August 14, 2008 | David F. Coppedge

Membrane Switches Keep Your Brain Humming

Tunnels with rotating gates and rocker switches – this sounds like mechanical engineering.  It’s the machinery that helps power your brain, reported scientists from UCLA and the Pasteur Institute.
    Their paper in Science described the structure of just one of many kinds of membrane channels.1  Cell membranes are lined with elaborate one-way gates.  This one binds a sodium ion to a galactose sugar molecule and brings it inside the cell.  It’s a key player in the process that brings fuel to the brain.  Karpowich and Wang brought it home in their review of the paper in the same issue of Science:2

The average Western adult metabolizes hundreds of grams of carbohydrates per day, half of which is used as an energy source for the brain.  To benefit from these ingested carbohydrates, they must first be broken down into simple sugars, such as glucose, and absorbed through the epithelial cells of the intestine.  The glucose must then be reabsorbed in the kidneys.  On page 810 of this issue, Faham et al. report a major advance in elucidating the molecular mechanism by which this highly effective absorption is realized.

The wording in this statement reveals the stage that molecular biology is in.  Scientists have known about the chemistry of biological processes for decades.  Only now, however, are scientists revealing the mechanics behind that chemistry.  And mechanics it is: the paper describes gates made of protein that rotate open and closed to let the proper molecules in.  Other gates that are members of some of the other 250 families of membrane transporters use other mechanisms.  One of them in a simplified illustration in Karpowich and Wang’s review looks like a rocker switch: the cargo drops into a V-shaped mechanism, which when properly authenticated, inverts into an upside-down V and ejects the cargo outside the cell.
    The sodium galactose transporter studied by Faham et al looks more like a cylindrical gumball machine.  As the outside gate rotates, the cargo drops in.  Once safely enclosed, the inside gate rotates open and out falls the cargo into the cytoplasm.  Faham et al described this an “alternating-access mechanism.”  Since they act as one-way gates, Karpowich and Wang called these “symmetric transporters for asymmetric transport.
    What did the scientists think of these clever machines?  For one thing, the researchers noticed that there are other families of transporters that use similar mechanical methods, but have nothing in common in terms of their protein sequences.  “This structural homology is surprising,” they said.  “….These findings support classification of proteins using criteria such as topological arrangement, molecular function, and unique structural features involved in mechanism, rather than solely on the basis of primary sequence.”  The statement implies that evolutionary relationships are less useful in classifying the machines than functional descriptions.  In fact, evolution was never mentioned in either paper. 

1.  Faham, Watanabe et al, “The Crystal Structure of a Sodium Galactose Transporter Reveals Mechanistic Insights into Na+/Sugar Symport,” Science, 8 August 2008: Vol. 321. no. 5890, pp. 810-814, DOI: 10.1126/science.1160406.
2.  Karpowich and Wang, “Symmetric Transporters for Asymmetric Transport,” Science, 8 August 2008: Vol. 321. no. 5890, pp. 781-782, DOI: 10.1126/science.1161495.

Riddle: where would Darwinism go if it entered a cell by one of these transporter machines?  Answer: first, it would be tagged as foreign and dangerous contraband.  Then, a kinesin would carry it down a microtubule to a proteasome, where it would be cut up into little bits, then ejected outside where it belongs.  Where would Intelligent Design go?  It doesn’t need the transporter, because it’s already in the nucleus, encoded as DNA.

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