Proteins are lairge biological molecules, or macromolecules, consistin o ane or mair chains o amino acid residues. Proteins perform a vast array o functions athin organisms, includin catalysin metabolic reactions, DNA replication, respondin tae stimuli, an transportin molecules frae ane location tae anither. Proteins differ frae ane anither primarily in thair sequence o amino acids, that is dictatit bi the nucleotide sequence o thair genes, an that uisually results in protein foldin intae a speceefic three-dimensional structur that determines its acteevity.
A linear cheen o amino acid residues is cried a polypeptide. A protein conteens at least ane lang polypeptide. Short polypeptides, conteenin less nor 20–30 residues, are rarely conseedert tae be proteins an are commonly cried peptides, or whiles oligopeptides. The individual amino acid residues are bondit thegither bi peptide bonds an adjacent amino acid residues. The sequence o amino acid residues in a protein is defined bi the sequence o a gene, that is encodit in the genetic code. In general, the genetic code specifees 20 staundart amino acids; houiver, in certaint organisms the genetic code can include selenocysteine an—in certaint archaea—pyrrolysine. Shortly efter or even in synthesis, the residues in a protein are eften chemically modifee'd bi post-translational modification, that alters the pheesical an chemical properties, fauldin, stability, activity, an ultimately, the function o the proteins. Whiles proteins hae non-peptide groups attached, that can be cried prosthetic groups or cofactors. Proteins can an aa wirk thegither tae achieve a pairteecular function, an thay eften associate tae form stable protein complexes.
Ance formed, proteins anerly exeest for a certaint period o time an are then degradit an recycled bi the cell's machinery throu the process o protein turnower. A protein's lifespan is meisurt in terms o its hauf-life an covers a wide range. Thay can exist for minutes or years wi an average lifespan o 1–2 days in mammalian cells. Abnormal or misfauldit proteins are degradit mair rapidly aither due tae bein targetit for destruction or due tae bein unstable.
Lik ither biological macromolecules sic as polysaccharides an nucleic acids, proteins are essential pairts o organisms an pairteecipate in virtually ivery process within cells. Mony proteins are enzymes that catalyse biochemical reactions an are vital tae metabolism. Proteins an aa hae structural or mechanical functions, sic as actin an myosin in muscle an the proteins in the cytoskeleton, that form a system o scaffauldin that maintains cell shape. Ither proteins are important in cell signaling, immune responses, cell adhesion, an the cell cycle. In ainimals, proteins are needed in the diet to provide the essential amino acids that canna be synthesised. Digestion braks the proteins doun for uise in the metabolism.
Proteins mey be purified frae ither cellular components uisin a variety of techniques sic as ultracentrifugation, precipitation, electrophoresis, an chromatography; the advent o genetic ingineerin haes made possible a nummer o methods tae facilitate purification. Methods commonly uised tae study protein structur an function include immunohistochemistry, site-directed mutagenesis, X-ray crystallografie, nuclear magnetic resonance an mass spectrometry.