Heteromtoxin (HmTx), a novel heterodimeric phospholipase A(2) from Heterometrus laoticus scorpion venom

Heteromtoxin (HmTx) is a group III phospholipase A(2) produced in Heterometrus laoticus, in Thailand. In this study, HmTx was purified from venom by separation chromatography, and the PLA(2) activity of the fractions was determined by lecithin agar assay. The enzyme is an acidic protein with a pI of 5.6 and an apparent molecular weight of 14018.4 Da. The nucleotide sequence of HmTx contains 649 bp, and the mature protein is predicted to have 131 amino acid residues-104 of which make up the large subunit, and 27 of which make up the small subunit. The subunit structure of HmTx is highly similar to that of the other toxin, Pandinus imperator imperatoxin I (IpTx(i)) and to Mesobuthus tamulus phospholipase A(2) (MtPLA(2)). The 3D-structure of HmTx consists of three conserved alpha-helices: h1 (Lys24-His34), h2 (Cys59-Asp71), and h3 (Ala80-Phe89). The beta-sheet consisted of a single stranded anti-parallel beta-sheet (b1.1 at Glu43-Lys45 and b1.2 at Lys48-Asn50) that was highly similar to the conserved sequences (-CGXG-, -CCXXHDXC- and CXCEXXXXXC-) of Apis mellifera (bee) phospholipases.