Comparison between apo and complexed structures of bothropstoxin-I reveals the role of Lys122 and Ca2+-binding loop region for the catalytically inactive Lys49-PLA2s

Phospholipases A2 (Asp49-PLA2s) are enzymes responsible for cellular membrane disruption through Ca2+-dependent hydrolysis of phospholipids. A class of these proteins (Lys49-PLA2s) does not show catalytic activity but can exert a pronounced local myotoxic effect that is not neutralized by serum therapy. In this work, we present five structures of Lys49-PLA2s from snakes of the Bothrops genus in apo form, complexed with PEG molecules and chemically modified by p-bromofenacil bromide (BPB), a classic inhibitor of PLA2. We present herein an extensive structural analysis including: (i) the function of hydrophobic long-chain molecules as Lys49-PLA2s inhibitors, (ii) the role of Lys122, previously indicated as being responsible for Lys49-PLA2s catalytic inactivity and, (iii) a structural comparison of the Ca2+-binding loop region between Lys49 and Asp49-PLA2s. The Lys122 analysis of 30 different monomers for apo and complexed Lys49-PLA2s structures shows that this residue is very flexible and may bind to different carboxyl groups giving stability to the crystal structures. The structural comparisons of the Ca2+-binding loop region between Lys49 and Asp49-PLA2s reveal the importance of the Tyr28 residue conservation in Asp49-PLA2s to the integrity of this loop. The Tyr28 residue stabilizes this region by an interaction with Gly35 residue. In Lys49-PLA2s and low-catalytic Asp49-PLA2s this interaction does not occur, preventing the binding of Ca2+.