Protein

Protein is a natural polymer with a repeating unit of  -NHCH(R)COO- . The type of the R group is depending on amino acid. Its chemical structure and biosynthesis diagram are shown in Figure 2. If the n is 1 or more, that is peptide; if is less than 10, that is oligopeptides; if is larger than 10 but less than 50, that is polypeptides; if is larger than 50, that is protein.

Soy protein isolates are the most abundant protein among the plant proteins, and commonly used in food industries. Zein is a pure protein extracted from corn and has many excellent properties such water resistance, film-forming properties. Keratin is main component of claw, horns and feathers, which contains much amount of cysteine (16%). Collagen protein shows a triple helical structure and forms strong and tough fibers. Collagen is cooked and will produce water-soluble gelatin. Casein is the main protein in milk. 
In nature, enzymes under the instruction of related genes synthesize lots of proteins.