The Role of Ion Transition from the Pore Cavity to the Selectivity Filter in the Mechanism of Selectivity and Rectification in the Kv1.2 Potassium Channel: Transfer of Ion Solvation from Cavity Water to the Protein and Water of the Selectivity Filter

Potassium channels generally have a selectivity filter that includes the sequence threonine-threonine-valine-glycine-tyrosine-glycine (TTVGYG). The last five amino acids are conserved over practically the entire range of evolution, so the sequence obviously is necessary to the function of the channel. Here we show by quantum calculations on the upper part of the channel "cavity" (aqueous compartment between the gate and selectivity filter) and lower part of the selectivity filter (SF) how the channel with two sets of four threonines (the channel is fourfold symmetric) effects rectification and selectivity. The threonines are at the location in which the ion transfers from the cavity into the SF; in this calculation they play a key role in selectivity. The channel is also a rectifier. The wild type channel with K+ and three other cases are considered: 1) the upper set of four threonines is replaced by serines. 2) and 3) Related computations with the Na+ and NH4+ ions help to clarify the important factors in moving the ion from the cavity to the SF. In particular, one set of angles (not bond angles, O(T373-C=O) - O(T374-OH) - H(T374-OH)) flips a hydrogen into and out of the ion path, allowing the K+ to go forward but not back. This is essentially a ratchet and pawl mechanism, with the ratchet driven by electrostatics. This also allows a clear path forward for K+ but not for Na+ or NH4+, nor for K+ in a T[->]S mutant. Atomic charges in the lowest positions in the SF are the driving force moving the ion forward, but the O - O - H angle just specified is key to making the "knock-on" mechanism move the ions forward only, using the ratchet with the pawl formed by the hydrogen in the bonds that flip. A water interacts with threonine hydroxyls to allow ion passage, and another water moves together with the K+.