ing medium is minimal and therefore the
energy of the cation is highest. Thus, simple
electrostatic considerations allow us to
understand the functional significance of
the cavity and its strategic location. The
cavity overcomes the electrostatic destabilization
resulting from the low dielectric
bilayer by simply surrounding an ion with
polarizable water. A second feature of the
K1 channel structure also stabilizes a cation
at the bilayer center. The four pore
helices point directly at the center of the
cavity (Fig. 3, A, B, and D). The amino to
carboxyl orientation of these helices will
impose a negative electrostatic (cation attractive)
potential via the helix dipole
effect (18). The ends of the helices are
rather far (;8 Å) from the cavity center,
but all four contribute to the effect.
Therefore, two properties of the structure,
the aqueous cavity and the oriented helices,
help to solve a fundamental physical
problem in biology—how to lower the
electrostatic barrier facing a cation crossing
a lipid bilayer. Thus, the diffuse electron
density in the cavity center (Fig. 6C)
likely reflects a hydrated cation cloud
rather than an ion binding site (Fig. 7).
ing medium is minimal and therefore theenergy of the cation is highest. Thus, simpleelectrostatic considerations allow us tounderstand the functional significance ofthe cavity and its strategic location. Thecavity overcomes the electrostatic destabilizationresulting from the low dielectricbilayer by simply surrounding an ion withpolarizable water. A second feature of theK1 channel structure also stabilizes a cationat the bilayer center. The four porehelices point directly at the center of thecavity (Fig. 3, A, B, and D). The amino tocarboxyl orientation of these helices willimpose a negative electrostatic (cation attractive)potential via the helix dipoleeffect (18). The ends of the helices arerather far (;8 Å) from the cavity center,but all four contribute to the effect.Therefore, two properties of the structure,the aqueous cavity and the oriented helices,help to solve a fundamental physicalproblem in biology—how to lower theelectrostatic barrier facing a cation crossinga lipid bilayer. Thus, the diffuse electrondensity in the cavity center (Fig. 6C)likely reflects a hydrated cation cloudrather than an ion binding site (Fig. 7).
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