Crystal structure of bovine antithrombin III
Abstract
The crystal structure of bovine antithrombin III has been solved by molecular replacement, using a~-antitrypsin (30% sequence homology with human antithrombin III) as a model. The protein crystallizes with two molecules in the asymmetric unit. The use of different resolution ranges was essential in determining the true orientations of the two molecules , because these were the orientations that appeared most consistently in the rotation function, albeit with different scores and slightly different values. Accuracy and correctness of the orientations of the two independent molecules was crucial for the success of the translation function. Stripping off surface atoms from the trial model resulted in a better signal-to-noise ratio in the Crowther-Blow translation function, which turned out to be very discriminative, even though the translational search was performed with less than 30% of the asymmetric unit. In this way, the orientation and position of each of the two independent molecules was separately determined; the crystal packing of the corresponding dimer showed no bad contacts. Phases calculated with this model allowed the determination of heavy-atom sites in a derivative which had previously resisted interpretation. The current R factor after energy minimization and a l ps molecular-dynamics simulation is 32% at 3.6 ,~ resolution.