Many drugs have enzymes as drug target. On the one hand, a drug can be a substrate for an enzyme and be biotransformed into another substance.
Examples: An enzyme is able to convert a certain substrate into a certain product. This is a normal conversion or biotransformation. An enzyme also can be a very useful tool for activation of a drug. When a drug is unable to pass the stomach or when it is unstable, a prodrug can be taken. This prodrug will then be converted into the active drug by an enzyme in the body. A drug can also compete with the endogenous substrate for the enzyme. In this way the normal product will be produced in a lesser extent, because the enzyme is also busy with the conversion of the drug into another product. When the drug is preferred above the normal substrate by the enzyme, it is called competitive inhibition. In this
case the normal product is hardly made, while the abnormal product is synthesized in large amounts. On the other hand, a drug can bind an enzyme (without acting as a substrate), resulting in an altered enzyme activity. In this case an enzyme is a receptor.
Examples: The enzyme activity can decrease: due to the subsequent inability of the enzyme to achieve the normal conversion rate, less product is formed. When drug binding to an enzyme completely blocks the enzyme activity, no product will be formed. A drug can also enhance the activity of the enzyme. Drug binding to an enzyme can also result in a conformational change that enhances the activity of the enzyme and thus leads to production of more product. Examples of drugs that inhibit enzymes are ACE-inhibitors, methotrexate, thionamides, and oral anticoagulants.