Reduced pH induces an inactive non-native conformation of the monomeric bothropstoxin-I (Lys49-PLA2)

Venomous Peptides: Molecular Origin of the Toxicity of Snake Venom PLA2-like Peptides

Snakebite envenoming claims 81-138 thousand lives annually, with vipers responsible for many of those. Phospholipase A2 (PLA2) enzymes and PLA2-like proteins are among the most important viper venom toxins. The latter are particularly intriguing, as three decades after their discovery, their molecular mechanism of toxicity is still poorly understood at best. PLA2-like proteins destabilize eukaryotic cell membranes through an unknown mechanism, causing an uncontrolled influx of Ca2+ ions and ultimately triggering cell death. It is now clear that the C-terminal segment is fundamental to the toxicity, as 13-mer peptides with the same sequence exhibit most or all of the activities of the complete PLA2-like proteins. To finally clarify the mechanism of toxicity of these venom peptides, we have simulated their interaction with model cell membranes. Molecular dynamics simulations showed that peptides initially dispersed across the cell membrane quickly and spontaneously migrated, aggregated, induced membrane thinning, and formed clear and transient membrane pores. We calculated the potentials of the mean force for Ca2+ transfer across the cell membranes through the transient pores. The pores significantly lower the free energy barrier for Ca2+ translocation, an effect that grows with the size of the peptide aggregates and, thus, with the pore radius. Ca2+ flowed across the membrane through the largest pores with almost no barrier. The permeability of Ca2+ through the largest pores exceeded the permeability of pharmaceutical drugs by 4 orders of magnitude, revealing the easiness by which Ca2+ overflows the intracellular medium. These results elucidate the illusive molecular origin of the toxicity of this famous class of snake venom-derived peptides.

BthTX-I, a phospholipase A2-like toxin, is inhibited by the plant cinnamic acid derivative: chlorogenic acid

Snakebite is a significant health concern in tropical and subtropical regions, particularly in Africa, Asia, and Latin America, resulting in more than 2.7 million envenomations and an estimated one hundred thousand fatalities annually. The Bothrops genus is responsible for the majority of snakebite envenomings in Latin America and Caribbean countries. Accidents involving snakes from this genus are characterized by local symptoms that often lead to permanent sequelae and death. However, specific antivenoms exhibit limited effectiveness in inhibiting local tissue damage. Phospholipase A2-like (PLA2-like) toxins emerge as significant contributors to local myotoxicity in accidents involving Bothrops species. As a result, they represent a crucial target for prospective treatments. Some natural and synthetic compounds have shown the ability to reduce or abolish the myotoxic effects of PLA2-like proteins. In this study, we employed a combination approach involving myographic, morphological, biophysical and bioinformatic techniques to investigate the interaction between chlorogenic acid (CGA) and BthTX-I, a PLA2-like toxin. CGA provided a protection of 71.8% on muscle damage in a pre-incubation treatment. Microscale thermophoresis and circular dichroism experiments revealed that CGA interacted with the BthTX-I while preserving its secondary structure. CGA exhibited an affinity to the toxin that ranks among the highest observed for a natural compound. Bioinformatics simulations indicated that CGA inhibitor binds to the toxin's hydrophobic channel in a manner similar to other phenolic compounds previously investigated. These findings suggest that CGA interferes with the allosteric transition of the non-activated toxin, and the stability of the dimeric assembly of its activated state.

Lys49 myotoxins, secreted phospholipase A2-like proteins of viperid venoms: A comprehensive review

Muscle necrosis is a potential clinical complication of snakebite envenomings, which in severe cases can lead to functional or physical sequelae such as disability or amputation. Snake venom proteins with the ability to directly damage skeletal muscle fibers are collectively referred to as myotoxins, and include three main types: cytolysins of the "three-finger toxin" protein family expressed in many elapid venoms, the so-called "small" myotoxins found in a number of rattlesnake venoms, and the widespread secreted phospholipase A₂ (sPLA₂) molecules. Among the latter, protein variants that conserve the sPLA₂ structure, but lack such enzymatic activity, have been increasingly found in the venoms of many viperid species. Intriguingly, these sPLA₂-like proteins are able to induce muscle necrosis by a mechanism independent of phospholipid hydrolysis. They are commonly referred to as "Lys49 myotoxins" since they most often present, among other substitutions, the replacement of the otherwise invariant residue Asp49 of sPLA₂s by Lys. This work comprehensively reviews the historical developments and current knowledge towards deciphering the mechanism of action of Lys49 sPLA₂-like myotoxins, and points out main gaps to be filled for a better understanding of these multifaceted snake venom proteins, to hopefully lead to improved treatments for snakebites.

BthTX-II from Bothrops jararacussu venom has variants with different oligomeric assemblies: An example of snake venom phospholipases A2 versatility

Phospholipases A₂ (PLA₂s) are found in almost every venomous snake family. In snakebites, some PLA₂s can quickly cause local myonecrosis, which may lead to permanent sequelae if antivenom is administered belatedly. They hydrolyse phospholipids in membranes through a catalytic calcium ions-dependent mechanism. BthTX-II is a basic PLA₂ and the second major component in the venom of Bothrops jararacussu. Herein, using the software SEQUENCE SLIDER, which integrates crystallographic, mass spectrometry and genetic data, we characterized the primary, tertiary and quaternary structure of two BthTX-II variants (called a and b), which diverge in 7 residues. Crystallographic structure BthTX-IIa is in a Tense-state with its distorted calcium binding loop buried in the dimer interface, contrarily, the novel BthTX-IIb structure is a monomer in a Relax-state with a fatty acid in the hydrophobic channel. Structural data in solution reveals that both variants are monomeric in neutral physiological conditions and mostly dimeric in an acidic environment, being catalytic active in both situations. Therefore, we propose two myotoxic mechanisms for BthTX-II, a catalytic one associated with the monomeric assembly, whereas the other has a calcium independent activity related to its C-terminal region, adopting a dimeric conformation similar to PLA₂-like proteins.

