Editing N-terminus (section)

{{short description|Start of a polypeptide}}
{{Redirect|N-term|the divergence test|Term test}}
{{refimprove|date=January 2017}}
<!-- Please expand this article if you have any more info -->
[[Image:Tetrapeptide structural formulae v.1.png|thumb|350px|A [[tetrapeptide]] (example: [[Valine|Val]]-[[Glycine|Gly]]-[[Serine|Ser]]-[[Alanine|Ala]]) with <span style="color:green;" title="Valine">'''green'''</span> highlighted N-terminal α-amino acid (example: L-[[valine]]) and <span style="color:blue;" title="Alanine">'''blue'''</span> marked C-terminal α-amino acid (example: L-[[alanine]]). This tetrapeptide could be encoded by the mRNA sequence 5'-[[Valine|GUU]] [[Glycine|GGU]] [[Serine|AGU]] [[Alanine|GCU]]-3'.]]
The '''N-terminus''' (also known as the '''amino-terminus''', '''NH<sub>2</sub>-terminus''', '''N-terminal end''' or '''amine-terminus''') is the start of a [[protein]] or [[polypeptide]], referring to the free [[amine]] group (-NH<sub>2</sub>) located at the end of a polypeptide. Within a peptide, the amine group is bonded to the [[carboxylic acid|carboxylic]] group of another amino acid, making it a chain.  That leaves a free carboxylic group at one end of the peptide, called the [[C-terminus]], and a free amine group on the other end called the N-terminus. By convention, peptide sequences are written N-terminus to C-terminus, left to right (in [[Writing system#Directionality|LTR writing systems]]).<ref>{{Cite web|url=https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/protein2.htm|title=Peptides & Proteins|last=Reusch|first=William|date=5 May 2013|website=Michigan State University Department of Chemistry}}</ref> This correlates the [[translation (biology)|translation]] direction to the text direction, because when a protein is translated from [[messenger RNA]], it is created from the N-terminus to the C-terminus, as amino acids are added to the carboxyl end of the protein.{{cn|date=July 2024}}