Editing Protein structure (section)

===Primary structure===
{{Main|Protein primary structure}}
The [[primary structure]] of a protein refers to the sequence of [[amino acid]]s in the polypeptide chain. The primary structure is held together by [[peptide bonds]] that are made during the process of [[protein biosynthesis]]. The two ends of the [[polypeptide chain]] are referred to as the [[carboxyl terminus]] (C-terminus) and the [[amino terminus]] (N-terminus) based on the nature of the free group on each extremity. Counting of residues always starts at the N-terminal end (NH<sub>2</sub>-group), which is the end where the amino group is not involved in a peptide bond. The primary structure of a protein is determined by the [[gene]] corresponding to the protein. A specific sequence of [[nucleotide]]s in [[DNA]] is [[transcription (genetics)|transcribed]] into [[mRNA]], which is read by the [[ribosome]] in a process called [[translation (biology)|translation]]. The sequence of amino acids in insulin was discovered by [[Frederick Sanger]], establishing that proteins have defining amino acid sequences.<ref>{{cite journal | vauthors = Sanger F, Tuppy H | title = The amino-acid sequence in the phenylalanyl chain of insulin. I. The identification of lower peptides from partial hydrolysates | journal = The Biochemical Journal | volume = 49 | issue = 4 | pages = 463–481 | date = September 1951 | pmid = 14886310 | pmc = 1197535 | doi = 10.1042/bj0490463 }}</ref><ref>{{cite journal | vauthors = Sanger F | title = Chemistry of insulin; determination of the structure of insulin opens the way to greater understanding of life processes | journal = Science | volume = 129 | issue = 3359 | pages = 1340–1344 | date = May 1959 | pmid = 13658959 | doi = 10.1126/science.129.3359.1340 | bibcode = 1959Sci...129.1340G }}</ref> The sequence of a protein is unique to that protein, and defines the structure and function of the protein. The sequence of a protein can be determined by methods such as [[Edman degradation]] or [[Mass spectrometry#Protein identification|tandem mass spectrometry]]. Often, however, it is read directly from the sequence of the gene using the [[genetic code]]. It is strictly recommended to use the words "amino acid residues" when discussing proteins because when a peptide bond is formed, a [[water molecule]] is lost, and therefore proteins are made up of amino acid residues. [[Post-translational modification]]s such as [[phosphorylation]]s and [[glycosylation]]s are usually also considered a part of the primary structure, and cannot be read from the gene. For example, [[insulin]] is composed of 51 amino acids in 2 chains. One chain has 31 amino acids, and the other has 20 amino acids.