Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
3.3.3 Gated Channels
Besides controlling nanopore size and shape, individual molecular transport channels can be gated either mechanically (e.g., ligand gating)2348 or electrically (e.g., voltage gating).1050 Either method might usefully be employed to control molecular transport through the surfaces of medical nanodevices in a process that could very loosely be described as molecular transistor gating.
A good example of mechanical gating in biology is the nicotinic acetylcholine receptor channel, probably the best understood ligand-gated channel.391,396 Nerve impulses are communicated across neuromuscular junctions and autonomic ganglia via neurotransmitters such as acetylcholine. STM images419 confirm that the receptor itself is cylindrical, a bundle of 5 rod-shaped polypeptide subunits arranged like barrel staves with outside diameter ~6.5 nm. The receptor protrudes 6 nm on the synaptic side of the membrane and 2 nm on the cytoplasmic side. The water-filled channel pore lies along the symmetry axis, lined by 5 a-helices, with a 2.2 nm wide mouth on the synaptic surface, a 0.65 nm waist where the structure dives through the cell membrane, and a 2 nm wide cytosolic exit.
Normally, the channel is closed and no ions may pass. In this closed state, the channel is occluded at the waist by a ridge of large residues forming a tight hydrophobic ring. Each subunit has a bulky leucine at the bend in the a-helix, a critical position. When two acetylcholine molecules bind to the receptor, these helices allosterically tilt, shifting the position of the ridges. The pore becomes open because it is now lined with small polar residues rather than by large hydrophobic ones. This conformational change allows 2.5 x 107 Na+ ions/sec to flow through the channel, about 10% of the diffusion-limited rate. (Anions like Cl- cannot enter the pore because they are repelled by rings of negatively charged residues positioned at either end of the receptor.)
Acetylcholine binding opens the gate in less than 100 microsec under physiologic conditions. Subsequent rapid destruction of acetylcholine by acetylcholinesterase, an enzyme tethered to the membrane surface by a covalently attached glycolipid group, closes the gate in ~1 millisec. Much faster gating action (~10-8 - 10-6 sec) could be achieved by nanodevices operating variable-scale nanopores (Section 3.3.2) in response to sensor data or other control signals. Such signals could drive the insertion or retraction of diamondoid rods, wedges, or trapdoors across the channel lumen to regulate the transmission of molecules having specific sizes, shapes, and charge distributions.
Transport channels through nanodevice surfaces may also be gated electrically.392 In contrast to the acetylcholine receptor, which is relatively nondiscriminating and allows both inorganic and organic cations to pass, the voltage-gated calcium channel has a highly discriminating mechanism with a Ca++:Na+ permeation ratio on the order of 1000:1. (The high specificity of the voltage-gated Ca++ channel is a consequence of a single-file pore mechanism involving a pair of specific Ca++ ion binding sites. Selectivity is assured if either of the two sites is occupied by Ca++, as monovalent ions do not bind strongly enough to the free site or generate sufficient electrostatic repulsion to push the first Ca++ ion through the channel.395) Potassium channels1311,3435-3437 are 100 times more permeable to K+ than to Na+, and sodium channels favor the passage of Na+ over K+ by a factor of 12. All three of these voltage-gated channels are important in the generation and conduction of neural action potentials.
A nerve impulse is an electrical signal produced by the flow of ions across the plasma membrane of a neuron. Neuron interiors have high concentrations of K+ and low concentrations of Na+. The resting potential of a neuron is 60 mV. An action potential may be generated when the membrane potential is slightly depolarized to 40 mV. This opens the Na+ voltage-gated channels, rapidly accelerating depolarization to a peak of +30 mV in ~1 millisec. Then Na+ channels close and K+ channels open, allowing K+ ions to exit the cell, restoring the 60 mV resting potential. Only ~1 ion of every ~106 Na+ and K+ ions present in the local extracellular medium and the axoplasm participate in each such nerve impulse.
The sodium channel is a single polypeptide chain with four repeating units. Each repeating unit folds into six transmembrane alpha helices, including one that is positively charged called the S4 helix. The S4 helix is the voltage sensor that triggers the opening of the gate. Three positively charged residues on each S4 helix are paired at the resting membrane potential with negative charges on other transmembrane helices in a staircase geometry. The initial small depolarization event produces a spiral motion of each S4 accompanied by the net movement of one or two charges to the extracellular side of the membrane, essentially turning this left-handed hydrogen-bonded "molecular screw" through a ~60° rotation.395 This outward 0.5-nm translation of the four S4 segments opens the sodium gate by removing a steric barrier to ion flow. The energy cost of moving ~6 electrical charges (~10-18 coul) from the cytosolic to the extracellular side of the membrane against a ~100 mV potential (thus opening the gate in ~75 microsec) is ~100 zJ. Quantum tunneling activation of sodium channels, taking 1-1000 microsec, has been analyzed by Chancey.679
Artificial ion-gated polymer membranes were reported in 1982,393 protein engineering of switchable pore-forming proteins is well-known,880 and "intelligent gels" are being developed that can change size and molecular porosity in response to chemical, electrical or thermal stimuli. In 1998, however, electroporation was a more commonly used method in biological research and a useful technique for "transfecting" cells in genetic studies. Electroporation employs a brief intense pulse of electricity to provide a force that opens cellular pores, enabling the insertion of macromolecules like DNA into cells of interest; laser pulses reduce cell loss to 10% by using a square-wave pulse to effect rapid and reversible pore formation.1295 Artificial pH-gated 200-nm diameter nanopores2335 and natural pH-gating of virion pores in the cowpea chlorotic mottle virus2391 were demonstrated in 1998.
The first true voltage-gated nanomembrane was fabricated by Charles Martin and colleagues in 1995.394 This membrane consists of cylindrical gold nanotubules with inside diameters as small as 1.6 nm. When the tubules are positively charged, cations are excluded and only negative ions are transported through the membrane. When the membrane receives a negative voltage, only positive ions are transported through the tubules. Nanodevices may combine voltage gating with pore size and electrosteric constraints to achieve precision transport control with moderate molecular specificity at diffusion-limited throughput rates. In 1997, an ion channel switch biosensor with sub-picomolar sensitivity and quantitative detection time of ~600 sec was demonstrated by an Australian research group.3039,3040
Last updated on 7 February 2003