Competitive vs Non-Competitive Inhibition

## Introduction

The distinction between competitive and non-competitive inhibition is central to understanding drug potency, surmountability, and therapeutic strategy. These concepts apply to both enzyme inhibitors and receptor antagonists, and they predict how the body's own substrate or agonist concentrations affect drug efficacy.

## Competitive Inhibition

A competitive inhibitor binds the same site as the substrate or endogenous agonist. Because binding is mutually exclusive, increasing substrate concentration can displace the inhibitor and restore full activity. On a Lineweaver-Burk plot, competitive inhibition increases the apparent Km without changing Vmax—lines intersect on the y-axis.

At receptors, competitive antagonists produce a rightward parallel shift of the agonist dose-response curve without reducing the maximum response. The Schild equation (dose ratio = 1 + [B]/KB) quantifies the shift, and a Schild plot with slope of 1 confirms pure competitive antagonism.

## Non-Competitive Inhibition

A non-competitive inhibitor binds a different site from the substrate (or binds irreversibly to the orthosteric site), reducing catalytic activity regardless of substrate concentration. Vmax decreases while Km remains unchanged. The inhibition cannot be overcome by adding more substrate.

At receptors, non-competitive antagonists reduce the maximum achievable response (depress the top of the dose-response curve). This can occur through allosteric binding, irreversible orthosteric binding, or downstream pathway interruption. Ketamine acts as a non-competitive NMDA antagonist by blocking the open channel pore.

## Mixed Inhibition

Mixed inhibitors bind both the free enzyme and the enzyme-substrate complex with different affinities. Both Km and Vmax change simultaneously. Mixed inhibition is actually the most common form in biological systems; pure competitive and pure non-competitive inhibition represent the two limiting cases of this general model.

## Clinical Significance

Understanding the inhibition type helps predict drug behavior during disease states where endogenous ligand levels fluctuate:

- **Losartan** (competitive AT1 antagonist): High angiotensin II can partially overcome blockade
- **Valsartan** (insurmountable AT1): Slow dissociation renders it functionally non-competitive
- **Phenoxybenzamine** (irreversible alpha-blocker): Used pre-operatively for pheochromocytoma catecholamine surges

## Key Takeaways

- Competitive inhibitors bind the same site and can be overcome by excess substrate
- Non-competitive inhibitors reduce maximal response regardless of substrate level
- Kinetic parameters (Km and Vmax changes) definitively distinguish inhibition type
- Clinical surmountability depends on mechanism and drug-target residence time