The nature of the receptor site for the reversible K+ channel blocking by aminopyridines

This work presents a theoretical study aimed at the identification of the receptor site for the blocking of the voltage dependent K+ channels by protonated aminopyridines. Thus, the density functional theory (DFT) at the B3LYP/6-311 G (d,p) level is applied, both in vacuum and in solution, to a series of active (protonated) compounds: 2-aminopyridine, 3-aminopyridine, 4-aminopyridine, 3,4-diaminopyridine, and 4-aminoquinoleine. Analysis of the X-ray structure of the alpha-subunit of the channel shows that charged aminopyridines can interact electrostatically with a glutamic acid residue in the outside of the pore, or through a cation-pi interaction in the inside. To test both possibilities, model complexes are built using as nucleophiles a carboxylic group and an ethylene molecule, respectively. The three-dimensional electrostatic potential distribution of the protonated aminopyridines shows that an approaching nucleophile will be oriented toward the N-H (protonated) bond. Interaction with the carboxylic residue leads to a proton transfer, with the aminopyridine-carboxylic acid linked by a hydrogen bond. The observed breaking of the equivalence of the Laplacian of the charge density, the relative energy variation for the complexes, and the interaction with only one of the carboxylic residues in the fourfold alpha-subunit of the K+ channel are not compatible with the observed in vitro activity variation of aminopyridines. On the other hand, the study on the ethylene complexes shows, in vacuum and solution, a cation-pi interaction, clearly characterized by the atoms in molecules (AIM) theory. The variation of relative energy in solution is very small, but approaches the variation of in vitro activity. Our results, the pharmacophoric characteristics of aminopyridines, and the analysis of the three-dimensional internal structure of the K+ channel alpha-subunit suggest two putative receptor sites. One is formed by the four Thr-Thr-Val chains conforming the entrance to the narrow part of the inner K+ channel. The other is defined by four Thr residues within the pore.