Vmax and Km in competitive versus non-competitive inhibition

[medicalschoolislife]

Hi everyone,

I was having trouble understanding the Kaplan explanation for why Vmax is unchanged in competitive inhibition, but it decreases with non-competitive inhibition.

I searched on Google, and found this explanation:

"Vmax is the maximum velocity of the enzyme. Competitive inhibitors can only bind to E and not to ES. They increase Km by interfering with the binding of the substrate, but they do not affect Vmax because the inhibitor does not change the catalysis in ES because it cannot bind to ES."

If there is only a limited amount of enzyme, then wouldn't taking up a bunch of the active sites by the competitive inhibitor cause Vmax to decrease? This would happen since there's less enzyme to bind the actual substrate.

In non-competitive inhibition, the inhibitor can bind to enzyme-substrate complexes, so I understand why Vmax decreases there. But I don't see why the same wouldn't happen for competitive inhibition, since a certain amount of the enzyme is also out of commission by being inhibited by the competitive inhibitor.

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[Matthew9Thirtyfive]

So this requires a little more information about what Vmax actually is. It's the maximum velocity of the enzyme, yes, but what it specifically measures is the release of the ES complex into the enzyme and the products. A competitive inhibitor is designed to occupy the active site. The rest is probability. So long as the competitive inhibitor does not **** with the ES complex, it will not affect Vmax. It will, however, affect Km because it specifically affects the ability of substrate to bind into the active site. These terms are just measuring different things, which is why the different inhibitors affect them differently.

Non-competitive inhibitors affect the ability of the ES complex to form a product, forcing it to either return to the enzyme-inhibitor complex or to release the inhibitor. Since it is affecting the ES complex, it changes Vmax. It does not affect the enzyme's ability to bind the substrate, and so it doesn't affect Km.