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N-terminus-Wikipedia

N-terminus

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Start of a polypeptide

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

protein

or

polypeptide

referring to the free

amine

group (-NH2) located at the end of a polypeptide. Within a peptide, the amine group is bonded to another

carboxylic

group in a protein to make it a chain, but since the end amino acid of a protein is only connected at the carboxy- end, the remaining free amine group is 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 N-terminus to C-terminus – amino acids are added to the carboxyl end).

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Chemistry[

edit

]

Each amino acid has an

amine

group and a

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

translated

from

messenger RNA

, it is created from N-terminus to C-terminus. The amino end of an amino acid (on a charged

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

chloroplasts

, the modified version

N-formylmethionine

, fMet). However, some proteins are modified

posttranslationally

, for example, by cleavage from a

protein precursor

, and therefore may have different amino acids at their N-terminus.

Function[

edit

]

N-terminal targeting signals[

edit

]

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 codes

” 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[

edit

]

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

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[

edit

]

The N-terminal mitochondrial

targeting peptide

(mtTP) allows the protein to be imported into the

mitochondrion

.

Chloroplast targeting peptide[

edit

]

The N-terminal chloroplast targeting peptide (cpTP) allows for the protein to be imported into the

chloroplast

.

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N-terminal modifications[

edit

]

Protein N-termini can be modified co – or posttranslationally. Modifications include the removal of initiator methionine (iMet) by

aminopeptidases

, attachment of small chemical groups such as

acetyl

, propionyl and

methyl

, and the addition of membrane anchors, such as palmitoyl and myristoyl groups

[3]

N-terminal acetylation

N-terminal acetylation

is a form of protein modification that can occur in both

prokaryotes

and

eukaryotes

. It has been suggested that N-terminal acetylation can prevent a protein from following a secretory pathway.

[4]

N-Myristoylation[

edit

]

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[

edit

]

The N-terminus can also be modified by the addition of a

fatty acid

anchor to form N-acylated proteins. The most common form of such modification is the addition of a

palmitoyl

group.

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See also[

edit

]

  • C-terminus

  • TopFIND

    , a scientific database covering

    proteases

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

References[

edit

]

  1. ^

    Reusch, William (5 May 2013).

    “Peptides & Proteins”

    . Michigan State University Department of Chemistry.

  2. ^

    Voet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2013). Fundamentals of Biochemistry: Life at the Molecular Level (4th ed.). Hoboken, NJ: Wiley.

    ISBN

     

    978-0470547847

    .

  3. ^

    Varland (April 21, 2015).

    “N-terminal modifications of cellular proteins:The enzymes involved, their substrate specificities and biological effects”

    . Proteomics. 15 (14): 2385–401.

    doi

    :

    10.1002/pmic.201400619

    .

    PMC

     

    4692089

    .

    PMID

     

    25914051

    .

  4. ^

    Arnesen, Thomas (May 31, 2011).

    “Towards a Functional Understanding of Protein N-Terminal Acetylation”

    . PLOS Biology. 9 (5): e1001074.

    doi

    :

    10.1371/journal.pbio.1001074

    .

    PMC

     

    3104970

    .

    PMID

     

    21655309

    .

Retrieved from “

https://en.wikipedia.org/w/index.php?title=N-terminus&oldid=1004779085

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