N-terminus

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A tetrapeptide (example: Val-Gly-Ser-Ala) with green highlighted N-terminal α-amino acid (example: L-valine) and blue marked C-terminal α-amino acid (example: L-alanine). This tetrapeptide could be encoded by the mRNA sequence 5'-GUU GGU AGU GCU-3'.

The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a

polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to the 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 LTR writing systems).[1] This correlates the 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.

Chemistry

Each amino acid has an

carboxylic group. Amino acids link to one another by peptide bonds 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 polypeptide chain. The chain has two ends – an amine group, the N-terminus, and an unbound carboxyl group, the C-terminus.[2]

When a protein is

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

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

secretory pathway. In eukaryotic cells, these proteins are synthesized at the rough endoplasmic reticulum. In prokaryotic cells, the proteins are exported across the cell membrane. In chloroplasts, signal peptides target proteins to the thylakoids
.

Mitochondrial targeting peptide

The N-terminal mitochondrial 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

myristoyl groups[3]

N-terminal acetylation

Main article:
N-terminal acetylation

secretory pathway.[4]

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 proteases, their cleavage site specificity, substrates, inhibitors and protein termini originating from their activity

References

  1. ^ Reusch, William (5 May 2013). "Peptides & Proteins". Michigan State University Department of Chemistry.
  2. ISBN 978-0470547847
    .
  3. PMID 25914051
    .
  4. PMID 21655309
    .
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