Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Shortly after or even during synthesis, the residues in a protein are often chemically modified by posttranslational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins. In general, the genetic code specifies 20 standard amino acids however, in certain organisms the genetic code can include selenocysteine-and in certain archaea-pyrrolysine. The sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form in a biologically functional way.
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