Enzymes: Inhibitors (A-level Biology)

Enzymes: Inhibitors

Inhibition of Enzymes

Often, it is necessary to inhibit the activity of an enzyme. In many biological processes, enzymes function in a negative feedback loop. In this loop, the product of the enzyme will inhibit the enzyme once enough product has been made in order to stop the enzyme from producing more product than what is needed. Additionally, many pharmaceutical drugs are designed to inhibit enzymes for therapeutic purposes. In this section, we will discuss the two major mechanisms by which enzymatic activity can be inhibited.

Competitive Inhibition

• Competitive inhibitors compete with the substrate for the active site. The inhibitor molecule, which has a similar shape to the substrate molecule, competes with the substrate to bind to the active site of an enzyme. When the inhibitor binds, it occupies the active site without causing a reaction.

• Increasing substrate concentration can reduce inhibition. If there is a greater concentration of substrate, the substrate will outcompete the inhibitor for the active site, and vice versa. Therefore increasing the substrate concentration will increase the rate of reaction (up to a certain point, after which the inhibitor is outnumbered and has a negligible effect).

Non-competitive Inhibition

• Non-competitive inhibitors do not bind to the active site. The inhibitor molecule can bind to a site on an enzyme that is not the active site.
• Non-competitive inhibitors change the shape of the active site. Binding of the inhibitor to the alternative site results in a conformational change (change in shape) of the active site. Because of this conformational change, the enzyme is no longer able to bind to its substrate.
• Increasing substrate concentration has no effect. The inhibitor is binding to an alternative site, so adding more substrate is useless.

• Non-competitive inhibition is common. Non-competitive inhibition is a common mechanism by which negative feedback loops work in many biochemical processes.

Reversible and Non-Reversible Inhibition

• Inhibition of enzymes can be reversible or non-reversible. Some inhibitors bind reversibly to enzymes (i.e. their inhibiting effect on the enzyme can be reversed) while others bind irreversibly. 
• Reversible inhibitors form weak bonds with the enzymes. Reversible inhibitors typically form hydrogen or ionic bonds with their enzymes which are weak and can easily be broken.
• Irreversible inhibitors form strong bonds with the enzymes. Irreversible inhibitors typically form covalent bonds with their enzymes which are much stronger and cannot be easily broken.

Drugs and Metabolic Poisons as Enzyme Inhibitors

• Medicinal drugs can function as enzyme inhibitors. Certain medicines work by inhibiting enzymes of different bacteria and viruses:
• • Penicillin inhibits the enzyme that helps proteins form in bacterial cell walls. Penicillin is an antibiotic that inhibits the enzyme transpeptidase in a bacteria’s cell walls, which weakens it and causes the bacteria to eventually rupture.
• • Antiviral drugs can inhibit the replication of viral DNA. Antiviral drugs are used to treat viral infections. Some antivirals inhibit the enzyme reverse transcriptase, which is necessary for the virus to replicate its DNA. As a result, the virus can no longer replicate inside the host.
• Metabolic poisons can inhibit enzymes. Some metabolic poisons can inhibit enzymes in our body and disrupt metabolic reactions, which damages our cells and can even be fatal. Examples of highly toxic poisons which inhibit cellular respiration include:
• • Cyanide.  It irreversibly inhibits the enzyme cytochrome c oxidase.
• • Arsenic. It inhibits the enzyme pyruvate dehydrogenase.
• • Malonate. It inhibits the enzyme succinate dehydrogenase.

End-Product Inhibition

• Many enzymes can partake in a large metabolic pathway. Most metabolic reactions in the body take place in the form of a metabolic pathway, a set of interconnected metabolic reactions. Products from the first reaction are needed for the second reaction to take place, and so on. Each step of this pathway can involve a different enzyme.
• End-product inhibition is when the final product inhibits an enzyme involved in the initial reactions. At the end of the metabolic pathway, the final product may be able to inhibit the enzyme responsible for catalysing the initial reaction. This causes the whole metabolic pathway to stop.
• End-product inhibitors are typically reversible inhibitors. When many products are being produced, the enzymes of the earlier pathways are being inhibited, and this causes the pathway to stop functioning. When the products are used up, there are no longer any inhibitors for the enzymes and the metabolic pathway can operate again, leading to more products being produced once more. This is how metabolic pathways are commonly regulated.

Inactive Precursors

• Enzymes can initially be produced in an inactive form. Some enzymes in the body are inactive when they are first produced. These inactive forms are called “precursors“. They are kept inactive by a reversible inhibitor component, which gets removed in the right environment, to allow the enzyme to become active.
• Precursors prevent unnecessary damage to cells. The purpose of initially producing enzymes in an inactive form is to prevent them from breaking down the cellular environment they are made in. An example would be the enzyme trypsin, which breaks down protein in the small intestine. It is initially produced by the pancreas as an inactive precursor to prevent it from damaging the pancreatic cells.

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