: 220
Four levels build upon each other: Primary = amino acid sequence (determined by gene, held by peptide bonds). One amino acid change can cause disease (sickle cell: Glu→Val). Secondary = local folding patterns held by backbone H-bonds. Alpha-helix: right-handed coil, H-bonds between C=O(i) and N-H(i+4), 3.6 residues per turn, R-groups point outward. Beta-sheet: extended chains with H-bonds between adjacent strands (parallel or antiparallel). Proline disrupts alpha-helix (rigid ring constrains phi angle). Tertiary = overall 3D fold of single polypeptide, stabilized by: hydrophobic interactions (dominant driving force — non-polar side chains cluster in protein interior), disulfide bonds (Cys-S-S-Cys, covalent), ionic bonds (Asp COO-...NH3+ Lys), hydrogen bonds between side chains. Quaternary = assembly of multiple subunits (only in multi-chain proteins). Hemoglobin: 2alpha + 2beta subunits, each with a heme group carrying O2. Denaturation disrupts 2°/3°/4° structure while 1° (peptide bonds) remains intact. Caused by: heat, pH extremes, urea, heavy metals, organic solvents. Results in loss of biological activity. Anfinsen's ribonuclease experiment proved that primary structure alone determines the 3D fold (renaturation upon removing denaturant).