A Pseudoenzymatic Regulatory Role of the Noncatalytic Subunit of the Neurotoxin Vipoxin at Arachidonic-Acid-Containing Membrane Interfaces

Secreted phospholipases A2 (sPLA2s) act at lipid interfaces where enzymatic turnover is strongly influenced by membrane packing and interfacial physicochemical conditions. Vipoxin, a heterodimeric neurotoxin from Vipera ammodytes meridionalis, is one such complex, comprising a catalytically active sPLA2 subunit (VBC) and a catalytically impaired homolog, VAC, suggesting a pseudoenzymatic regulatory role. Using SAPC Langmuir monolayers as a model of arachidonic-acid-containing membranes, we monitored the compensated monolayer area change, ΔA(t), under barostatic conditions as an integrated readout of the interfacial behavior of Vipoxin and its isolated subunits. The responses revealed pronounced modulation by surface pressure and by the acidic acetate versus basic Tris-HCl subphase environment: VBC retained high catalytic competence under both conditions, whereas Vipoxin displayed greater environmental sensitivity, consistent with VAC-dependent modulation of enzyme–membrane coupling. VAC, although lacking canonical catalytic activity, produced measurable interfacial effects under acidic conditions and high lateral pressure. Analysis using the interfacial quality parameter Qm demonstrated that VAC modifies the pressure dependence of the heterodimer and stabilizes interfacial accommodation of VBC. These findings indicate that VAC functions as a pseudoenzymatic regulatory subunit whose role emerges from dynamic coupling between enzymatic activity and lipid interfacial organization.