An important molecule in biochemistry: proteins are linear chains of amino acids, with various sequences of amino acids forming a variety of proteins. The 22 amino acids naturally incorporated into polypetides are called “standard” or proteinogenic, whilst 9 are labelled “essential” as they cannot be produced by the human body.When in the human body, the 22 standard amino acids are either used to synthesize biomolecules such as proteins, or are oxidised to urea and carbon dioxide as an energy source.When oxidised, amino acids can be converted into glucose and ketone bodies – when ingested sugars and carbohydrate levels are low, these are used as a source of energy.