Enzyme Inhibitor - Concepts
The rates of enzyme catalysed reactions are decreased by specific inhibitors. Inhibitors are compounds that combine with enzymes and prevent enzyme and substrate from forming ES complex. The toxicity of many compounds such as hydrogen cyanide and hydrogen sulphide results from their action as enzyme inhibitors. Many drugs also act to inhibit specific enzymes. Thus, knowledge of enzyme inhibitors is vital to understand drug action and toxic agents.
Compounds which convert the enzymes into inactive substances and then adversely affect the rate of enzyme catalysed reactions are called as enzyme inhibitors. Such a process is known as enzyme inhibition. Two broad classes of enzyme inhibitors are generally recognized. They are reversible and irreversible inhibitors. This depends on whether the inhibition can be reversed or not.
A reversible enzyme inhibitor dissociates very rapidly from its target enzyme because it becomes very loosely bound with the enzyme. Three general types of reversible inhibition is observed: competitive, noncompetitive and un-competitive, depending on the following factors.
· Whether the inhibition is over come or not by increasing the concentration of the substrate.
· Whether the inhibitor binds with the active site or site other than the active site (allosteric site).
· Whether the inhibitor binds with the free enzyme only or with the enzyme substrate complex only or with either of the two.
Competitive inhibitors can combine reversibly with the active site of enzyme and compete with the substrate for binding with the active site. If the site is occupied by the inhibitor it is unavailable for the binding of the substrate (Fig 9.6). The competitive inhibitor always resembles the structure of the substrate. In some cases competitive inhibitors are exact structural analogues of the substrates. The combination of a competitive inhibitor (I) with enzyme (E) can be written in the same manner as combination with substrate, although the inhibitor cannot be chemically transformed to products.
Ki is equal to the dissociation constant for the enzyme – inhibitor complex EI.
The degree of inhibition depends upon the relative concentration of the substrate and the inhibitor. It also depends on the relative affinity of inhibitor towards enzyme active site. Thus, by increasing the substrate concentration we can decrease the degree of inhibition keeping inhibitor concentration at constant level.
The classic example is the inhibition of succinate dehydrogenase by malonate and other dicarboxylic acids. Succinate dehydrogenase is a member of the group of enzymes catalyzing the Krebs tricarboxylic acid cycle.
It catalyzes the removal of two hydrogen atoms from the two methylene carbon atoms of succinate. Succinate dehydrogenase is inhibited by malonate, which resembles succinate in having two ionized carboxyl groups.
Many micro organisms like bacteria synthesize the vitamin folic acid from para-aminobenzoic acid. Sulphanilamide and other sulfa drugs are structural analogs of para-aminobenzoic acid. So, sulfa drugs act as competitive inhibitor and occupy the active site of some bacterial enzyme catalyzing this reaction. When this reaction is affected, it blocks the folic acid biosynthesis which is essential for the growth of micro organisms, ultimately results in the death of the micro organisms. Thus, many sulfa drugs act as antibiotics
This type of inhibition occurs when an inhibitor combines reversibly only with ES to form ESI which cannot yield the products.
Ki = dissociation constant of ESI complex.
An un-competitive inhibitor also binds at an allosteric site and the binding takes place only in enzyme substrate complexes and not with the free enzyme molecule (Fig 9.7).
In this type of inhibition no competition occurs between the substrate and the inhibitor and the inhibitor has no structural resemblance with the substrate and it binds with the enzyme at a place other than the active site. Since I and S may combine at different sites, formation of both EI and ESI complexes takes place (Fig 9.8). The enzyme is inactivated when inhibitor is bound, whether the substrate is present or not. Non competitive inhibition in contrast to competitive inhibition cannot be overcome by increasing substrate concentration. For example various heavy metal ions such as Ag2+, Hg2+, Pb2+ inhibit the activity of a variety of enzymes. Urease can be inactivated by any one of these metal ions.
Irreversible inhibitors are those that combine with or destroy a functional group on the enzyme that is essential for its activity. The irreversible inhibitor dissociates very slowly from its target enzyme because it becomes very tightly bound to its active site, thus inactivating the enzyme molecule. The bonding between the inhibitor and the enzyme may be covalent or noncovalent.
· Alkylating agents such as iodoacetamide, irreversibly inhibit the catalytic activity of some enzymes by modifying cysteine and other side chains.
· Organo phosphorous compounds such as diisopropyl phosphoflouridate are potential irreversible inhibitors of enzymes that have active seryl residues at their catalytic sites.