Deadly poisons
Many poisonous substances have their effects because they inhibit or even overactivate enzymes. Potassium cyanide, for example, inhibits cell respiration. It does this because it is a non-competitive inhibitor for a vital respiratory enzyme called cytochrome oxidase, found in mitochondria. Inhibition of this enzyme decreases the use of oxygen, so ATP cannot be made. The organism can only respire anaerobically, which leads to a build up of lactic acid in the blood.
Only 100-200mg of cyanide must be absorbed in order to make an adult lose consciousness. This can occur in as little as 10 seconds. If untreated, the body goes into a coma in around 45 minutes, and death results after around 2 hours.

Replacement Enzymes and Cystic Fibrosis
One of the symptoms of the inherited disease cystic fibrosis is that the passage of digestive enzymes, normally secreted from the pancreas into the gut, is blocked. This means that individuals with the condition have problems digesting their food. Doctors prescribe enzymes in tablet form to individuals with cystic fibrosis in order to overcome this problem. The enzymes are packaged in an acid-resistant coat so that they are not destroyed by the acid and protein-digesting enzymes in the stomach.

Ethylene glycol poisoning
Ethylene glycol is found in the antifreeze used in car engines. Ethylene glycol is not itself poisonous, but if taken into the body, it is broken down in the liver by the enzyme alcohol dehydrogenase. The breakdown product, oxalic acid, is extremely toxic and ethylene glycol poisoning can lead to death.
If ethylene glycol intake is suspected, the individual can be given a massive dose of ethanol. This leads to servere alcohol intoxication. The ethanol acts as a competitive inhibitor of the alcohol dehydrogenase. This reduces the rate of production of oxalic acid, allowing the ethylene glycol to be excreted harmlessly.

Snake venom
Snake venom is a mixture of toxins and different enzymes. Phosphodiesterases are present in most snake venoms. These interfere with the working of the prey's heart, causing a fall in blood pressure.
Snake venom also contains an inhibitor of the enzyme acetyl cholinesterase. This enzyme is involved nerver transmission, and its inhibition results in paralysis.
The enzyme hyaluronidase is a digestive enzyme that breaks down connective tissue and so helps the toxins to penetrate tissues quickly. Snake venom also often contains ATP-ases, which are used for breaking down ATP to disrupt the prey's use of energy.

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Undergraduate Biology (Biological Molecules) Notiz am Interfering with Enzymes - Poisons and Drugs, erstellt von siobhan.quirk am 30/05/2013.