Binding site of AChR: Difference between revisions

There is a 13 amino acids high affinity peptide(<scene name='68/688431/Hap/2'>HAP</scene>) which corresponding to residues 187-199 of the AChR that can inhibits the binding of α-BTX to AChR. And the high affinity and specific interaction of α-bungarotoxin (<scene name='68/688431/Structure_of_btx/1'>α-BTX</scene>) with AChR has been of considerable importance in the study of the binding site of AChR.<ref>PMID:11683996</ref> The little peptide can bind to α-BTX as competitive inhibitors of α-BTX biding to AChR. So the complex between α-BTX and this little peptide(HAP) maybe can used as a model to study the binding site of AChR.

The ligand binding site of AChR is mainly located at the α-subunits. The acetylcholine binding protein(<scene name='68/688431/Achbp/2'>AChBP</scene>) is most closely related to the α-subunits of the nAChR. AChBP is a soluble protein found in the snail [http://en.wikipedia.org/wiki/Lymnaea_stagnalis Lymnaea stagnalis]. Nearly all residues that are conserved within the nAChR family are present in AChBP, including those that are relevant for lignad binding.<ref>PMID:11357122</ref> And AChBP can also bind with α-Neurotoxins. So the AChBP structure is obviously an ideal candidate for testing the relevance of the conformation of the HAP when bound to α-BTX, to that of the corresponding binding region in AChR.<ref>PMID:11683996</ref>

The 13-mer <scene name='68/688431/Hap/2'>HAP</scene> assumes an antiparallel β hairpin structure, which can be used as a model to study the binding site of AChR. It is held snugly between <scene name='68/688431/Figure_1234/3'>fingers 1,2 and 4</scene> of α-BTX. The shortest and most numerous interactions are formed with <scene name='68/688431/Figure_1234/2'>finger 2</scene> of α-BTX. The two arms of the HAP hairpin assume a β sheet conformation, with residues Leu2 (corresponding to position 188 in AChR)-Tyr4 (corresponding to position 190 in AChR ) making an <scene name='68/688431/Residues_between_btx_and_hap/4'> intermolecular interaction </scene> with α-BTX residues Val39-Glu41 on a loop region. Tyr3 (corresponding to position 189 in AChr) of HAP forms a sung fit into a loop region of α-BTX. The formation of <scene name='68/688431/H_bond_between_hap_and_btx/2'>two H bonds</scene> from its hydroxyl to residues Thr8 and lle11 of α-BTX makes the tyrosine at that position an ideal candidate for forming binding interactions with α-BTX. Indeed, this tyrosine is known to play a crucial role in α-BTX binding. <ref>PMID:11683996</ref>  

In nAChR, the ligand-binding site is located at the interface between two subunits. The homopentameric α7 receptor contains five identical ligand binding sites. In these sites acrtylcholine is expected to bind through [http://en.wikipedia.org/wiki/Cation%E2%80%93pi_interaction cation-π interactions], where the positive charge of the quaternary ammonium of acetylcholine interacts with the electron-rich aromatic side chains.<ref>PMID:11357122</ref> The ACh binding site in AChBP was assigned by the localization of a solvent molecule (positively charge HEPES) seen near residues corresponding to the 187-199 loop of the AChR α subunit and stacking on the corresponding Trp 143.<ref>PMID:11683996</ref> <scene name='68/688431/Hepes_five_subunits/2'>HEPES</scene> can be refined in the current AChBP structure, it does not make any specific hydrogen bonds with the protein, it stacks with its quaternary ammonium onto <scene name='68/688431/Hepes_trp143/1'>Trp 143</scene> making cation-π interactions as expected for nicotinic agonists.<ref>PMID:11357122</ref>  

The superimposed model of AChBP and α-BTX suggests that the putative agonist HEPES seen in the AChBP structure is blocked from entering or leaving the AChBP interface cleft by the insertion of <scene name='68/688431/Hepes_black_loop_2/1'>loop 2</scene> of α-BTX into that cleft. This clarifies and explains the strong inhibition of AChR function by the toxin.<ref>PMID:11683996</ref> The superposition of the HAP on loop 182-193 of AChBP(Fig.5)show that the major interaction between α-BTX and AChR α subunit, occur in residues187-192 of that sununit.