A representation o the 3D structur o the proteinmyoglobin shawin turquoiseα-helices. This protein wis the first tae hae its structur solved biX-ray crystallografie. Taewart the richt-centre amang the buchts, aprosthetic group cried aheme group (shawn in gray) wi a boond oxygen molecule (reid).
A linear cheen o amino acid residues is cried apolypeptide. 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 criedpeptides, or whilesoligopeptides. The individual amino acid residues are bondit thegither bipeptide bonds an adjacent amino acid residues. Thesequence o amino acid residues in a protein is defined bi thesequence o agene, that is encodit in thegenetic code. In general, the genetic code specifees 20 staundart amino acids; houiver, in certaint organisms the genetic code can includeselenocysteine an—in certaintarchaea—pyrrolysine. Shortly efter or even in synthesis, the residues in a protein are eften chemically modifee'd bipost-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 criedprosthetic groups orcofactors. Proteins can an aa wirk thegither tae achieve a pairteecular function, an thay eften associate tae form stableprotein complexes.
Ance formed, proteins anerly exeest for a certaint period o time an are thendegradit an recycled bi the cell's machinery throu the process oprotein turnower. A protein's lifespan is meisurt in terms o itshauf-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.
