Main focus of this lecture is to present on Enzymes; here briefly examine what enzymes are and describe how they work. They are globular proteins that catalyse chemical reactions in living organisms. The enzyme binds to the substrates by its active site. The active site is a pocket formed by the folding of the protein where the substrates bind. The active site involves a small number of key residues that actually bind the substrates. The rest of the protein structure is needed to maintain these residues in position.