Editing N-terminus

{{short description|Start of a polypeptide}}
{{Redirect|N-term|the divergence test|Term test}}
{{refimprove|date=January 2017}}
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[[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}}

==Chemistry==
Each amino acid has an [[amine]] group and a [[carboxylic group]]. Amino acids link to one another by [[peptide bond]]s which form through a [[dehydration reaction]] that joins the carboxyl group of one amino acid to the [[amine]] group of the next in a head-to-tail manner to form a [[Peptide|polypeptide]] chain. The chain has two ends – an amine group, the N-terminus, and an unbound carboxyl group, the [[C-terminus]].<ref name="Voet, Voet, Pratt">{{cite book|last1=Voet|first1=Donald|last2=Voet|first2=Judith G.|last3=Pratt|first3=Charlotte W.|title=Fundamentals of Biochemistry: Life at the Molecular Level|date=2013|publisher=Wiley|location=Hoboken, NJ|isbn=978-0470547847|edition=4th}}</ref>

When a protein is [[translation (biology)|translated]] from [[messenger RNA]], it is created from N-terminus to C-terminus. The amino end of an amino acid (on a charged [[transfer RNA|tRNA]]) during the elongation stage of translation, attaches to the carboxyl end of the growing chain. Since the [[start codon]] of the [[genetic code]] codes for the amino acid [[methionine]], most protein sequences start with a [[methionine]] (or, in bacteria, [[mitochondria]] and [[chloroplast]]s, the modified version [[N-Formylmethionine|''N''-formylmethionine]], fMet). However, some proteins are modified [[posttranslational modification|posttranslationally]], for example, by cleavage from a [[protein precursor]], and therefore may have different amino acids at their N-terminus.

==Function==

===N-terminal targeting signals===
The N-terminus is the first part of the protein that exits the [[ribosome]] during [[protein biosynthesis]]. It often contains [[signal peptide]] sequences, "intracellular [[postal code]]s" that direct delivery of the protein to the proper [[organelle]]. The signal peptide is typically removed at the destination by a signal [[peptidase]]. The N-terminal amino acid of a protein is an important determinant of its half-life (likelihood of being degraded). This is called the [[N-end rule]].

====Signal peptide====
{{main article|Signal peptide}}
The N-terminal signal peptide is recognized by the [[signal recognition particle]] (SRP) and results in the targeting of the protein to the [[secretory pathway]]. In [[eukaryote|eukaryotic cells]], these proteins are synthesized at the rough [[endoplasmic reticulum]]. In [[prokaryote|prokaryotic cells]], the proteins are exported across the [[cell membrane]]. In [[chloroplast]]s, signal peptides target proteins to the [[thylakoid]]s.

====Mitochondrial targeting peptide====
The N-terminal mitochondrial [[target peptide|targeting peptide]] (mtTP) allows the protein to be imported into the [[mitochondrion]].

====Chloroplast targeting peptide====
The N-terminal chloroplast targeting peptide (cpTP) allows for the protein to be imported into the [[chloroplast]].

==N-terminal modifications==
Protein N-termini can be modified co - or post-translationally. Modifications include the removal of initiator methionine (iMet) by [[aminopeptidase]]s, attachment of small chemical groups such as [[Acetyl group|acetyl]], [[propionyl]] and [[Methyl group|methyl]], and the addition of membrane anchors, such as [[Palmitoyl group|palmitoyl]] and [[myristoyl group]]s<ref>{{Cite journal|title = N-terminal modifications of cellular proteins:The enzymes involved, their substrate specificities and biological effects|last = Varland|date = April 21, 2015|journal = Proteomics|doi = 10.1002/pmic.201400619|pmid = 25914051 |volume=15|issue = 14|pmc=4692089|pages=2385–401}}</ref>

===N-terminal acetylation===
{{main article|N-terminal acetylation}}
[[N-terminal acetylation]] is a form of protein modification that can occur in both [[prokaryote]]s and [[eukaryote]]s. It has been suggested that N-terminal acetylation can prevent a protein from following a [[secretory pathway]].<ref>{{Cite journal|title = Towards a Functional Understanding of Protein N-Terminal Acetylation|last = Arnesen|first = Thomas|date = May 31, 2011|journal = PLOS Biology|doi = 10.1371/journal.pbio.1001074|pmid = 21655309|volume=9|issue = 5|pages=e1001074|pmc=3104970 | doi-access=free }}</ref>

===N-Myristoylation===

{{main article|Myristoylation}}
The N-terminus can be modified by the addition of a myristoyl anchor. Proteins that are modified this way contain a consensus motif at their N-terminus as a modification signal.

===N-Acylation===

{{main article|Palmitoylation}}
The N-terminus can also be modified by the addition of a [[fatty acid]] anchor to form N-acetylated proteins. The most common form of such modification is the addition of a palmitoyl group.

==See also==
* [[C-terminus]]
* [[TopFIND]], a scientific database covering [[protease]]s, their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity

==References==
{{Reflist}}

[[Category:Post-translational modification]]
[[Category:Proteins]]
[[Category:Protein structure]]