The activity of any enzyme is extremely sensitive to the presence of specific chemicals that bind to the enzyme. Inhibitors are capable of switching off the activity of enzyme. 

When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor. Competitive inhibitors compete with the substrate for the substrate binding site of the enzyme and block the activity.

The inhibition of succinic dehydrogenase enzyme by malonate which closely resembles the substrate succinate in structure is an example of competitive inhibition.