N terminus

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. Normally the amine group is bonded to another carboxylic group in a protein to make it a chain, but since the end of a protein has only 1 out of 2 areas chained, the free amine group is referred to the N-terminus. By convention, peptide sequences are written N-terminus to C-terminus, left to right in LTR languages. 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 carbonyl end).

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.

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.

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