Thermodynamics of Fusion Peptide−Membrane Interactions

Abstract

The fusion peptides of viral membrane fusion proteins play a key role in the mechanism of viral spike glycoprotein mediated membrane fusion. These peptides insert into the lipid bilayers of cellular target membranes where they adopt mostly helical secondary structures. To better understand how membranes may be converted to high-energy intermediates during fusion, it is of interest to know how much energy, enthalpy and entropy, is provided by the insertion of fusion peptides into lipid bilayers. Here, we describe a detailed thermodynamic analysis of the binding of analogues of the influenza hemagglutinin fusion peptide of different lengths and amino acid compositions. In small unilamellar vesicles, the interaction of these peptides with lipid bilayers is driven by enthalpy (-16.5 kcal/mol) and opposed by entropy (-30 cal mol(-1) K(-1)). Most of the driving force (deltaG = -7.6 kcal/mol) comes from the enthalpy of peptide insertion deep into the lipid bilayer. Enthalpic gains and entropic losses of peptide folding in the lipid bilayer cancel to a large extent and account for only about 40% of the total binding free energy. The major folding event occurs in the N-terminal segment of the fusion peptide. The C-terminal segment mainly serves to drive the N-terminus deep into the membrane. The fusion-defective mutations G1S, which causes hemifusion, and particularly G1V, which blocks fusion, have major structural and thermodynamic consequences on the insertion of fusion peptides into lipid bilayers. The magnitudes of the enthalpies and entropies of binding of these mutant peptides are reduced, their helix contents are reduced, but their energies of self-association at the membrane surface are increased compared to the wild-type fusion peptide.

Locking the Kink in the Influenza Hemagglutinin Fusion Domain Structure

“…Using the enthalpy and free energy changes, the entropy changes were calculated from ⌬G ϭ ⌬H Ϫ T⌬S ( Table 1). As noted before (12), the fusion domain-lipid interactions are driven by enthalpy and opposed by entropy. The enthalpy change, which includes contributions from folding and lipid insertion, provides the driving force for fusion domain insertion.…”

“…S1C and S1D). Heats of mixing and the contribution of the binding of the hydrophilic solubility tag are subtracted by a control measurement with the tag peptide AcH7 alone (12), resulting in the values of ⌬⌬H in Table 1. WT, F9A, and I10A had similar enthalpy changes on the order of Ϫ15 to Ϫ16 kcal/mol, while the fusion-deficient mutants F9A/I10A and W14A had significantly smaller negative enthalpy changes on the order of Ϫ8 to Ϫ9 kcal/mol.…”

“…At pH 5, the domain adopts a shallow angle boomerang structure, with an ␣-helix in the N-terminal arm and a short 3 10 -helix in the C-terminal arm. The domain docks into lipid bilayers with the N-terminal and C-terminal arms inserting into the hydrophobic core of the bilayer, while the apex of the kink, Asn 12 , is located in the hydrophilic head group region of the bilayer. The structure and lipid interaction of the non-fusogenic mutant, W14A, is very different and its location in the bilayer also differs significantly from that of the wild-type structure (4).…”

“…Four hydrogen bonds are observed in all conformers and are thought to stabilize the structure in the kink region. These are the hydrogen bonds between the amide NHs of Asn 12 Fig. 5), there are subtle differences in the kink region.…”

Folding Amphipathic Helices Into Membranes: Amphiphilicity Trumps Hydrophobicity

“…They reported a value of only −0.14 kcal mol −1 per residue for ΔG residue . Subsequently, Li et al 26 published a value of −0.25(±0.05) kcal mol −1 per residue, using model host-guest fusion peptides. We show here that such differences in ΔG residue can arise in part from differences in μ H .…”