What are the digestive enzymes humans lack?

Humans lack cellulase, the enzyme required to break down cellulose. This is the reason why humans are unable to digest cellulose. 

Cellulose is a complex substance that needs the enzyme cellulose for proper degradation. In humans, because cellulase is absent, the cellulose remains indigestible. This undigested cellulose is the fiber that helps the intestinal tract function more smoothly.