Role of membrane lipids in peptide hormone function: binding of enkephalins to micelles

In the course of their biological function, peptide hormones must be transferred from an aqueous phase to the lipid-rich environment of their membrane-bound receptor proteins. We have investigated the possible influence of phospholipids in this process, using 360-MHz 1H and 90-MHz 13C NMR spectroscopy to examine the association of the opioid peptides [Met]- and [Leu]enkephalins (Tyr-Gly-Gly-Phe-Met/Leu) with phospholipid micelles. Binding of peptides to lipid was monitored in NMR spectra by selective chemical shift movements (e.g., the Phe aromatic ring protons) and residue-specific line broadening (e.g., of Met/Leu carbonyl- and alpha-carbon resonances). Results established that the zwitterionic hormones associate hydrophobically both with a neutral lipid (lysophosphatidylcholine) and (also electrostatically) with a negative lipid (lysophosphatidylglycerol). An association constant of Ka = 3.7 X 10(1) M-1 was calculated for the hydrophobic binding of enkephalin to lysophosphatidylcholine. NMR data suggested that enkephalin binds to the lipid with Met/Leu, Phe, and likely Tyr side-chain substituents associated with nonpolar interior regions of the micelle, whereas the COOH-terminal carboxylate moiety of the peptide is located in the surface of the lipid particle. An "attraction-interaction" model is proposed for hormone-lipid association wherein negative lipids attract the hormone electrostatically, while site-specific hydrophobic contacts facilitate its entry, concentration, and orientation into the lipid phase.