Protein structure levels
MCAT glossary · Biological & Biochemical Foundations of Living Systems
Definition
Primary (sequence), secondary (α-helix, β-sheet via backbone H-bonds), tertiary (3D fold via side-chain interactions), quaternary (subunit assembly).
Protein structure levels in plain English
Proteins are the tiny machines that do most jobs in your body, and they get their powers from how they fold into specific 3D shapes — describable at four zoom levels. 'Primary' is just the bead order of the chain (the exact sequence of amino acids). 'Secondary' is local crimping into neat repeating patterns — coils (α-helices) or pleats (β-sheets) — held by weak hydrogen bonds along the backbone. 'Tertiary' is how that whole chain balls up into one full 3D shape via its side chains, and 'quaternary' is when several finished chains ('subunits') clip together into a bigger team, like hemoglobin's four pieces.
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