Panacea within a Pandora's box: the antiparasitic effects of phospholipases A2 (PLA2s) from snake venoms

Parasitic diseases affect millions of individuals worldwide, mainly in low-income regions. There is no cure for most of these diseases, and the treatment relies on drugs that have side effects and lead to drug resistance, emphasizing the urgency to find new treatments. Snake venom has been gaining prominence as a rich source of molecules with antiparasitic potentials, such as phospholipases A₂ (PLA₂s). Here, we compile the findings involving PLA₂s with antiparasitic activities against helminths, Plasmodium, Toxoplasma, and trypanosomatids. We indicate their molecular features, highlighting the possible antiparasitic mechanisms of action of these proteins. We also demonstrate interactions between PLA₂s and some parasite membrane components, shedding light on potential targets for drug design that may provide better treatment for the illnesses caused by parasites.

Gallic acid anti-myotoxic activity and mechanism of action, a snake venom phospholipase A2 toxin inhibitor, isolated from the medicinal plant Anacardium humile

Snakebite envenoming is the cause of an ongoing health crisis in several regions of the world, particularly in tropical and neotropical countries. This scenario creates an urgent necessity for new practical solutions to address the limitations of current therapies. The current study investigated the isolation, phytochemical characterization, and myotoxicity inhibition mechanism of gallic acid (GA), a myotoxin inhibitor obtained from Anacardium humile. The identification and isolation of GA was achieved by employing analytical chromatographic separation, which exhibited a compound with retention time and nuclear magnetic resonance spectra compatible with GA's commercial standard and data from the literature. GA alone was able to inhibit the myotoxic activity induced by the crude venom of Bothrops jararacussu and its two main myotoxins, BthTX-I and BthTX-II. Circular dichroism (CD), fluorescence spectroscopy (FS), dynamic light scattering (DLS), and interaction studies by molecular docking suggested that GA forms a complex with BthTX-I and II. Surface plasmon resonance (SPR) kinetics assays showed that GA has a high affinity for BthTX-I with a K_D of 9.146 × 10^-7 M. Taken together, the two-state reaction mode of GA binding to BthTX-I, and CD, FS and DLS assays, suggest that GA is able to induce oligomerization and secondary structure changes for BthTX-I and -II. GA and other tannins have been shown to be effective inhibitors of snake venoms' toxic effects, and herein we demonstrated GA's ability to bind to and inhibit a snake venom PLA₂, thus proposing a new mechanism of PLA₂ inhibition, and presenting more evidence of GA's potential as an antivenom compound.

The allosteric activation mechanism of a phospholipase A2-like toxin from Bothrops jararacussu venom: a dynamic description

The activation process of phospholipase A₂-like (PLA₂-like) toxins is a key step in their molecular mechanism, which involves oligomeric changes leading to the exposure of specific sites. Few studies have focused on the characterization of allosteric activators and the features that distinguish them from inhibitors. Herein, a comprehensive study with the BthTX-I toxin from Bothrops jararacussu venom bound or unbound to α-tocopherol (αT) was carried out. The oligomerization state of BthTX-I bound or unbound to αT in solution was studied and indicated that the toxin is predominantly monomeric but tends to oligomerize when complexed with αT. In silico molecular simulations showed the toxin presents higher conformational changes in the absence of αT, which suggests that it is important to stabilize the structure of the toxin. The transition between the two states (active/inactive) was also studied, showing that only the unbound BthTX-I system could migrate to the inactive state. In contrast, the presence of αT induces the toxin to leave the inactive state, guiding it towards the active state, with more regions exposed to the solvent, particularly its active site. Finally, the structural determinants necessary for a molecule to be an inhibitor or activator were analyzed in light of the obtained results.

PLA2-like proteins myotoxic mechanism: a dynamic model description

Phospholipase A₂-like (PLA₂-like) proteins contribute to the development of muscle necrosis in Viperidae snake bites and are not efficiently neutralized by current antivenom treatments. The toxic mechanisms of PLA₂-like proteins are devoid of catalytic activity and not yet fully understood, although structural and functional experiments suggest a dimeric assembly and that the C-terminal residues are essential to myotoxicity. Herein, we characterized the functional mechanism of bothropic PLA₂-like structures related to global and local measurements using the available models in the Protein Data Bank and normal mode molecular dynamics (NM-MD). Those measurements include: (i) new geometric descriptions between their monomers, based on Euler angles; (ii) characterizations of canonical and non-canonical conformations of the C-terminal residues; (iii) accessibility of the hydrophobic channel; (iv) inspection of ligands; and (v) distance of clustered residues to toxin interface of interaction. Thus, we described the allosteric activation of PLA₂-like proteins and hypothesized that the natural movement between monomers, calculated from NM-MD, is related to their membrane disruption mechanism, which is important for future studies of the inhibition process. These methods and strategies can be applied to other proteins to help understand their mechanisms of action.