Enzymes at Work - Inhibitors of Action

Inhibitors of enzyme-controlled reactionsInhibitors are defined as substances that reduce the reaction rate in an enzyme-controlled reaction because they have some effect on the enzyme molecule. A wide range of enzyme inhibitors exist. Some have their effect on the active site of just one enzyme. Others affect another part of the molecule, indirectly causing a change in the shape of the active site. Some inhibitors can affect a number of different enzymes. Competitive InhibitorsCompetitive inhibitors molecules have a similar shape to that of the substrate molecules. This means they can occupy the active site, forming enzyme-inhibitor complexes. These complexes do not lead to the formation of products because the inhibitor is not identical to the substrate. The enzyme does not catalyse a reaction. Enzyme inhibition occurs because whenever an inhibitor molecule is occupying an enzyme's active site, a substrate molecule cannot enter. So the number of the enzyme-substrate complexes is reduced and reaction rate slows down. The level of inhibition depends on the concentrations of inhibitor and substrate. Where the numbers of substrate molecules are increased, the level inhibition decreases because a substrate molecule is more likely than an inhibitor molecule to collide with an active site.Non-competitive InhibitorsNon-competitive inhibitors do not compete with substrate molecules for a place in the active site. Instead, they attach to the enzyme molecule in a region away from the active site. The attachment of a non-competitive inhibitor molecule to an enzyme molecule distorts the tertiary structure of the enzyme molecule. This leads to a change in shape of the active site. This means the substrate no longer fits into the active site, so enzyme-substrate complexes cannot form and the reaction rate decreases.The level of inhibitition depends on the number of inhibitor molecules present. If there are enough inhibitor molecules to bind to all the enzymes molecule present, then the enzyme-controlled reaction will stop. Changing the substrate concentratopn will have no effect on this form of inhibition. Permanent InhibitorsMost competitive inhibitors do not bind permanently to the active site. They bind to the active site for a short period and then leave. Their action is described as reverisble, as removal of the inhibitor from the reacting mixture leaves the enzymes molecules unaffected.Many non-competitive inhibitors bind permanently to enzyme molecules. The inhibition is not reversible, and any enzymes molecules bound by inhibitor molecules and effectively denatured. The regulatoin of a number of metabolic pathways involves the inhibition of enzymes to control the reaction rates.

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