Mechanisms of Ion Pumps/Channels

[fafalone]

The currently accepted model for membrane transport of ions and other small molecules is a channel or a pump. However, it is not clearly understood how these are selective. Silicon rubber has demonstrated the same selectivity as a plasma membrane, which calls into question whether the channels and pumps actually do the transport. The issue of energy expenditure also is raised: Na⁺/K⁺ pumps alone would have to be using 40-50% of all ATP energy within the cell. What is everyones opinion of alternative hypotheses for the role of these so-called channel and pump proteins, especially considering they may function as receptors rather than channels or pumps at all?

[Giles]

Do you have a link so I can see more precisely what is being proposed? It was my understanding the pumps only generated a few mV anyway.

 

All i can say for now is that membrane selectivity can be altered by mutating the polypeptide sequence on the cytoplasmic side of the sodium and potassium influx channels, and that poisons inhibiting ATP synthesis also inhibit Na+ efflux.

[fafalone]

The idea is from a book I'm reading, Cells, Gels, and the Engines of Life by Gerald Pollack.

 

Journal articles cited relavent to the new idea are

Sachs and Qin, 1993

Lev et. al., 1993

Whittam, 1961

Ling, 1962

[Giles]

Is it a very new idea? i was told last year that the pump is only thought to account for a difference of few mV of membrane potential anyway.

 

the molecular structure of the potassium channel has been resolved in the last year or so, and it does appear to be a channel. the paper was published in Nature but i don't have a reference for you :/

[fafalone]

The voltage changes on the membrane were identical to the changes of silicon rubber (the first paper I cited)... and a channel is different than a pump.

[Giles]

Are you proposing that the membrane itself is selective in neurons etc?

 

heh, i know the pumps and channels are different, but you posted about both. the points in my post were unconnected :/

[fafalone]

I'm questioning whether pumps/channels pump/channel at all. The papers cited show identical voltage patterns (selectivity) whether its silicon rubber or normal membrane channel... how are we so confident pump/channel proteins aren't just receptors?

[Giles]

What do you mean by receptor? I assume you're referring to some alternate model for changing the selectivity of the membrane, but i can't imagine what it might be.

 

Surely this would also entail overturning conventional wisdom on the impermeability of plasma membranes to high charge concentrations? I got the impression this was a settled matter of chemistry but i could be mistaken, as I haven't the savvy to check that personally.

 

Is there a physical chemist in the house?

[fafalone]

Even the papers that established these ideas talked about getting some unexpected results... we're not currently able to observe in real time an ion moving across the membrane. It occurs in a matter of picoseconds, so we can't actually see exactly where its going through; only how much gets into a cell (by using radioactive tracing).

 

I'll bring more information over following weeks; these are the ideas we're talking about once a week in my cell and molecular biology book club with the director of honors research in biology here.