Practice question

Geometrical constraints of fluid phospholipid bilayer dictate minimal span length for alpha-helical membrane proteins. Hydrophobic thickness between carbonyl regions of liquid-crystalline phosphatidylcholine bilayer averages three point zero nanometers. An alpha helix advances zero point fifteen nanometers per residue along axis with three point six residues per turn, pitch zero point five four nanometers stabilized by intramolecular hydrogen bonds between carbonyl i and amide i plus four. Dividing three nanometers by zero point fifteen per residue yields about twenty residues required for perpendicular crossing without exposure of polar backbone. Surveys of high-resolution structures and hydropathy analyses reveal hydrophobic stretches of nineteen to twenty three residues predominantly leucine, isoleucine, valine and phenylalanine flanked by interfacial aromatic belt tryptophan tyrosine at glycerol region and positively charged lysine arginine following positive-inside rule determining orientation. Glycophorin A helix of nineteen residues exemplifies minimal length with dimerization via GXXXG motif. Longer helices tilt up to thirty degrees accommodating mismatch or contain proline kinks influencing channel gating and receptor activation.