title: Crystal structure of a myotoxic Asp49-phospholipase A2 with low catalytic activity: Insights into Ca2+-independent catalytic mechanism.
abstract: A myotoxic Asp49-phospholipase A2 (Asp49-PLA2) with low catalytic activity (BthTX-II from Bothrops jararacussu venom) was crystallized and the molecular-replacement solution has been obtained with a dimer in the asymmetric unit. The quaternary structure of BthTX-II resembles the myotoxic Asp49-PLA2 PrTX-III (piratoxin III from B. pirajai venom) and all non-catalytic and myotoxic dimeric Lys49-PLA2S. Despite of this, BthTX-II is different from the highly catalytic and non-myotoxic BthA-I (acidic PLA2 from B. jararacussu) and other Asp49-PLA2S. BthTX-II structure showed a severe distortion of calcium-binding loop leading to displacement of the C-terminal region. Tyr28 side chain, present in this region, is in an opposite position in relation to the same residue in the catalytic activity Asp49-PLA2S, making a hydrogen bond with the atom O delta 2 of the catalytically active Asp49, which should coordinate the calcium. This high distortion may also be confirmed by the inability of BthTX-II to bind Na+ ions at the Ca2+-binding loop, despite of the crystallization to have occurred in the presence of this ion. In contrast, other Asp49-PLA2S which are able to bind Ca2+ ions are also able to bind Na+ ions at this loop. The comparison with other catalytic, non-catalytic and inhibited PLA2S indicates that the BthTX-II is not able to bind calcium ions; consequently, we suggest that its low catalytic function is based on an alternative way compared with other PLA2S.
authors.affiliation: Departamento de Física e Biofísica, Instituto de Biociências, UNESP, Botucatu-SP, Brazil.
authors.forename: Luiz C
authors.initials: LC
authors.lastname: Corrêa
authors.forename: Daniela P
authors.initials: DP
authors.lastname: Marchi-Salvador
authors.forename: Adélia C O
authors.initials: AC
authors.lastname: Cintra
authors.forename: Suely V
authors.initials: SV
authors.lastname: Sampaio
authors.forename: Andreimar M
authors.initials: AM
authors.lastname: Soares
authors.forename: Marcos R M
authors.initials: MR
authors.lastname: Fontes
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017837:Bothrops; D002118:Calcium / Q000378:metabolism*; D002384:Catalysis; D018360:Crystallography, X-Ray / Q000379:methods; D019281:Dimerization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D054467:Phospholipases A2 / Q000737:chemistry* / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D012910:Snake Venoms / Q000201:enzymology*
chemical_list: D012910:Snake Venoms; D054467:Phospholipases A2; D002118:Calcium