Proteins are created from linear polymers of amino acids linked to each other via peptide bonds.
- Oligopeptides: small peptide polymers containing between 2 and 50 amino acids
- Polypeptides: large peptide polymers containing more than 50 amino acids
Phenylalanine Threonine Histidine
Valine Isoleucine Arginine
Tryptophan Methionine Leucine
Lysine
Proteins from animal sources contain all 10 essential amino acids and are therefore considered high quality. Proteins from plant sources, however, contain some but not all essential amino acids. A judicious combination of different plant sources can provide a balanced supply of high-quality protein.
Digestion and Absorption
The acidic pH (< 2) of the stomach denatures proteins and thus makes their polypeptide chains more susceptible to enzymatic degradation by peptidases, as follows:
- Pepsin: Proteins are initially digested in the stomach by the gastric enzyme pepsin. It is most effective at low pH and is inactive at a higher pH such as that observed in the duodenum (pH > 7).
- Pancreatic peptidases: The short polypeptides generated from the action of pepsin are further cleaved in the duodenum by pancreatic peptidases. Cleavage by these peptidases releases amino acids, which are absorbed by intestinal cells.
Proteins are a major source of fuel, enzymatic activity, and structural support (e.g., keratin, collagen). Their degradation into amino acids provides the building blocks for the synthesis of other important molecules (e.g., heme molecules and nucleotides).
