= Peptide sequencing
<Peptide sequencing> determines the order of <amino acid residues> in a <peptide>, conventionally read from its <N-terminus> toward its <C-terminus>. This is more information than <amino acid> composition: AGST and TSAG contain the same four residues but put different neighbours together.
Overlapping fragments make the distinction concrete. If two oriented fragments from one chain are AGS and GST, their shared GS segment supports the combined sequence AGST. This is a toy reconstruction, assuming the fragments overlap and repeated segments do not create ambiguity. A composition measurement alone cannot supply that order.
Sequencing can also locate a chemically identified site inside a <protein>. In a 1990 <1-aminocyclopropane-1-carboxylate synthase> experiment, a labelled active-site segment was isolated after <protein> digestion. Sequence analysis placed a <lysine> within it, while chemical analysis linked that residue to <pyridoxal phosphate>. The combination connected an ordered segment to a chemical function rather than merely listing the sample's <amino acids>.<Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase by W. K. Yip and others>[PNAS87, pp.7930–7934; Abstract, active-site peptide isolation and sequencing]{ref}
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