Selective Permeability of Protein Channels. Many
of the protein channels are highly selective for transport of
one or more specific ions or molecules. This results from
the characteristics of the channel itself, such as its diameter,
its shape, and the nature of the electrical charges and
chemical bonds along its inside surfaces.
Potassium channels permit passage of potassium ions
across the cell membrane about 1000 times more readily
than they permit passage of sodium ions. This high degree
of selectivity, however, cannot be explained entirely by
molecular diameters of the ions since potassium ions
are slightly larger than sodium ions. What is the mechanism
for this remarkable ion selectivity? This question
was partially answered when the structure of a bacterial
potassium channel was determined by x-ray crystallography.
Potassium channels were found to have a tetrameric
structure consisting of four identical protein subunits surrounding
a central pore (Figure 4-4). At the top of the
channel pore are pore loops that form a narrow
selectivity
filter. Lining the selectivity filter are carbonyl oxygens.
When hydrated potassium ions enter the selectivity filter,
they interact with the carbonyl oxygens and shed most of
their bound water molecules, permitting the dehydrated
potassium ions to pass through the channel. The carbonyl
oxygens are too far apart, however, to enable them to
interact closely with the smaller sodium ions, which are
therefore effectively excluded by the selectivity filter from
passing through the pore.