1 to α-KTx12.4 from Tityus (Buthidae) genus species, known as butantoxin-like peptides, and the two α-KTx21 peptides, Vm23 and Vm24, purified from Vaejovis mexicanus smithi, belonging to the Iurida suborder scorpion. Butantoxins inhibit high-conductance Ca2+-activated and Shaker-B K+

channels [7] and [20], whereas Vm24 selectively and irreversibly blocks Kv1.3 channels of human T lymphocytes at pM concentrations, and it is much less active on KCa3.1 and hKv1.2 channels [30]. Similarly to the vast majority of scorpion KTxs, OcyKTx2 reversibly blocks Shaker B K+ channels with a Kd of 82 nM and the Shaker-related Kv1 family member Kv1.3 channel with a Kd of 18 nM. Comparative analysis of OcyKTx2 amino acid sequence against check details those from databanks shows that it has a 70% identity to α-KTx6.10 (OcKTx5, UniProtKB Q6XLL5), a putative peptide identified in the transcriptome of Opistophthalmus carinatus. Indeed, in the phylogenetic tree ( Fig. 3), OcKTx5 is the most related peptide of OcyKTx2. On the other hand, OcyKTx2 presents 64% identity to Pi4 (α-KTx6.4, UniProtKB P84094), a K+ channel inhibitor purified from Pandinus imperator (Scorpionidae). Pi4 potently and reversibly

blocks Kv1.2, Shaker-B, Bortezomib cell line and small conductance (SK) KCa channels [21], but is has no effect on Kv1.1 and Kv1.3 channels [19]. Finally, and interestingly, the lowest identity (35%) of OcyKTx2 with other members of the α-KTx6 family peptides is the one with α-KTx6.16 (OcyC12), a precursor sequence

identified in the same scorpion, O. cayaporum, whose mature sequence has not yet been identified in the venom [27]. This distance between these latter two peptides identified from the same species (O. cayaporum) was also observed in the phylogenetic analysis (see Fig. 3). Despite structural similarities, α-KTx6 peptides differ in their pharmacological profiles. In general, α-KTx6 peptides have specific activity for the Shaker related voltage gated K+-channels. However, some GNA12 peptides act on one Kv1 channel subtype and also block calcium dependent K+-channels. HsTx1 (α-KTx6.3) potently blocks Kv1.1 and Kv1.3 whereas it does not compete with 125I-apamin binding onto SK channels from rat brain synaptosomes [16]. Anuroctoxin (α-KTx6.12) is a high-affinity blocker of human T lymphocytes Kv1.3 channels, and does not block the Ca2+-activated IKCa1 K+ channels either [2]. HgeTx1 (α-KTx6.14) blocks Shaker-B with a Kd of 52 nM [26]. MTX (α-KTx6.2) is a potent and selective inhibitor of the intermediate (IK) conductance Ca2+-activated and of Kv1.2 K+ channels [5], [14] and [15]. Pi1 is inactive on Kv1.1 and Kv1.3 up to micromolar concentrations, but acts on Kv1.2 and Shaker-B channels with nanomollar affinity. IsTX (α-KTx6.12), a peptide isolated from Opisthacanthus madagascariensis, binds to Kv1.3 with low (μM) affinity [31]. Most of the α-KTxs have a common functional dyad (e.g.