Protein

Protein is a type of large molecule (macromolecule) found in every living cell. It consists of a chain of amino acids linked together by bonds called peptide bonds. The type of protein and its function depend on the sequence of amino acids and the three-dimensional shape that the chain folds into.

Proteins perform various functions: some are enzymes that speed up chemical reactions in the body, others are structural components such as collagen and keratin, some transport substances (for example hemoglobin carries oxygen), and some are hormones and immune system cells such as antibodies. They also play a role in cell-to-cell signalling.

Chemical structure

Each amino acid has an amino group, a carboxyl group, and a side chain whose chemical properties define the amino acid. The 20 standard amino acids that make up proteins have four levels of molecular organization: primary structure (sequence), secondary structure (such as alpha helix and beta sheet), tertiary structure (the overall folded shape), and quaternary structure when multiple chains combine together.

How it is made

The information for making protein is encoded in DNA. First, DNA is transcribed into messenger RNA (mRNA), which is then read by the ribosome, which links amino acids together according to the genetic code. This process is called translation. After synthesis, the protein may undergo further modifications, such as the addition of sugars or phosphate groups, which affect its function.

Protein in food

Protein is one of the major nutrients in food, found in meat, fish, eggs, milk, beans, corn, lentils, and peas. When eaten, protein is broken down into amino acids in the stomach and intestines, and the body then uses these to build its own proteins. Some amino acids the body can manufacture; others must be obtained from food and are called essential amino acids.