Practice question

Thermodynamic and kinetic aspects of alpha-helical insertion illuminate hydrophobic driving forces. Nascent chain emerges from ribosome tunnel, signal recognition particle SRP binds hydrophobic signal anchor, delivers ribosome-nascent complex to SRP receptor at endoplasmic reticulum membrane, GTP hydrolysis releases chain to Sec61 translocon aqueous channel laterally gated by plug displacement. Partitioning of hydrophobic segment into lipid phase determined by apparent free energy Delta G apparent measured via reporter Lep assays where insertion efficiency correlates with hydrophobicity scale: leucine contributes minus zero point six, isoleucine minus zero point five, arginine plus two kilocalories reflecting cost dehydrating charged guanidinium. Releasing ordered clathrate water cages around nonpolar side chains into bulk increases entropy favoring spontaneous insertion without ATP hydrolysis, while translocon lowers barrier preventing aggregation. Interfacial aromatic residues tryptophan tyrosine anchor via partitioning and hydrogen bonding, cytosolic lysine arginine enforce orientation via positive-inside rule. Hence hydrophobic forces dominate embedding, van der Waals contacts refine packing, ionic interactions stabilize extramembranous loops and headgroup contacts.