The British Biophysical Society

Not Just Any Old Ion - How Proteins Impose Molecular Selectivity

Tony Wilkinson

 

A striking characteristic of the interactions of biological molecules with their target ligands is exquisite selectivity. This selectivity is now well-understood thanks to X-ray diffraction analysis of crystals of biological macromolecules which reveal their three-dimensional structures in atomic detail.

The transporter proteins of bacteria which shuttle molecules into and out of the cell illustrate this well. Like most living cells, bacteria are rather fussy about the types of molecules they admit. Most molecules that do enter bacteria pass through channels in the cell membrane, to which access is carefully controlled. One such transport system is the phosphate transporter which is specific for phosphate ions and will not handle sulphate ions, even though both species are similar in shape (tetrahedral), size and charge (anionic). Access to the phosphate channel is controlled by a receptor protein that captures extracellular phosphate, but not sulphate, and delivers it to the channel. The structural basis of this ion selectivity is apparent from an examination of the structure of the phosphate receptor with phosphate bound to it, determined by X-ray crystallography.

As shown in the top figure, the phosphate receptor completely surrounds the phosphate ion and makes twelve hydrogen bonds with it. Eleven of these involve a hydrogen bond donor group on the protein, interacting with one of the four phosphate oxygens which act as hydrogen bond acceptor groups (arrows pointing from protein to phosphate in the bottom figure).

The exception to this is the interaction of the -OH group of the phosphate which donates a hydrogen bond to an accepting carboxylate group on the protein (arrow pointing from phosphate to protein). The presence of this carboxylate group in the binding site is the cause of the discrimination against sulphate. Unlike phosphate which carries a proton and is best described as HPO₄^2- at physiological pH (around pH 7), sulphate, which is SO₄^2-, does not. Sulphate binding to the phosphate receptor protein would therefore lead to one of the four oxygens of the sulphate anion, which together share a double negative charge, being very close to the negatively charged carboxylate group on the protein. The charge-charge repulsion that would result, would be energetically highly unfavourable. As a result, sulphate binding to the phosphate receptor is not observed.

Reading List

Luecke, H. and Quiocho, F. A. Nature (1990) Vol 347, pages 257-260.

Professor Tony Wilkinson specialises in crystallography and protein engineering at the York Structural Biology Laboratory, York University.