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Unformatted text preview: cell also aid in stabilizing the configuration of the helix at the normal resting potential. Figure 4. The positively charged amino acids in the S4 segment are attracted to negatively charged amino acids (E&D) in S2 and S3. This holds them in place at rest. Depolarization of the cell overcomes this electrostatic attraction, thereby repelling the positively charged S4 segment, which then initiates channel opening. When the cell is depolarized, and the inside of the cell gains positive charges, the positive charges act to repel the positively charged helix and displace it upward into the extracellular space. Each charge on the outside of the helix is shifted upward to the position of the next charged neighbor where it is stabilized. The net effect is to cause the helix to rotate upwards into the extracellular fluid and thereby add positive charges to the outside of the membrane. This movement of positive charges in the membrane repels positive ions in the extracellular fluid and generates a small current, called the gating current. The gating current is evoked before the channel opens and thus it occurs before the ionic current that flows through the open channel. The upward displacement of the S4 segment is coupled to and thus opens the activation gate, as shown in Fig. 5. Recall that there the sodium channel has 4 domains, and each domain has an S4 segment. The S4 segments in domains I, II and III have to move upward to open the activation gate, which allows ionic current to flow through the pore of the channel (Fig. 7, top panel). 5 Figure 5. Schematic arrangement of the voltage sensor in S4 segment. In the resting state, the S4 segment is positioned in the membrane, attracted by the negative charges inside. When + charges enter the cell causing depolarization the + changes in the sensor are repelled by the + charges inside the depolarized cell. The repulsive + charges "push" the S4 segment upward into the extracellular space, which in turn repels + charges in the extracellular fluids, thereby generating the "gating current". The gating current is transient, and lasts only as long as long as the S4 segment moves. The upward movement of the S4 segment is coupled to other segments and thereby opens the activation gate, allowing ions to flow through the pore. The flow of ions through the pore is called the ionic current. The effects of repolarization are exactly the opposite: the increase in negative charges on the inside (and/or the increase in positive charges on the outside) then attract the positive charges on the S4 helix, causing it to move downward back to its normal position. That movement, in turn, is also coupled to and closes the activation gate. Inactivation Activation of Na+ channels, the opening of the channel so that ions flow through the pore, is always shortly followed by inactivation, where a second gate automatically closes and prevents ions from flowing through the pore, even though the cell is depolarized. One might then ask whether the voltage sensor also comprises a portion of the inactivation gate? The answer is no; the voltage sensor and activation gates are not part of the inactivation gate and are independent of it. This was shown in the 1970s when investigators injected the enzyme, pronase, into the axoplasm of the squid giant axon. When an experiment was conducted in voltage clamp, applying positive current to the inside of the cell caused an opening of the activation gate, and thus an inward current carried by Na, and when the applied current was stopped, the activation gate closed. Thus the operation of the activation gate was not affected by pronase. What was affected was inactivation; with pronase, there was no inactivation and inward current, carried by Na+, flowed so long as the cell was clamped at a positive membrane potential. In other words, the activation gate behaved normally but the inactivation gate was eliminated by intracellular pronase. This experiment showed something more; since the pronase was injected into the axon, the inactivation gate must be on the cytoplasmic side of the channel protein. Recent evidence shows that the inactivation gate is on the intracell...
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- Spring '08