Secondary chelation through shared water provides ion selectivity in bacterial sodium channels

Area of Science:

• Biophysics
• Structural Biology
• Ion Channel Physiology

Background:

• Selective ion transport through sodium channels is crucial for cellular function.
• Understanding the molecular basis of sodium (Na⁺) versus potassium (K⁺) selectivity in ion channels remains a key challenge.

Purpose of the Study:

• To elucidate the mechanism of Na⁺ over K⁺ selectivity in the bacterial sodium channel Na_vMs.
• To investigate the role of ion hydration shells in channel selectivity.

Main Methods:

• Molecular dynamics simulations were employed to study ion hydration and channel interactions.
• Analysis of ion-binding sites and the structural dynamics of Na⁺ and K⁺ within the channel.

Main Results:

• The Na_vMs channel's selectivity filter forms hydrogen bonds with the Na⁺ hydration shell in an octahedral geometry, mimicking bulk water.
• Na⁺ ions pass through the filter unhindered, while K⁺ ions experience energetic barriers due to their hydration shell being compressed.
• The channel utilizes "secondary chelation," interacting with the ion's hydration shell, unlike K⁺-channels that use "primary chelation".

Conclusions:

• The Na_vMs channel achieves Na⁺ selectivity by interacting with the Na⁺ hydration shell, facilitating high Na⁺ conductance.
• This "secondary chelation" mechanism is a novel explanation for ion selectivity and may be conserved in other channel proteins.