Final answer:
Competitive inhibition is when an inhibitor binds reversibly to the enzyme's active site, competing with the substrate. Noncompetitive inhibition occurs when an inhibitor binds to a different site, altering the active site shape. In reversible inhibition, the inhibitor can dissociate from the enzyme, whereas irreversible inhibition permanently inactivates the enzyme.
Step-by-step explanation:
Competitive inhibition is when an inhibitor reversibly binds to an enzyme at the enzyme active site; competing with the substrate for binding. A competitive inhibitor must be a molecule that is structurally similar to the substrate molecule, allowing it to interact with the enzyme active site through similar non-covalent interactions, but it does not, or cannot, undergo the same chemical reaction. When the inhibitor is bound to the active site, it blocks the correct substrate from binding and catalysis from occurring. However, as a reversible inhibitor, it can disassociate from the enzyme eventually allowing for the correct substrate to bind and the catalysis to occur. Because the inhibitor and substrate are in competition for the same active site, inhibition is concentration-dependent. As shown in the below plot of rate of reaction vs. substrate concentration (Figure 19.6.1), the competitive inhibitor slows the rate of reaction, but at higher substrate concentrations, the normal maximum rate can be reached.
A noncompetitive inhibitor can bind to either the free enzyme or the enzyme-substrate complex because its binding site on the enzyme is distinct from the active site. Binding of this kind of inhibitor alters the three-dimensional conformation of the enzyme, changing the configuration of the active site with one of two results. Either the enzyme-substrate complex does not form at its normal rate, or, once formed, it does not yield products at the normal rate. Because the inhibitor does not structurally resemble the substrate, nor is it competing with the substrate for the active site, the addition of excess substrate does not reverse the inhibitory effect.
Answers:
- It inactivates an enzyme by bonding covalently to a particular group at the active site.
- A competitive inhibitor structurally resembles the substrate for a given enzyme and competes with the substrate for binding at the active site of the enzyme. A noncompetitive inhibitor binds at a site distinct from the active site and can bind to either the free enzyme or the enzyme-substrate complex.
Irreversible Inhibition:
An irreversible inhibitor inactivates an enzyme by bonding covalently to a particular group at the active site. When the inhibitor is bound, the enzyme active site is blocked, the substrate does not bind, and catalysis cannot occur, similar to competitive inhibition. The difference here is that the inhibition is irreversible, meaning that the inhibitor remains bound and does not dissociate from the enzyme because the enzyme-inhibitor covalent bonds are not easily broken. In the presence of an irreversible inhibitor, the substrate cannot bind the active site at all, nor can high substrate concentrations outcompete the inhibitor, hence the enzyme is completely inactivated.
Non-competitive Inhibition:
A noncompetitive inhibitor does not bind at the active site. It attaches at some other site on the enzyme, and changes the shape of the protein. This shift in three-dimensional structure alters the shape of the active site so that the substrate will no longer fit in the site properly.
Reversible Inhibition:
Reversible enzyme inhibition can be competitive, noncompetitive, or uncompetitive, depending on where the inhibitor binds to the enzyme, substrate, or enzyme-substrate complex.
Noncompetitive Inhibition:
A noncompetitive inhibitor does not have a shape similar to the substrate and does not bind to the active site. It binds with the enzyme outside the active area but changes the folding patterns of the protein such that the active site can not acquire the proper shape. So, the substrate can not fit into the active site properly.
On the other hand, a noncompetitive (allosteric) inhibitor binds to the enzyme at an allosteric site, a location other than the active site, and still manages to block substrate binding to the active site by inducing a conformational change that reduces the affinity of the enzyme for its substrate.