Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Protein performs a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, which are differentiated by the nucleotide sequence of their genes.
A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides containing less than 20 to 30 residues are rarely considered proteins and are generally referred to as peptides or sometimes as oligopeptides. The individual amino acid residues are linked together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene encoded in the genetic code. In general, the genetic code states 20 standard amino acids; However, in certain organisms, the genetic code may contain selenocysteine and, in certain archaea, pyrrolysine. Soon after or even during synthesis, residues in a protein are often chemically modified by post-translational modifications, altering the physical and chemical properties, folding, stability, activity, and ultimately the function of proteins. Sometimes proteins have bound non-peptide groups, which may be termed prosthetic groups or cofactors. Proteins can also work together to achieve a particular function, and they often combine to form stable protein complexes.
