Protein

A protein is a large biological molecule built from amino acids joined by peptide bonds into long chains called polypeptides. Twenty kinds of amino acid are used, and the order in which they appear, set by the DNA sequence of a gene, makes each protein different. Proteins carry out most of a cell's work and take on a wider range of jobs than any other kind of biological molecule.

Each chain folds into a precise three-dimensional shape, and that shape decides what the protein can do. Biochemists describe four levels of structure: the primary sequence of amino acids, local secondary folds such as alpha helices and beta sheets, the overall tertiary shape, and, in proteins with several chains, the quaternary arrangement of those subunits. Heat, extreme pH or harsh chemicals can unfold, or denature, a protein and destroy its function, as happens to egg white when it is cooked.

Examples include enzymes such as amylase, transport proteins such as hemoglobin, hormones such as insulin, structural proteins such as keratin and actin, and antibodies that fight infection. A single amino acid change in hemoglobin causes sickle cell disease.