The correct answer is (2) K₂.

Introduction

Mechanism-based (suicide) inhibitors are among the most potent and specific enzyme inhibitors in biochemistry and pharmacology. Penicillin serves as a classical example, irreversibly inactivating transpeptidase enzymes during bacterial cell wall synthesis. Understanding exactly which step of the catalytic pathway suicide inhibition targets is essential for students and professionals interpreting enzyme kinetics, designing drugs, and analyzing metabolic control.

Enzyme Catalytic Steps and Suicide Inhibition

The typical enzyme-catalyzed reaction follows these steps:

1. Eff + S ⇌ [E.S] (K₁): Free enzyme binds substrate, forming enzyme-substrate complex.

2. [E.S] ⇌ [E.P] (K₂): Substrate is chemically transformed within the enzyme’s active site to enzyme-product complex—the key chemistry step.

3. [E.P] ⇌ Eff + P (K₃): Product is released, enzyme is regenerated for further catalysis.

What Is a Suicide (Mechanism-Based) Inhibitor?

• Suicide inhibitors are substrate analogs recognized and processed by the enzyme as if they were normal substrates.

• During the turnover (the ES→EP step), the enzyme is inactivated by covalent modification. The enzyme “commits suicide” in trying to catalyze the reaction with the inhibitor.

• This mechanism ensures highly specific and irreversible inhibition.

Why K₂ Is the Affected Step

• K₂ is the rate constant (or equilibrium constant) for the conversion of enzyme-substrate ([E.S]) to enzyme-product ([E.P]).

• Suicide inhibitors like penicillin covalently bind only after the initial ES complex forms and the chemistry step proceeds.

• Penicillin mimics substrate and, as the enzyme attempts to convert ES to EP, forms a stable, covalent complex that the enzyme cannot resolve, locking it in the EP form and stopping further catalysis.

Answer Choices Explained

• K₁: Substrate binding step – unaffected by suicide inhibitors, as they rely on the active site binding the substrate (or analog).

• K₂: The chemistry step where the suicide inhibitor irreversibly modifies the enzyme – correct answer.

• K₃: Product release – at this stage, the enzyme is already inactivated.

• K₄: Not shown or relevant in the given pathway (the standard model involves 3 key kinetic steps).

Biological and Clinical Relevance

• Penicillin and similar β-lactam antibiotics are successful because they exploit suicide inhibition, irreversibly blocking bacterial cell wall synthesis.

• Mechanism-based inhibitors are favored for drug design due to their high specificity and reduced side effects.

Summary Table

Conclusion

Suicide inhibitors like penicillin act during the ES → EP (K₂) step, exploiting the enzyme’s normal catalytic function to irreversibly deactivate it. This mechanism ensures maximal specificity and lasting inhibition, making K₂ the critical target step.