POTASSIUM-DEPENDENT MOLECULES 211
selectivity exhibited by these proteins [faithful discrimination between K +
(radius 1.33 Å ) and the smaller Na + (radius 0.95 Å ) ions, for instance], and (2)
how can the K + channels be highly selective and at the same time move ions
through membranes at extremely rapid rates (approaching the diffusion limit).
Potassium ion channels exclude smaller alkali metal cations Li + (radius 0.60 Å )
and Na + (radius 0.95 Å ) but allow permeation of the larger Rb + (radius 1.48 Å )
and Cs + (radius 1.69 Å ). In fact, Rb + is used as a K + analog within potassium
ion channel pores in this KcsA study because it is more electron - dense and
thus easier to visualize by X - ray diffraction. Both Rb + and Cs + were used to
show that the selectivity fi lter contains two ions about 7.5 Å apart. In this study,
diffuse electron density indicates a third ion, perhaps surrounded by a hydra-
tion shell, in the large diameter cavity ( ∼ 10 Å across) below the selectivity fi lter
and one or more poorly defi ned (by electron density) cations continuing
through the pore toward the cell ’ s interior. The ion found in the central cavity
overcomes the electrostatic destabilization resulting from its hydrophobic
environment by assuming a shell of polarizable water. In addition, four pore
helices point toward the center of the cavity in an orientation that imposes a
negative electrostatic potential — attractive to the cation — at the edges of the
cavity. These two features, the aqueous cavity and the helical orientation, solve
the problem of the electrostatic barrier to a cation crossing a lipid bilayer (the
membrane). The authors then ask an important question: What is the signifi -
cance of the internal pore ’ s hydrophobic lining? They believe the answer lies
in the high ion throughput necessary for the pore ’ s effi ciency. If the potassium,
or any other ion being transported, were to be attracted and held by hydro-
philic residues in the pore, rapid ion transport would be impossible. In other
words, the chemical and structural design of this part of the pore facilitate high
ion throughput in this longer segment of the pore. The rate - limiting step for
K+ ions traversing the channel is limited to the shorter 12 - Å length and ∼ 2.5 - Å
diameter of the selectivity fi lter. The selectivity fi lter itself exhibits two essen-
tial features: (1) The main chain carbonyl oxygens of the amino acid residues
(thr75, val76, gly77, tyr78, gly79, TVGYG) lining the fi lter form an oxygen
atom stack suitably positioned to coordinate the dehydrated K + ions as they
pass through, and (2) the valine and tyrosine residues in the conserved sequence
TVGYG have their side chains pointing out of the fi lter toward tryptophan
residues in the pore helices surrounding the fi lter. The aromatic residues inter-
act through van der Waals contacts and through hydrogen bonding — for
instance, between tyr hydroxyls and tryptophan nitrogens. The reference 15
authors state that the structure appears to behave “ like a layer of springs
stretched radially outward to hold the pore open at its proper diameter. ” In
fact, this structure prevents the accommodation of the smaller Li + and Na +
ions with their smaller radii. How does the selectivity fi lter move K + ions along
when attractive forces would keep them in the fi lter? The authors believe that
a single K + ion would be held tightly but that a second K + ion entering causes
a repulsion that allows the fi rst K + to move along.