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Most antimicrobial peptides (AMPs) exert their microbicidal activity through membrane permeabilization. The designed AMP EcDBS1R4 has a cryptic mechanism of action involving the membrane hyperpolarization of Escherichia coli , suggesting that EcDBS1R4 may hinder processes involved in membrane potential dissipation. We show that EcDBS1R4 can sequester cardiolipin, a phospholipid that interacts with several respiratory complexes of E. coli . Among these, F 1 F O ATP synthase uses membrane potential to fuel ATP synthesis. We found that EcDBS1R4 can modulate the activity of ATP synthase upon partition to membranes containing cardiolipin. Molecular dynamics simulations suggest that EcDBS1R4 alters the membrane environment of the transmembrane F O motor, impairing cardiolipin interactions with the cytoplasmic face of the peripheral stalk that binds the catalytic F 1 domain to the F O domain. The proposed mechanism of action, targeting membrane protein function through lipid reorganization may open new venues of research on the mode of action and design of other AMPs., Competing Interests: Authors declare no competing interests., (© 2023 The Authors.)

Septic or endotoxic shock is a common cause of death in hospital intensive care units. In the last decade numerous antimicrobial peptides and proteins have been tested in the search for an efficient drug to treat this lethal disease. Now in phase III clinical trials, rBPI(21), a recombinant N-terminal fragment of the bactericidal/permeability-increasing protein (BPI), is a promising drug to reduce lesions caused by meningococcal sepsis. We correlated structural and stability data with functional information of rBPI(21) bound to both model systems of eukaryotic and bacterial membranes. On interaction with membranes, rBPI(21) loses its conformational stability, as studied by circular dichroism. This interaction of rBPI(21) at membrane level was higher in the presence of negatively charged phospholipid relatively to neutral ones, with higher partition coefficients (K(p)), suggesting a preference for bacterial membranes over mammalian membranes. rBPI(21) binding to membranes is reinforced when its disulfide bond is broken due to conformational changes of the protein. This interaction is followed by liposome aggregation due to unfolding, which ensures protein aggregation, and interfacial localization of rBPI(21) in membranes, as studied by extensive quenching by acrylamide and 5-deoxylstearic acid and not by 16-deoxylstearic acid. An uncommon model of the selectivity and mechanism of action is proposed, where membrane induces unfolding of the antimicrobial protein, rBPI(21). The unfolding ensures protein aggregation, established by protein-protein interaction at membrane surface or between adjacent membranes covered by the unfolded protein. This protein aggregation step may lead to membrane perturbation.

The extracellular hemoglobin of Glossoscolex paulistus (HbGp) is constituted of subunits containing heme groups, monomers and trimers, and nonheme structures, called linkers, and the whole protein has a minimum molecular mass near 3.1 x 10(6) Da. This and other proteins of the same family are useful model systems for developing blood substitutes due to their extracellular nature, large size, and resistance to oxidation. HbGp samples were studied by dynamic light scattering (DLS). In the pH range 6.0-8.0, HbGp is stable and has a monodisperse size distribution with a z-average hydrodynamic diameter (D(h)) of 27 +/- 1 nm. A more alkaline pH induced an irreversible dissociation process, resulting in a smaller D(h) of 10 +/- 1 nm. The decrease in D(h) suggests a complete hemoglobin dissociation. Gel filtration chromatography was used to show unequivocally the oligomeric dissociation observed at alkaline pH. At pH 9.0, the dissociation kinetics is slow, taking a minimum of 24 h to be completed. Dissociation rate constants progressively increase at higher pH, becoming, at pH 10.5, not detectable by DLS. Protein temperature stability was also pH-dependent. Melting curves for HbGp showed oligomeric dissociation and protein denaturation as a function of pH. Dissociation temperatures were lower at higher pH. Kinetic studies were also performed using ultraviolet-visible absorption at the Soret band. Optical absorption monitors the hemoglobin autoxidation while DLS gives information regarding particle size changes in the process of protein dissociation. Absorption was analyzed at different pH values in the range 9.0-9.8 and at two temperatures, 25 degrees C and 38 degrees C. At 25 degrees C, for pH 9.0 and 9.3, the kinetics monitored by ultraviolet-visible absorption presents a monoexponential behavior, whereas for pH 9.6 and 9.8, a biexponential behavior was observed, consistent with heme heterogeneity at more alkaline pH. The kinetics at 38 degrees C is faster than that at 25 degrees C and is biexponential in the whole pH range. DLS dissociation rates are faster than the autoxidation dissociation rates at 25 degrees C. Autoxidation and dissociation processes are intimately related, so that oligomeric protein dissociation promotes the increase of autoxidation rate and vice versa. The effect of dissociation is to change the kinetic character of the autoxidation of hemes from monoexponential to biexponential, whereas the reverse change is not as effective. This work shows that DLS can be used to follow, quantitatively and in real time, the kinetics of changes in the oligomerization of biologic complex supramolecular systems. Such information is relevant for the development of mimetic systems to be used as blood substitutes.