Secondary & Tertiary Structure
Also known as: primary structure · secondary structure · tertiary structure · quaternary structure · protein folding
The levels of peptide/protein organisation: sequence (primary), local folds (secondary), 3-D shape (tertiary), assembly (quaternary).
A peptide or protein is described at several structural levels. Primary structure is the amino-acid sequence itself. Secondary structure is local, repeating folding — alpha-helices and beta-sheets — held by hydrogen bonds. Tertiary structure is the overall three-dimensional shape of one chain. Quaternary structure is how multiple chains assemble.
Short peptides may have little fixed secondary or tertiary structure, but even a small fold can be essential to activity. Shape follows directly from sequence, which is why a single substituted residue can change how a molecule works.
RELATED TERMS
Amino-Acid Sequence
The specific order of amino-acid residues in a peptide — its "primary structure" — which dictates shape and activity.
Peptide Bond
The covalent amide bond linking amino acids into a peptide chain — the defining structural feature of every peptide.
Disulfide Bond
A covalent bridge between two cysteine side chains that locks a peptide’s three-dimensional shape in place.
Protein
A large biological molecule of one or more long amino-acid chains; peptides are the same chemistry at shorter length.
Side Chain (R Group)
The variable "R group" on each amino acid that gives it its chemical character — charge, size and water-affinity.
Educational content, not medical advice. All products supplied for research purposes only — not for human consumption.