Proteins as Drugs and Drug Design Targets Enzymes

2 PROTEINS AS DRUGS AND DRUG DESIGN TARGETS ENZYMES 

A considerable number of enzymes occupy a central and crucial role in the activity of drugs. Dihydrofolate reductase, an enzyme involved in purine and amino acid biosynthesis, is the target of antibacterial sulfanilamides, which act both as bacteriostatics and antimalarials. These drugs act on the enzyme in different ways, some being so-called antimetabolites (i.e., reversible enzyme inhibitors). Some diuretics act on carbonic 

Enzymology is a large and complex subdivision of biochemistry, and no attempt will be made to cover it in this book. The reader who requires a review of enzyme kinetics and mechanisms is referred to the many currently available and excellent textbooks of biochemistry. 

Enzyme inhibitors inhibit the action of enzymes either reversibly or irreversibly. Since enzymes are such pervasive, powerful biological catalysts, inhibitors can act as potent drugs. Broadly categorized, enzyme inhibitors may be either irreversible or reversible. 

Irreversible inhibitors combine or destroy a functional group on the enzyme so that it is no longer active. They often act by covalently modifying the enzyme. Thus a new enzyme needs to be synthesized. Examples of irreversible inhibitors include acetylsal-icyclic acid, which irreversibly inhibits cyclooxygenase in prostaglandin synthesis. Organophosphates (e.g., malathion, 8.10) irreversibly inhibit acetylcholinesterase. Suicide inhibitors (mechanism-based inactivators) are a special class of irreversible inhibitors. They are relatively unreactive until they bind to the active site of the enzyme, and then they inactivate the enzyme. 

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