Interchain hydrogen-bonding interactions may facilitate translocation of K+ ions across the potassium channel selectivity filter, as suggested by synthetic modeling chemistry

Abstract

A 4-fold symmetric arrangement of TVGYG polypeptides forms the selectivity filter of the K+ channel from Streptomyces lividans (KcsA). We report the synthesis and properties of synthetic models for the filter, p-tert-butyl-calix[4]arene-(OCH(2)CO-XOBz)(4) (X = V, VG, VGY), 1-3. The first cation (Na+, K+) binds to the four -[OCH(2)CO]- units, a region devised to mimic the metal-binding site formed by the four T residues in KcsA. NMR studies reveal that cations and valine amide protons compete for the carbonyl oxygen atoms, converting NH(Val)...O=C hydrogen bonds to M+ ...O=C bonds (M+ = Na+ or K+). The strength of these interchain NH(Val)...O=C hydrogen bonds varies in the order 3 > 2 > 1. We propose that such interchain H-bonding may destabilize metal binding in the selectivity filter and thus help create the low energy barrier needed for rapid cation translocation.

Figure 1

View of the VGYG portion of the selectivity filter structure, drawn with coordinates from Doyle et al. (2), overlaid on that of p-tert-butyl-calix[4]arene-(OCH₂COCl)₄ showing how the four -{OCH₂CO}- calixarene units mimic the four T75 residues (see text). In some K⁺ channels, T75 is replaced by a serine. Red, blue, and black colors designate oxygen, nitrogen, and carbon atoms, respectively. Ion and water locators (green and red) are assigned as in the x-ray structure, but we do not claim identical locations in our synthetic model compounds.

Figure 2

¹H NMR spectra (500 MHz, CD₃CN, 283 K) of 3 in the absence of metal ions (Top) or presence of K⁺ picrate (Middle) presence of Na⁺ picrate (Bottom).

Figure 3

Illustration of interchain N—H_Val⋅⋅⋅O⩵C hydrogen bonding by M⁺⋅⋅⋅O⩵C-binding alternation in 1–3 [M⁺ = Na⁺ and K⁺; R = CH(CH(CH₃)₂)COCH₂Ph for 1; R = CH(CH(CH₃)₂)CONHCH₂COCH₂Ph for 2 and R = CH(CH(CH₃)₂)CONHCH₂CONHCH(CH₂C₆H₅OH)COCH₂Ph for 3]. Upper, side view. Lower, end view.

Figure 4

Schematic representation of interchain hydrogen bonding and metal binding proposed for 3 on the basis of proton chemical shift changes [R = CH(CH₃)₂, R′ = CH₂-C₆H₄-OH]. The significant chemical shift changes observed up to the tyrosine amide proton suggest movement of the metal from the -{OCH₂CO}- to the glycine site (see text).