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 examined the interfacial behavior of Vipoxin and its isolated subunits under barostatic conditions. ΔA(t) measurements revealed pronounced pressure- and pH-dependent modulation of activity. VBC maintained high catalytic competence, whereas Vipoxin showed enhanced pH sensitivity and 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.