Venom of Scorpio maurus [4]. The C-terminus of MTx is amidated and thus does not carry a negative charge at neutral pH. Figure 1A shows that the secondary structure of MTx contains an a-helix and two anti-parallel b-sheets. MTx has been shown to inhibit one subtype of voltage-gated K+ channels of the inhibitor Shaker family (Kv1.2) and calcium-activated K+ channels of intermediate-conductance (IKCa) with nanomolar affinities [4,5,6]. MTx is special in that its backbone is interconnected by four disulfide bridges (Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34), rather than three disulfide bridges commonly found in other Kv1 channel toxinblockers. MTx has a particular high affinity for Kv1.2 (IC50 = 0.8 nM), whereas its affinities for Kv1.1 (IC50 = 37 nM or .100) and Kv1.3 (IC50 = 150 nM or 3 mM) are significantly lower [4,5,7]. Here, two IC50 values measured from channels expressed in different cell lines are quoted for Kv1.1 and Kv1.3 (more details will be described below). This is in contrast to many other Kv1 channel blockers such as charybdotoxin (ChTx) [8], ShK [9] and 15481974 OSK1 [10], which are more effective for Kv1.3 or Kv1.1 than Kv1.2. MTx shows high selectivity for Kv1.2 over Kv1.1 and Kv1.3, although these channels differ in only several positions at the P-loop turret and near the selectivity filter (Figure 1B). A small ring of four aspartate residues at position 379 is located just above the selectivity filter of Kv1.2, whereas a larger acidic ring at position 355 of the P-loop turret is located about 10 ?A above it (Figure 1C). Due to the 
unique selectivity profile of MTx for Kv1.2 and IKCa, a number of experimental [5,6,7,11,12,13,14,15] as well as theoretical [16,17,18] studies have been carried out to understand the binding modes of MTx to K+ channels. These studies are consistent with Lys23 of MTx being the key residue which protrudes into the selectivity filter of Kv1.2 on binding. The mechanism of block by MTx has been believed to be similar to other peptide blockers such as ChTx which carries three disulfide bridges [11]. However, how MTx interacts with the outer vestibular wall of Kv1.2 and other channels has not been resolved. For example, Fu et al. [16] found that Lys30 of MTx is a key residue coupled with Asp379 of Kv1.2, whereas Yi et al. [17] suggested that Lys7 of MTx is the residue in contact with Asp379. Yet, Visan et al. [5] believe that Lys7 of MTx should be in close proximity to Asp363 of Kv1.2.Selective Block of Kv1.2 by MaurotoxinKv1.3 with micromolar affinities. The selectivity of MTx for Kv1.2 over Kv1.1 and Kv1.3 likely arises from the steric effects by residue 381 near the selectivity filter.Computational Methods Molecular Dynamics as a Docking MethodDifferent methods including rigid-body molecular docking [18,19,20], molecular docking with limited Epigenetic Reader Domain flexibility [21,22], Brownian dynamics simulation [23,24,25], and MD simulation 
with distance restraints (biased MD) [26], have been used to 12926553 predict the binding modes between various toxins and channels. In molecular docking methods and Brownian dynamics simulation, the flexibility of proteins and the entropy of water are ignored. In contrast, both protein flexibility and water entropy are taken into account in biased MD. However, biased MD requires at least one toxin-channel interaction residue pair to be identified from experimental data at the beginning of simulations. In biased MD, a harmonic potential is applied to maintain the distance between one or several.Venom of Scorpio maurus [4]. The C-terminus of MTx is amidated and thus does not carry a negative charge at neutral pH. Figure 1A shows that the secondary structure of MTx contains an a-helix and two anti-parallel b-sheets. MTx has been shown to inhibit one subtype of voltage-gated K+ channels of the Shaker family (Kv1.2) and calcium-activated K+ channels of intermediate-conductance (IKCa) with nanomolar affinities [4,5,6]. MTx is special in that its backbone is interconnected by four disulfide bridges (Cys3-Cys24, Cys9-Cys29, Cys13-Cys19 and Cys31-Cys34), rather than three disulfide bridges commonly found in other Kv1 channel toxinblockers. MTx has a particular high affinity for Kv1.2 (IC50 = 0.8 nM), whereas its affinities for Kv1.1 (IC50 = 37 nM or .100) and Kv1.3 (IC50 = 150 nM or 3 mM) are significantly lower [4,5,7]. Here, two IC50 values measured from channels expressed in different cell lines are quoted for Kv1.1 and Kv1.3 (more details will be described below). This is in contrast to many other Kv1 channel blockers such as charybdotoxin (ChTx) [8], ShK [9] and 15481974 OSK1 [10], which are more effective for Kv1.3 or Kv1.1 than Kv1.2. MTx shows high selectivity for Kv1.2 over Kv1.1 and Kv1.3, although these channels differ in only several positions at the P-loop turret and near the selectivity filter (Figure 1B). A small ring of four aspartate residues at position 379 is located just above the selectivity filter of Kv1.2, whereas a larger acidic ring at position 355 of the P-loop turret is located about 10 ?A above it (Figure 1C). Due to the unique selectivity profile of MTx for Kv1.2 and IKCa, a number of experimental [5,6,7,11,12,13,14,15] as well as theoretical [16,17,18] studies have been carried out to understand the binding modes of MTx to K+ channels. These studies are consistent with Lys23 of MTx being the key residue which protrudes into the selectivity filter of Kv1.2 on binding. The mechanism of block by MTx has been believed to be similar to other peptide blockers such as ChTx which carries three disulfide bridges [11]. However, how MTx interacts with the outer vestibular wall of Kv1.2 and other channels has not been resolved. For example, Fu et al. [16] found that Lys30 of MTx is a key residue coupled with Asp379 of Kv1.2, whereas Yi et al. [17] suggested that Lys7 of MTx is the residue in contact with Asp379. Yet, Visan et al. [5] believe that Lys7 of MTx should be in close proximity to Asp363 of Kv1.2.Selective Block of Kv1.2 by MaurotoxinKv1.3 with micromolar affinities. The selectivity of MTx for Kv1.2 over Kv1.1 and Kv1.3 likely arises from the steric effects by residue 381 near the selectivity filter.Computational Methods Molecular Dynamics as a Docking MethodDifferent methods including rigid-body molecular docking [18,19,20], molecular docking with limited flexibility [21,22], Brownian dynamics simulation [23,24,25], and MD simulation with distance restraints (biased MD) [26], have been used to 12926553 predict the binding modes between various toxins and channels. In molecular docking methods and Brownian dynamics simulation, the flexibility of proteins and the entropy of water are ignored. In contrast, both protein flexibility and water entropy are taken into account in biased MD. However, biased MD requires at least one toxin-channel interaction residue pair to be identified from experimental data at the beginning of simulations. In biased MD, a harmonic potential is applied to maintain the distance between one or several.
