title: Mechanism of tetrodotoxin block and resistance in sodium channels.
abstract: Tetrodotoxin (TTX) has been used for many decades to characterize the structure and function of biological ion channels. Yet, the precise mechanism by which TTX blocks voltage-gated sodium (NaV) channels is not fully understood. Here molecular dynamics simulations are used to elucidate how TTX blocks mammalian voltage-gated sodium (Nav) channels and why it fails to be effective for the bacterial sodium channel, NaVAb. We find that, in NaVAb, a sodium ion competes with TTX for the binding site at the extracellular end of the filter, thus reducing the blocking efficacy of TTX. Using a model of the skeletal muscle channel, NaV1.4, we show that the conduction properties of the channel observed experimentally are faithfully reproduced. We find that TTX occludes the entrance of NaV1.4 by forming a network of hydrogen-bonds at the outer lumen of the selectivity filter. The guanidine group of TTX adopts a lateral orientation, rather than pointing into the filter as proposed previously. The acidic residues just above the selectivity filter are important in stabilizing the hydrogen-bond network between TTX and NaV1.4. The effect of two single mutations of a critical tyrosine residue in the filter of NaV1.4 on TTX binding observed experimentally is reproduced using computational mutagenesis.
authors.affiliation: Research School of Biology, Australian National University, Canberra ACT 0200, Australia. Electronic address: rong.chen@anu.edu.au.
authors.forename: Rong
authors.initials: R
authors.lastname: Chen
authors.affiliation: Research School of Biology, Australian National University, Canberra ACT 0200, Australia.
authors.forename: Shin-Ho
authors.initials: SH
authors.lastname: Chung
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001665:Binding Sites; D003198:Computer Simulation; D006801:Humans; D006860:Hydrogen Bonding; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D062553:NAV1.4 Voltage-Gated Sodium Channel / Q000737:chemistry* / Q000187:drug effects* / Q000235:genetics; D011487:Protein Conformation; D026941:Sodium Channel Blockers / Q000737:chemistry / Q000494:pharmacology*; D055672:Static Electricity; D013779:Tetrodotoxin / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D062553:NAV1.4 Voltage-Gated Sodium Channel; C106251:SCN4A protein, human; D026941:Sodium Channel Blockers; D013779:Tetrodotoxin
keywords: Hydrogen bond network; Ion channel block; Molecular dynamics; Na(V)Ab; Sodium channel