Enzyme inhibitors are molecules that prevent the normal functioning of enzymes, inhibiting their usual function. Typically this means slowing their reactions down, or halting them altogether.

While enzymes are vital for life, and many enzyme inhibitors are potent poisons (cyanide for example), enzyme inhibitors are also vital for regulating the function of enzymes, by preventing enzymes working at their full rate at all times, they can be used in the transport of enzymes from areas where their activity isn't wanted to areas where it is.

There are three types of enzyme inhibitor, competitive inhibition, uncompetitive inhibition, and non-competitive inhibition.

Competitive Inhibition

In competitive inhibition, a compound will bind to the active site of the enzyme, but not undergo any reaction. As in such in a situation the enzyme has reduced access to the substrate (due to the enzyme active site being blocked by the inhibitor) the result is the equivalent of less enzyme being present in the reaction mixture, as some of the enzyme molecules are being prevented from accessing substrate due to the presence of the inhibitor (Figure 1).

Figure 1: In competitive inhibition the inhibitor molecule can bind to the active site of the enzyme in the same way that the substrate can, but unlike the substrate cannot be metabolized into a product. This creates an alternative reaction pathway leading to the enzyme-inhibitor complex, which is a dead end in the reaction pathway. Like the formation of the enzyme-substrate complex, this complex formation is reversible, so if substrate concentrations are high enough, they can overcome the effect of the inhibitor to achieve the same V_max as the uninhibited enzyme, but the K_M will increase, as it will take longer to reach this rate.

As a competitive inhibitor competes with the substrate when binding to the active site, the enzyme can eventually achieve the same V_max, but at higher substrate concentrations than without the enzyme. As a higher concentration of substrate is needed to achieve the same reaction rate, even though the V_max remains the same. This means that the k_M is affected by a competitive inhibitor, but not the V_max, as shown by the Michaelis-Menten and Lineweaver-Burk plots (Figure 2).

Figure 2: The effect of a competitive inhibitor on the function of an enzyme can be seen most effectively on a Michaelis-Menten (left), and especially a Lineweaver-Burk (right) plot. In a competitive inhibition, the K_M increases, while the V_max remains the same, which can be seen in the Michaelis-Menten plot as the initial slope of the line in the presence of the inhibitor being much less steep, but the line still approaches the same maximum rate. In the Lineweaver-Burk plot, this is reflected by both lines having the same y-intercept, but the competitive inhibitor having a greater slope and a different x-intercept, reflecting the change in K_M.

Competitive inhibition is a common form of enzyme inhibition, examples are drugs like penicillin (an inhibitor of bacterial enzymes) and methotrexate (an anti-cancer drug that inhibits important enzyme in cancer cells). This also serves to regulate the function of enzymes when their substrates are at low concentrations, while allowing them to function as normal at higher substrate concentrations.

Uncompetitive Inhibition

A rare type of inhibition is uncompetitive inhibition. In uncompetitive inhibition, the inhibitor doesn't bind to the active site of the enzyme directly, instead, it can only bind to the enzyme-substrate complex. This doesn't prevent the enzyme-substrate complex formation like the competitive inhibitor does, but it does prevent the reaction proceeding towards product formation (Figure 3).

Figure 3: In uncompetitive inhibition, the inhibitor can only bind to the enzyme-substrate (ES) complex, and in doing so creates an enzyme-substrate-inhibitor (ESI) complex. This ESI cannot be metabolized into product, and so this pathway is a dead end in the reaction pathway, and because the formation of this complex cannot be overcome by increasing substrate concentration, the V_max is reduced. As the ESI complex is formed from the ES complex, reducing the concentration of the latter, this affects the equilibrium of the E + S to ES reaction, thus increasing the affinity of the enzyme for its substrate, and so reducing the K_M.

The effect of binding to the enzyme-substrate complex by the inhibitor is to slow down the rate of product formation, as some of the enzyme-substrate complex forms an enzyme-substrate-inhbitor complex instead. As the inhibitor does not compete with the substrate, increasing the substrate concentration will not eventually overcome the effect of the inhibitor on the rate of reaction, thus an uncompetitive inhibitor, unlike a competitive inhibitor reduces the V_max. However as the inhibitor doesn't affect the rate of enzyme-substrate complex formation, and may even increase the affinity for the substrate, the k_M actually increases, as shown by the Michaelis-Menten and Lineweaver-Burk plots (Figure 4).

Figure 4: The effect of an uncompetitive inhibitor on the function of an enzyme can be seen most effectively on a Michaelis-Menten (left), and especially a Lineweaver-Burk (right) plot. In the case of uncompetitive inhibition, the V_max is reduced, but the concentration of substrate required to reach half that reduced V_max increases, and hence the K_M increases. This is because the presence of the inhibitor increases the affinity of the enzyme for its substrate. This can be seen in the Lineweaver-Burk plot, where the inhibitor forms a parallel line to the uninhibited enzyme, reflecting an increase in both the V_max and the K_M.

Uncompetitive inhibition is a rare type of enzyme inhibition, but one example is hydrazine, a highly toxic compound that causes many health problems.

Non-Competitive Inhibition

Non-competitive inhibition is more common than uncompetitive inhibition. In non-competitive inhibition, the inhibitor binds to the enzyme away from the substrate, and furthermore can bind to the enzyme and enzyme-substrate complex with equal affinity (Figure 5).

Figure 5: In non-competitive inhibition, the inhibitor binds to a site away from the active site of the enzyme, and is able to bind to both the enzyme alone, and the enzyme-substrate complex. Like other forms of inhibitor, the enzyme-substrate-inhibitor complex is not able to complete the reaction, and thus this is a dead end in the reaction pathway. As the inhibitor can bind to both the enzyme alone and the enzyme-substrate complex, and in doing both affect the progression of the enzyme catalyzed reaction, it affects both the V_max by slowing down the rate of product formation, however the K_M remains the same, as the inhibitor has no effect on the enzyme affinity for its substrate.

As the enzyme binds to both the enzyme, and the enzyme-substrate complex, it has effects of both a competitive and an uncompetitive inhibitor. Like with the uncompetitive inhibitor, the non-competitive inhibitor does not compete with the enzyme substrate, and so increased substrate concentrations will never overcome the effect of the enzyme, thus the V_max is reduced. However because the effect of the inhibitor does not change the affinity of the enzyme for its substrate, as the inhibitor can bind to the enzyme in whatever state it is in, the k_M remains the same. This is shown in the Michaelis-Menten and Lineweaver-Burk plots (Figure 6)

Figure 6: The effect of a non-competitive inhibitor on the function of an enzyme can be seen most effectively on a Michaelis-Menten (left), and especially a Lineweaver-Burk (right) plot. With the V_max reduced (from 100 to 70) by the non-competitive inhibitor, the y-intercept changes. However because the K_M remains the same at 10mM, so too does the x-intercept.

Perhaps the most famous enzyme inhibitor of all is a non-competitive inhibitor. Cyanide non-competitively inhibits an enzyme vital for energy production in the body's cells. This is what makes cyanide such a potent poison.