Question

1. Explain the difference between competitive and noncompetitive inhibition. I am having a hard time understanding the difference between the two. Could someone please explain the difference between the two?

2. Could someone also give an example of one drug that acts as an enzyme inhibitor and explain its mechanism of action.

Answer 1

1. Imagine a lock (enzyme), whose keyhole is tampered with some dirt (inhibitor), so that the genuine key cannot fit in it. That is competitive inhibition for you - an inhibitor competing with the substrate (reactant) for the same active site. If the inhibitor binds to the active site, it blocks the binding of the substrate.

Imagine the same lock, banged with a hammer from the outside (not necessarily at the keyhole area), so that the lock is deformed and cannot function. That is non-competitive inhibition for you. The inhibitor does not compete for the active site, but binds at some other place in the enzyme, different from the active site, and brings some conformational change to it, so that it can no longer function.

(a) Normal binding (b) Competitive inhibition(c) Noncompetitive inhibition Substrate Active site Competitive inhibitor Enzyme Noncompetitive inhibitor

2. The best example is penicillin. The structure of this molecule resembles to the transition state (the transient state of the substrate just after enzyme-substrate complex formation, and just before the product formation) of the peptide bond formation between (D-alanine)-(D-alanine) - a constituent of the cell wall. So, penicillin is able to bind to the enzyme DD-transpeptidase that synthesizes (D-alanine)-(D-alanine) at the active site and blocks cell wall synthesis.