The K+-Channel: High Selectivity, High Flux and Energy Minimization

Diffusion of K⁺ across membranes occurs at a very high rate (10⁸ K⁺/sec) despite the fact that there is a large energetic barrier to transferring a charged species across the  ~40 Å hydrophobic core of the membrane.  The presence of a protein, the K⁺ channel, permits the passive diffusion of K⁺ into a cell.

Structure. The K⁺-channel is a tetrameric protein, the four primarily alpha-helical polypeptides being arranged with 4-fold symmetry around the ion conduction channel, or pore. 

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Side View	Top View

The pore opening is very restricted, being only slightly larger than the ionic diameter of  K⁺  (2.66 Å).  Clustered around the pore opening are 4 aspartates (D80), one from each subunit, creating a favorable negatively charged electrostatic environment (see below).  However, at the outer periphery on the top of the protein lie 4 arginines, which provide a repulsive potential, directing the K⁺ toward the center of the top surface. 

Being an integral membrane protein, the  K⁺ - channel has charged, hydrophilic surfaces exposed to water, and a large hydrophobic membrane-spanning region. (In the molecular surface representations:  Red = negative electrostatic potential; Blue = positive electrostatic potential; White =  non-charged ).

The electrostatic environment around the ion pore opening is very negative (Left Figure). A cut-away view of the protein shows the interior of the ion conduction channel,  ~ 18 Å long, and the extension of the negative electrostatic potential into the pore ( Center and Right Figures).  Also shown is a large open cavity, which presumably is filled with water that extends from the bottom of the protein, to ~20 Å into the interior of the protein.  This has important consequences for the overall energetics (see below).

Selectivity:  The positively charged K⁺ is bound to the pore opening, and  waters of hydration must be stripped from the ion.  Selectivity is achieved by the fact that other cations, such as Na⁺ (diameter = 1.9 Å), do not precisely fit into the pore opening.  Binding of K⁺ also changes the electrostatic potential from negative to positive in the immediate vicinity of the pore opening, however, a negative potential still exists which can attract other K ⁺ ions . 

K+ bound, but not shown	After Binding K+

Stabilization:Interestingly, the ion conduction channel is hydrophobic, the sequence being Gly -Tyr-Gly-Val  with the peptide backbone atoms forming the channel surface.  Initially, stabilization of the K⁺  occurs via chelation with the backbone (shown in white) carbonyl oxygen atoms.  (For sake of simplicity, only two subunits are shown).   Also shown in the figure is that each subunit contributes a short helical segment whose C-termini points to the bottom of the channel.  Thus, the K⁺ can be “drawn” to the bottom of the channel by  the attractive force of the negatively charged end of these helices dipole moment.  

K+ at the top	K+ at the bottom

The movement of the K⁺ along the conduction channel results in a change in the electrostatic potential  in the channel.

Release of the K⁺ into the water filled cavity is achieved when a second K⁺ binds to the pore opening, which results in a repulsive overlap of positive electrostatic potentials, thus “popping” the bottom K⁺ out of the channel.

Minimization: The water filled cavity plays a very important role in the high flux through this protein.  First, it provides waters of hydration for the K⁺ leaving the channel.  Second, by extending so far into the protein interior, the distance which the K⁺ ion travels through a hydrophobic environment is decreased from  ~40 Å to 18 Å.  Thus, this cavity dramatically decreases the large, unfavorable energetic barrier which would exist if the ion were to travel directly through the membrane.Thus, the K⁺ - channel elegantly demonstrates how  the three dimensional structure of an integral membrane protein can exert a high degree of ion selectivity, stabilization of this ion in a hydrophobic environment,  and minimization of the distance, hence decreasing the energetic barrier, that the ion must travel through a hydrophobic environment.