An inhibitor is any agent that interferes with the activity of an enzyme. Inhibitors may affect the binding of enzyme to substrate, or catalysis (via modification of the enzyme's active site), or both. Researchers use enzyme inhibitors to define metabolic pathways and to understand enzyme reaction mechanisms. Many drugs are designed as inhibitors of target enzymes. Inhibition is also a natural phenomenon. Cells regulate metabolic pathways by specific inhibition of key enzymes.