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( N-terminus)
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) refers to the end of a protein or polypeptide terminated by an amino acid with a free amine group (-NH2). The convention for writing peptide sequences is to put the N-terminus on the left and write the sequence from N- to C-terminus. When the protein is translated from messenger RNA, it is created from N-terminus to C-terminus. Each amino acid has a carboxyl group and an amine group, and amino acids link to one another to form a chain by a dehydration reaction by joining the amine group of one amino acid to the carboxyl group of the next. Thus polypeptide chains have an end with an unbound carboxyl group, the C-terminus, and an end with an amine group, the N-terminus. When the 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 or nascent chain. Since the start codon of the genetic code codes for the amino acid methionine, most protein sequences start with a methionine (more specifically 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. The N-terminus is the first part of the protein that exits the ribosome during protein biosynthesis. It often contains sequences that act as targeting signals, basically intracellular zip codes, that allow for the protein to be delivered to its designated location within the cell. The targeting signal is usually cleaved off after successful targeting by a processing peptidase
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