Structure of fasciculin 2 from green mamba snake venom: evidence for unusual loop flexibility

Abstract

The crystal structure of the snake toxin fasciculin 2, a potent acetylcholinesterase inhibitor from the venom of the green mamba (Dendroaspis angusticeps), has been determined by the molecular-replacement method, using the fasciculin 1 model and refined to 2.0A resolution. The introduction of an overall anisotropic temperature factor improved significantly the quality of the electrondensity map. It suggests, as it was also indicated by the packing, that the thermal motion along the unique axis direction is less pronounced than on the (ab) plane. The final crystallographic R factor is 0.188 for a model having r.m.s, deviations from ideality of 0.016,~, for bond lengths and 2.01' for bond angles. As fasciculin 1, fasciculin 2 belongs to the three-finger class of Elapidae toxins, a structural group that also contains the aneurotoxins and the cardiotoxins. Although the two fasciculins have, overall, closely related structures, the conformation of loop I differs appreciably in the two molecules. The presence of detergent in crystallization ,tnedium in the case of fasciculin 2 appears to be responsible for the displacement of the loop containing Thr9. This conformational change also results in the formation of a crystallographic dimer that displays extensive intermolecular interactions.

Crystal structure of a Cbtx–AChBP complex reveals essential interactions between snake α-neurotoxins and nicotinic receptors

“…Superimposition of an NmmI model, designed from the structure of its erabutoxin homolog, with the Cbtx molecule bound to AChBP suggests that NmmI His32 and Arg33 are positioned to mimic the orientation and contacts of Cbtx Phe29 and Arg33 within the AChBP binding pocket (not shown). However, NmmI Lys27 is too distant to be in contact with an AChBP residue, and NmmI loop II would have to move laterally while the tip of loop I would have to retract, or swing out as does loop I in Fas2 compared with fasciculin 1 (le Du et al , 1996), to avoid steric hindrance with the open loop C. This suggests a somewhat different orientation of NmmI at the subunit interface of the muscle‐type nAChR with Arg33 acting as a pivot. This is consistent with pairwise interaction energies that predicted proximity of NmmI Arg33 to Tyr190 in the α‐subunit and Leu119 in the γ‐subunit of the α1 2 βγδ receptor (Malany et al , 2000).…”

Nature: A Substantial Source of Auspicious Substances with Acetylcholinesterase Inhibitory Action

“…In another study [87], the crystal structure of FAS-II-mouse AChE complex delineated a large contact area consistent with the low dissociation constant of the complex and only a few of the amino acid residues which make up the overall interactive site of the FAS-II provided strong interactions needed for high binding affinity and inhibition of the enzyme. In more publications, FASs were reported to bind to the gorge entrance of AChE with excellent complementarity through many polar and hydrophobic interactions and to inhibit AChE by combined allosteric and dynamical means [88-92]. …”

Conformational Transitions in Protein-Protein Association: Binding of Fasciculin-2 to Acetylcholinesterase

“…Fasciculin-2 (FAS2), a three-fingered peptide isolated from snake venoms, is a picomolar inhibitor of acetylcholinesterase (AChE). Crystal structures (2)(3)(4) of the peptide reveal significant flexibility in loop I. This 61-residue peptide is an excellent model for conformational dynamics on the submicrosecond timescale, particularly the conformational changes of loop I that may be involved with its binding to AChE.…”