Thursday, December 20, 2007

Transmembrane pump structure

We are clearly now fully in the era of the membrane protein crystal structure--it's clear that even being in the greasy environment of the cell membrane isn't going to protect proteins much longer from the X-ray beams of determined structural biologists. This time, the structure is of the sodium potassium pump, also known as the Na+/K+ ATPase, which sets up the sodium and potassium gradients necessary for numerous cellular functions, from simple maintenance of osmotic balance to uptake of nutrients and nerve impulse transmission. Pharmacologically speaking, this protein is also the target of the simultaneously highly toxic and medically useful cardiac glycosides.

I am not nearly as surprised at the publication of this structure as I was at the adrenoceptor structure mentioned in my last post, or the structure of the amino acid transporter LeuT mentioned on here over a year ago, since multiple structures of a functionally very similar ion pump, namely the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase, had been published years ago (interestingly, some newer structures of this Ca2+ ATPase were just released as well).

I skimmed through the paper in which the Na+/K+ ATPase structure is described, and curiously the authors make absolutely no mention of the cardiac glycoside binding site, despite the fact that there is a clearly visible pocket among the amino acid side chains known to be involved in the binding of these inhibitors. Maybe, a discussion of this site was omitted either due to the modest resolution (3.5 A), and/or because the K+ occupied form of the protein, which is (assumed to be) mimicked by the rubidium ions in the crystal structure, is known to have a lower affinity for cardiac glycosides than the unoccupied form. It seems to me that groups like Qiu et. al. were very much on the right track, though.