Proteins are highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. They are large molecules with high molecular weights, composed of varying amounts of amino acids, which in the intact protein are united through covalent chemical linkages called peptide bonds.

A protein with a molecular weight of 100.000 would contain about 850 amino acid residues. At least 20 of these amino acids are present in both plant and animal proteins. Amino acids are classified as essential (including dispensable), and conditionally essential (conditionally indispensable).

The amino acids, linked together, form linear unbranched polymeric structures called polypeptide chains. Such chains may contain hundreds of amino-acid residues, arranged in specific order for a given protein.

Reaction of amino acids to form a peptide bond

Depending on the source, the protein has a different structure and combination of amino acids in its chemical make-up. Every protein molecule has a characteristic, three-dimensional shape or confirmation that determines its properties.