Drugs are
chemical agent’s which produces a measurable reaction in a biological system.

Drugs are administrated to target protein structures in order to produce a
pharmacological effect. Enzymes are proteins which act as biological catalysts
which alter the rate of biological reaction and are unchanged at the end of the
reaction, this allows reaction to occur at sufficient speed. Enzymes can be the
target of drugs in order to catalyse reactions which lead to formation of metabolites
which can be the biologically active form and bring about a pharmacological
effect and also aid the breakdown and elimination of the drug. This is the case
in the P450 cytochrome enzyme’s which is present in the liver; drugs encounter these
enzymes when undergoing first pass Metabolism. A drug can also have an
inhibitory effect on enzyme this is the case in the anti-cancer drug Tyrosine Kinases
Inhibitor which inhibits the enzyme tyrosine kinases preventing the
phosphorylation of tyrosine residue substrates effecting cell proliferation.

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This effect may be exhibited when drug binds to enzyme linked receptor, this
alters the shape, leading to signal to be transduced and enzymatic activation
or inhibition to occur. An enzyme may be targeted by an enzyme linked receptor
or a GPCR which acts as a relay to activate enzymes to produce a second
messenger which lead to the intended response.