Which enzyme is primarily involved in protein digestion?

Pepsin is the enzyme primarily involved in protein digestion. It is secreted by the stomach in an inactive form called pepsinogen, which is then activated by the acidic environment of the stomach. This activation is crucial because it allows pepsin to break down proteins into smaller peptides, facilitating their further digestion in the small intestine.

This process is essential for protein metabolism, as it enables the body to absorb amino acids, which are the building blocks of proteins. Unlike other enzymes listed, pepsin has a specific role in proteolysis—the breakdown of proteins—which is not shared by the others.

Amylase, for instance, is involved in the digestion of carbohydrates, lipase focuses on breaking down fats, and cellulase aids in the digestion of cellulose in plant materials. Therefore, among the options provided, pepsin stands out as the key enzyme responsible for initiating protein digestion in the digestive system.