Ca gated potassium channel

A model for the Ca++ gated potassium channel is shown in Figure 3.5, after Schumacher and Adelman [22] based on [23] and [24]. Here the shaded horizontal slabs represent are the upper (outside) and lower (inside) cell walls. The envisioned ion channel, as closed, is shown on the left (a). The structure, which extends across the cell membrane, has an upper "selectivity filter" (which passes only potassium ions), a central cavity, and a Ca++-controlled gate (lower) which opens the channel (right view, b). In this condition the channel will pass potassium ions, but not other ions. The nanometer scale dimensions of the ancient molecular structure that is modeled in Figure 3.5 are similar to those of the rotary motors shown above.

Figure 3.5 Model for Ca++-gated K channel, after [22] Voltage-gated potassium channel

The long-standing interest in the voltage-gated K+ channel, stems in part from the discovery of Hodgkin and Huxley [25], that nerve impulses are generated by the flux of ions across the lipid membrane of a neuron. These ion channels are responsible for bringing a nerve impulse to an end so a neuron can prepare to fire again. This unit is key to the operation of the central nervous system. It is clear that the voltage difference across the cell membrane serves to open and shut the channel to the flow of potassium ions.

Two competing contemporary models for this channel are depicted in Figure 3.6 [26,27]. In the currently accepted conventional model (upper) the open pore configuration (left) is closed by the lifting of positively cylindrical (helical) units upward by the electric potential difference. In the competing model (lower), closing of the pore to potassium flow results from rotation of "paddle" structures in response to the potential difference.

In both cases the dimensions are nanometer scale and flows of electrical currents are controlled by potential differences. These are the functions of a transistor in elec-

a Conventional model ipttpii tfes | ' *

b New model b New model

tronics. These ancient biological devices have dimension typically 10 nm, and are thus nanoscale natural versions of transistors.

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