Enzymes are basically proteins, except for ribozymes. They show quaternary structures which are stabilised by various types of interactions, like H-bond, disulphide bonds, etc.
Enzymes generally function in a narrow range of temperature and pH.
Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.
An inhibitor is any chemical substance which can stop the enzyme from action.
When the inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive inhibitor. The inhibition of succinic dehydrogenase by malonate is an example of this category.