Phospholipase A2 homolog

General Catalytically inactive snake venom phospholipases A2 (svPLA2s) present a high degree of amino acid sequence similarity to viperid snake PLA2s (group II subfamily). In these PLA2 variants, the aspartic acid residue at position 49 (Asp49) that is invariable in catalytically active PLA2s, is substituted by a lysine residue (Lys49). The epsilon-amino group of the Lys49 is located in the position normally occupied by the Ca 2+ cofactor in the Asp49-PLA2, and Lys49-PLA2s are catalytically inactive partly as a consequence of the loss Ca 2+ cofactor binding.

Although the Lys49-PLA2s do not show catalytic activity, they have a Ca 2+-independent membrane-damaging activity that does not involve the hydrolysis of membrane phospholipids. Nevertheless, the in vivo activities of the Lys49-PLA2s include the formation of edema and local myonecrosis, cytolysis of a wide range of cell types, bactericidal activity and local inflammation and pain.

Important residues and proposed mode of action
Several studies have shown that the C-terminal regions of different PLA2-like proteins, which include the 115-129 segment, are responsible for the myotoxic activity, due to a variable combination of positively charged and hydrophobic residues (Chioato et al., 2002, Chioato et al., 2007 ). Later, based on structural and functional studies, a myotoxic mechanism has been proposed that involves an allosteric transition and two distinct sites for interaction with the cell membrane, including: 1) a membrane-docking site (MDoS) formed by Lys-20; Lys-115 and Arg-118 residues and 2) a membrane-disruption site (MDiS) formed by Leu-121 and Phe-125 residues (Fernandes et al., 2013, Fernandes et al., 2014 ). This residues are conserved for all bothropic PLA2-like toxins and, similar putative sites are found in all PLA2-like toxins (Fernandes et al., 2014 ). A model of myotoxic mechanism has been proposed: an apo Lys49-PLA2 is activated by the entrance of a hydrophobic molecule (e.g. fatty acid) at the hydrophobic channel of the protein leading to a reorientation of a monomer. This reorientation causes a transition between ‘inactive’ to ‘active’ states, causing alignment of C-terminal and MDoS side-by-side and putting the MDiS in the same plane, exposed to solvent and in a symmetric position for both monomers. The MDoS region stabilizes the toxin on membrane by the interaction of charged residues with phospholipid head groups. Subsequently, the MDiS region destabilizes the membrane with penetration of hydrophobic residues. This insertion causes a disorganization of the membrane, allowing an uncontrolled influx of ions (i.e. calcium and sodium), and eventually triggering irreversible intracellular alterations and cell death (Fernandes et al., 2013, Borges et al., 2017 ).