Proteins are the major structural materials of animal tissue. Proteins are also involved in the maintenance and regulation of life processes.

Enzymes are proteins which act as biological catalysts.

Amino acids, the building blocks from which proteins are formed, are relatively small molecules which all contain an amino group, $-NH_2$, and a carboxyl group, $-COOH$.

Proteins are made of many amino acid molecules linked together by condensation reactions.

In condensation reactions to form proteins, the amino group of one amino acid and the carboxyl group of another amino acid join, with the elimination of water.

The link which forms between two amino acids is known as a peptide link, $-CONH$, or also as an amide link.

Proteins which fulfil different roles in the body are formed by linking together differing sequences of amino acids.

The body cannot make all of the amino acids required for protein synthesis. Certain amino acids, known as essential amino acids, must be acquired from the diet.

During digestion, enzyme hydrolysis of protein produces amino acids.

Within proteins, the long-chain molecules form spirals, sheets, or other complex shapes.

Proteins chains are held in these forms by intermolecular bonding between the side chains of the constituent amino acids.

When proteins are heated, the intermolecular bonds between the side chains of the constituent amino acids are broken, allowing the proteins to change shape (denature).

The denaturing of proteins in foods causes the texture to change when it is cooked.