Gated pores

It functions by generating a gated pore in the membrane to permit passage of glucose the pore is conformationally dependent on the presence of glucose and can oscillate rapidly (about 900 times/s). 

Anions of lipid-soluble phenols such as 2,4-dini-trophenol can serve as effective carriers of protons (Chapter 18). However, proteins usually serve as the natural carriers, both of protons and of other ions. A protein is sometimes pictured as rotating to present the solute-binding surface first to one side, then to the other side of a membrane. However, gated pores or channels are probable for most biological transport. 

Cation channels. When a nerve impulse passes along an axon gated pores or channels specifically permeable to Na+ and K+ open for short periods of... 

Sokolov S, Scheuer T, Catterall WA. Gating pore current in an inherited ion channelopathy. Nature 2007 446 76-78. 

Struyk AF, Cannon SC. A Na+Channel mutation linked to hypokalemic periodic paralysis exposes a proton-selective gating pore. J. Gen. Physiol. 2007 130 11-20. 

Deviation from expected kinetic behavior for the gated pore model has also been observed in the case of the two electrogenic transporters, the glutamate/aspartate and the adenine nucleotide carriers. 

On the other hand, in the m state, the exposure of some lysines change as would be expected for a gated pore carrier. Thus, lysine 42 switches from the cytosolic side to the matrix side. Lysines 91, 93 and 95 appear on the cytosoUc face, and 106 disappears. Lysine 146 is exposed only on the matrix side and 162 appears from the cytosolic side. A rather unsatisfactory aspect of the model is the necessity for the hydrophilic segment 230-259 to cross the membrane. This appears necessary if hydrophobic segment V spans the membrane, 198 and 205 are cytosolic, and 259, 262 and 267 are also cytosohc. Since these last lysines are very weakly reactive it is possible they are deep in a pocket, not shown in the model. 

Nevertheless, most aspects of the data are in excellent accord with a very asymmetric gated-pore model. It is tempting to imagine a third dimension by curving this two-dimensional model out of its plane in such a way that lysine residues 42, 48,106 and 146 are all contained within the same channel. It is apparent that the data are most satisfactorily explained by channels in the structure. An important unanswered question is whether each monomer forms an independent channel with an independent substrate binding area, or whether the dimeric structure is required for formation of a single channel. 

In general, mechanistic studies of the mitochondrial metabolite transporters carried out in the last five years have provided evidence supporting a gated-pore, sequential model for transport. Structural studies of the adenine nucleotide carrier are more advanced than studies of the other transporters, and these provide the most clear cut evidence. 

Islas LD, Sigworth F J 2001 Electrostatics and the gating pore of Shaker potassium channels. J Gen Physiol 117 69—89... 

Water-soluble poly(acrylic acid)-based nanocapsules with reversible pH- and ionic strength-dependent swelling transition were prepared by Meier et al. [224], During this transition gated pores in the spherical polymer shells are opened (closed), which enables free molecular exchange between the interior of the hollow sphere and the bulk medium. This pH-switchable control of permeability can trigger release of encapsulated cargo from the polyelectrolyte spheres. 

Connexin protein is unique in forming a gated pore through two membranes. Two questions particular to this structure are (1) how is the junctional channel assembled (e.g., how do the two hemichannels find each other and interact to form a stable structure) and (2) what is the interaction between... 

The Na, K -ATPase spans the plasma membrane, much like a gated pore, with a binding site for 3 Na ions open to the intracellular side (Fig. 10.12). Energy from... 

Fig. 27.11. Facilitative transport. Transport of glucose occurs without rotation of the glucose molecule. Multiple groups on the protein bind the hydroxyl groups of glucose and close behind it as it is released into the cell (i.e., the transporter acts like a gated pore ). O = outside I = inside.

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