Structure-Function Relationship of Phospholipase A₂ From Snake Venoms

Abstract

The enzyme phospholipase A₂ (EC 3.1.1.4; PLA₂) is widely distributed among various species in the animal kingdom, notably in the pancreatic tissues of mammals and animal venoms of reptiles and insects. It catalyzes efficiently and specifically the hydrolysis of the 2-acyl ester bond in 3-sn-phosphoglycerides. The primary sequences of more than 100 PLA₂ from various tissues of different animal families have been solved. Based on their primary structure, PLA₂s are classified into two groups. Group I comprises those from mammalian pancreas and the venoms of snake families Elapidae and Hydrophidae. The Group II PLA₂s those from the venoms of Crotalidae and Viperidae, have a short extension at the carboxyl terminus that terminates in a half-cystine linked to a half-cystine near the active site at position 50. Despite these structural differences, the PLA_2S are homologous proteins which show considerable sequence similarity, particularly in the Ca²⁺-binding and active site regions.

Besides a catalyst for the hydrolysis of phospholipids, PLA_2S from snake venoms have been shown to possess a wide variety of pharmacological activities, including neurotoxic, cardiotoxic, hemolytic, anticoagulant, and myonecrotic actions, among others. From a structural biology perspective, such a functional diversity within this group of structurally similar proteins raises questions of the relationship between their structure and their effect. Two PLA₂ enzymes have been chosen for the study because they show considerable sequence homology but differ in charge and toxicity: the Naja nigricollis enzyme is basic and relatively toxic while the acidic Naja naja atra PLA₂ is less toxic. Chemical modification of two highly potent presynaptic neurotoxins β₁-bungarotoxin and notexin was also carried out in the hope of elucidating the molecular features contributing to the biological activities of presynaptic PLA₂ neurotoxins.