Noncompetitive inhibitor
Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme and binds equally well to the enzyme whether or not it has already bound the substrate.
The inhibitor may bind to the enzyme whether or not the substrate has already been bound, but if it has a higher affinity for binding the enzyme in one state or the other, it is called a mixed inhibitor.
It is important to note that while all non-competitive inhibitors bind the enzyme at allosteric sites (i.e. locations other than its active site)—not all inhibitors that bind at allosteric sites are non-competitive inhibitors. In fact, allosteric inhibitors may act as competitive, non-competitive, or uncompetitive inhibitors.
Many sources continue to conflate these two terms, or state the definition of allosteric inhibition as the definition for non-competitive inhibition.
Non-competitive inhibition models a system where the inhibitor and the substrate may both be bound to the enzyme at any given time. When both the substrate and the inhibitor are bound, the enzyme-substrate-inhibitor complex cannot form product and can only be converted back to the enzyme-substrate complex or the enzyme-inhibitor complex. Non-competitive inhibition is distinguished from general mixed inhibition in that the inhibitor has an equal affinity for the enzyme and the enzyme-substrate complex.
The most common mechanism of non-competitive inhibition involves reversible binding of the inhibitor to an allosteric site, but it is possible for the inhibitor to operate via other means including direct binding to the active site. It differs from competitive inhibition in that the binding of the inhibitor does not prevent binding of substrate, and vice versa, it simply prevents product formation for a limited time.
This type of inhibition reduces the maximum rate of a chemical reaction without changing the apparent binding affinity of the catalyst for the substrate (Kmapp – see Michaelis-Menten kinetics).
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