Enzyme

An enzyme (from Greek énsimo (??????), formed by én = at or in and simo = leaven or yeast) is a protein that catalyzes, or speeds up, a chemical reaction.

Inhibition

Enzymes reaction rates can be changed by competitive inhibition, non-competitive inhibition, uncompetitive inhibition and mixed inhibition.

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Competitive inhibition

The inhibitor may bind to the substrate binding site as shown in the figure above, thus preventing substrate binding. An example for competitive inhibition is the enzyme succinate dehydrogenase by malonate. Succinate dehydrogenase catalyses the oxidation of succinate to fumarate.

Related Topics:
Malonate - Succinate - Fumarate

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Uncompetitive inhibition

Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme-substrate complex, not to the free enzyme, the enzyme-inhibitor-substrate (EIS) complex is catalytically inactive. This mode of inhibition is rare.

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Non-competitive inhibition

Non-competitive inhibitors never bind to the active center, but to other parts of the enzyme that can be far away from the substrate binding site, consequently, there is no competition between the substrate and inhibitor for the enzyme. The extent of inhibition depends entirely on the inhibitor concentration and will not be affected by the substrate concentration. However, these inhibitors bind only loosely with the enzyme and can be removed to resume the enzymatic activities. For example, cyanide combines with the copper prosthetic groups of the enzyme cytochrome c oxidase, thus inhibiting respiration.

Related Topics:
Cyanide - Copper - Cytochrome c oxidase - Respiration

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By changing the conformation (the three-dimensional structure) of the enzyme, the inhibitors either disable the ability of the enzyme to bind or turn over its substrate. The EI and EIS-complex have no catalytic activity.

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Partially competitive inhibition

The mechanism of partially competitive is similar to that of non-competitive inhibition, except that the EIS-complex has catalytic activity, which may be lower or even higher (partially competitive activation) than that of the ES-complex.

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Irreversible inhibitors

Some inhibitor bind irreversibly with the enzyme molecules, inhibiting the catalytic activities permanently. The enzymatic reactions will stop sooner or later and are not affected by an increase in substrate concentration. These are irreversible inhibitors. Examples are heavy metal ions including silver, mercury and lead ions.

Related Topics:
Silver - Mercury - Lead

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Another example of irreversible inhibition is provided by the nerve gas diisopropylfluorophosphate (DFP) designed for use in warfare. It combines with the amino acid serine (contains the ?SH group) at the active site of the enzyme acetylcholinesterase. The enzyme deactivates the neurotransmitter acetylcholine. Neurotransmitters are needed to continue the passage of nerve impulses from one neurone to another across the synapse. Once the impulse has been transmitted, acetylcholinesterase functions to deactivate the acetycholine almost immediately by breaking it down. If the enzyme is inhibited, acetylcholine accumulates and nerve impulses cannot be stopped, causing prolonged muscle contration. Paralysis occurs and death may result since the respiratory muscles are affected. Some insecticides currently in use, including those known as organophosphates (e.g. parathion), have a similar effect on insects, and can also cause harm to nervous and muscular system of humans who are overexposed to them.

Related Topics:
Nerve gas - Diisopropylfluorophosphate - Warfare - Amino acid - Serine - Acetylcholinesterase - Neurotransmitter - Acetylcholine - Nerve impulse - Neurone - Synapse - Paralysis - Death - Respiratory muscle - Insecticides - Organophosphate - Parathion - Nervous - Muscular system - Human

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