As I understand it, the variety of K+ channels can be attributed to
the fact that voltage-gated K+ channels, as opposed to Ca++ and Na+,
are made up of 4 domains from separate proteins. That is, the 4 domains
which go to make up any voltage-gated ion channel are different
proteins in K+ channels. Thus, because these 4 domains can each be any
of 20 possible protein structures, a whole lot of K+ channels could
theoreticaly exist. In fact, only about 20 or so voltage-gated ones
are seen (I think).
In regards to function, depending on which of these domains are used,
channel open-duration will vary. That is, the time which a single K+
channel is open varies depending on what kind of domains it's got. Such
variance influences the single channel conductance, the microscopic
current flow, &c.
