Protein Structure

Proteins fold in four levels. Primary is the amino acid sequence joined by peptide bonds. Secondary is local backbone folding into alpha-helices and beta-sheets, held by hydrogen bonds. Tertiary is the full 3D shape of one chain, driven mainly by the hydrophobic effect along with disulfides, salt bridges, and hydrogen bonds. Quaternary is the assembly of several subunits, as in hemoglobin.

Three modes: Which level (place a feature into 1, 2, 3, or 4), Which interaction (name the bond or force from a description), and Concepts (peptide bond, alpha-helix, beta-sheet, denaturation, and more).

Denaturation unfolds the higher levels (secondary, tertiary, quaternary) but leaves the primary sequence intact.