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TheN-terminus (also known as theamino-terminus,NH2-terminus,N-terminal end oramine-terminus) is the start of aprotein orpolypeptide, referring to the freeamine group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to thecarboxylic group of another amino acid, making it a chain. That leaves a free carboxylic group at one end of the peptide, called theC-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 (inLTR writing systems).[1] This correlates thetranslation direction to the text direction, because when a protein is translated frommessenger RNA, it is created from the N-terminus to the C-terminus, as amino acids are added to the carboxyl end of the protein.[citation needed]
Each amino acid has anamine group and acarboxylic group. Amino acids link to one another bypeptide bonds which form through adehydration reaction that joins the carboxyl group of one amino acid to theamine group of the next in a head-to-tail manner to form apolypeptide chain. The chain has two ends – an amine group, the N-terminus, and an unbound carboxyl group, theC-terminus.[2]
When a protein istranslated frommessenger RNA, it is created from N-terminus to C-terminus. The amino end of an amino acid (on a chargedtRNA) during the elongation stage of translation, attaches to the carboxyl end of the growing chain. Since thestart codon of thegenetic code codes for the amino acidmethionine, most protein sequences start with amethionine (or, in bacteria,mitochondria andchloroplasts, the modified versionN-formylmethionine, fMet). However, some proteins are modifiedposttranslationally, for example, by cleavage from aprotein precursor, and therefore may have different amino acids at their N-terminus.
The N-terminus is the first part of the protein that exits theribosome duringprotein biosynthesis. It often containssignal peptide sequences, "intracellularpostal codes" that direct delivery of the protein to the properorganelle. The signal peptide is typically removed at the destination by a signalpeptidase. The N-terminal amino acid of a protein is an important determinant of its half-life (likelihood of being degraded). This is called theN-end rule.
The N-terminal signal peptide is recognized by thesignal recognition particle (SRP) and results in the targeting of the protein to thesecretory pathway. Ineukaryotic cells, these proteins are synthesized at the roughendoplasmic reticulum. Inprokaryotic cells, the proteins are exported across thecell membrane. Inchloroplasts, signal peptides target proteins to thethylakoids.
The N-terminal mitochondrialtargeting peptide (mtTP) allows the protein to be imported into themitochondrion.
The N-terminal chloroplast targeting peptide (cpTP) allows for the protein to be imported into thechloroplast.
Protein N-termini can be modified co - or post-translationally. Modifications include the removal of initiator methionine (iMet) byaminopeptidases, attachment of small chemical groups such asacetyl,propionyl andmethyl, and the addition of membrane anchors, such aspalmitoyl andmyristoyl groups[3]
N-terminal acetylation is a form of protein modification that can occur in bothprokaryotes andeukaryotes. It has been suggested that N-terminal acetylation can prevent a protein from following asecretory pathway.[4]
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.
The N-terminus can also be modified by the addition of afatty acid anchor to form N-acetylated proteins. The most common form of such modification is the addition of a palmitoyl group.