Practice question

Membrane protein extraction and purification require detergents amphipathic molecules that solubilize lipid bilayer while ideally preserving native conformation, oligomeric state and enzymatic activity for functional studies. Non-ionic detergents such as Triton X-100 octylphenoxy polyoxyethylene, n-dodecyl-beta-D-maltoside DDM, digitonin steroidal glycoside and n-octyl-beta-D-glucoside possess uncharged hydrophilic head groups composed of polyoxyethylene or sugar moieties and hydrophobic alkyl tail. They insert into bilayer at low critical micellar concentration, fragment it into mixed lipid-detergent micelles, and surround transmembrane hydrophobic segments with gentle micellar environment, shielding from water without disrupting intra-protein electrostatic interactions and hydrogen bonds. Consequently proteins remain folded, often active, retaining ligand binding, ion channel gating and transporter turnover, enabling co-immunoprecipitation, activity assays, reconstitution into proteoliposomes and structural studies via X-ray crystallography and cryo-electron microscopy. In contrast ionic detergents like sodium dodecyl sulfate SDS carry charged sulfate headgroup that binds cooperatively along polypeptide backbone at ratio about one per two residues, imparting large negative charge and unfolding secondary structures into rod-like micelles, denaturing proteins for SDS-PAGE but permanently losing function. Hence non-ionic agents are preferred for purification of functional membrane proteins, while ionic used analytically.