This statement regarding enzyme inhibition is correct

The correct answer is competitive inhibition is observed when substrate and inhibitor compete for the active site on enzyme.

To understand why this statement is correct, let us break down the concepts of enzyme inhibition step-by-step:

1. Enzymes and Active Sites:
Enzymes are biological catalysts that speed up chemical reactions. They have a specific region called the active site where the substrate (the reactant molecule) binds to undergo the chemical reaction.

2. Competitive Inhibition:
In competitive inhibition, the inhibitor molecule closely resembles the chemical structure of the natural substrate. Because of this structural similarity, both the substrate and the inhibitor compete directly for binding to the same active site on the enzyme. If the inhibitor binds to the active site, it physically blocks the substrate from binding, thereby reducing the rate of the reaction. This directly supports the correct option.

3. Why the other options are incorrect:
- "non-competitive inhibitors often bind to the enzyme irreversibly": This is incorrect because non-competitive inhibition is typically reversible; the inhibitor binds to an allosteric site (a site other than the active site), and this binding can be reversible or irreversible depending on the specific inhibitor, but it is not defined by irreversibility.
- "non-competitive inhibition of an enzyme can be overcome by adding a large amount of substrate": This is incorrect. Since non-competitive inhibitors bind to a site other than the active site, adding more substrate will not displace the inhibitor. Increasing substrate concentration can only overcome competitive inhibition.
- "competitive inhibition is observed when substrate competes with an enzyme to bind to an inhibitor protein": This is incorrect because the competition is between the substrate and the inhibitor for binding to the enzyme, not between the substrate and the enzyme to bind to the inhibitor.