Fragment (e.g. 176-191)

A fragment is an uninterrupted section of a larger parent protein, labelled with the residue positions it covers. The numbering should be taken literally: 176-191 denotes residues 176 to 191 in the parent’s numbering, counted from the N-terminus of the mature protein. HGH Fragment 176-191 is thus a 16-residue segment from the C-terminal region of the 191-residue human growth hormone.

A fragment is not a small parent

The key idea: a fragment does not behave like a shrunken version of its parent. Growth hormone signals via a receptor that must dimerise across a wide contact surface, something a 16-residue segment cannot replicate; published studies of the 176-191 region looked at lipolytic activity ascribed to that particular domain, not at growth-hormone signalling. The same reasoning holds for TB-500, an actin-binding segment of the 43-residue thymosin beta-4 protein, and for IGF-1 DES, a shortened variant of IGF-1.

This has two practical implications. Precursor and mature proteins are numbered differently, so a fragment name remains ambiguous until the reference sequence is given. And findings reported for a parent protein must never be applied to one of its fragments. See HGH Fragment 176-191 explained and why a fragment is not the whole protein.

Related terms

analog · native sequence · linear peptide