are useful in pathways where it is more important to control the overall rate of reaction rather than the affinity for substrates.