Educational resource
Structural classification
Classification on each compound is a taxonomy term describing covalent topology. Currently the primary split is linear versus cyclic. It does not mean “forms a helix in water” or “is biologically rigid.”
Linear
A linear peptide, in this database, is a chain (or set of chains) whose only repeating covalent backbone links are peptide bonds, with no additional ring-closing covalent bond between residues or termini.
Linear peptides may still carry:
- C-terminal amides or ethylamides
- N-terminal pyroglutamate or acetyl groups
- side-chain lipids (e.g. palmitoylated lysines)
- non-canonical residues
Those features are modifications, not rings. Examples typically filed as linear: glucagon, angiotensin II, and exenatide.
Cyclic
Cyclic means at least one covalent ring is present in the mature structure:
- Disulfide — oxytocin, somatostatin-14, atrial natriuretic peptide (ANP), insulin
- Multiple disulfides — endothelin-1 and many knotted peptides
- Head-to-tail peptide bond — cyclosporine A
- Thioether / carba bridge — carbetocin-type analogues
- Side-chain lactam — designed conformational constraints between Lys and Asp/Glu
Why insulin is cyclic here
Insulin consists of two linear sequences covalently crosslinked by disulfides (plus an intrachain disulfide on the A chain). The architecture is a cystine-linked two-chain structure, so it is filed as cyclic. The sequence field still shows both chains and the length field sums both chains’ residues.
Macrocycles versus “cyclic” labels in literature
Authors sometimes call a peptide “cyclic” only when it is head-to-tail cyclized, and call disulfide peptides “bridged” or “looped.” This database uses a broader topological definition: any covalent ring qualifies as cyclic for filtering. Read display notation and structure notes when you need the specific ring chemistry.
Filtering strategy
On the Compounds page:
- Choose linear or cyclic classification.
- Add modification tags to narrow further (for example cyclic + amidated).
- Optionally constrain molecular weight or length.
Classification is curated. If a record’s ring chemistry is ambiguous in the source, the structure notes should say so — and registry links remain the identity check of last resort.