Structural analysis of vitamin A transport system

The method of isoraorphous replacement has been used to determine the crystal structure of human plasma prealbumin (Mwt. 56 000) Three derivatives were measured to a resolution of 2.5 Å and a mercury derivative was re-measured with a long x-ray count time to improve the quality of the estimates of anomalous scatter. A protein Fourier synthesis has been calculated using phases determined from the anomalous and isomorphous differences. The conformation of the two polypeptide chains in the asymmetric unit of P21212 (the crystal space group) has been determined. The chains in each monomer have an apparently identical conformation They are nearly fully extended and form an extensive network of (beta)-structure, Eight strands of (beta)-sheet in each monomer form a flattened cup shape the interior of which appears to be entirely hydrophobic. The ends and bottom of the cup are closed by loops in a random coil conformation. One loop contains a right-handed (alpha)-helix of about two turns. The top of the cup is closed by the second monomer,so that the front of this unit is a continuous 8-stranded (beta)-pleated sheet. The back of this is also formed by an 8-stranded p-sheet,but this is less regular. The dimers formed in this way interact so that their front sheets are opposed at a distance of 10 Å. The thyroxine-binding site has been identified within the channel formed by the (beta)-sheets of opposed dimers. There are two possible binding sites, only one of which may be occupied in one molecule (on chemical evidence).</p