Abstract
An X-ray study of orthorhombic crystals of cytosolic aspartate transaminase from chicken heart has been carried out at 5 Å resolution. The crystals belong to space group P2 12 12 1, with unit cell dimensions a = 62.7 A ̊ , b = 118.1 A ̊ , c = 124.5 A ̊ . The electron density map has been calculated on the basis of five heavy-atom derivatives. The model of the molecule derived from this map revealed clearly two subunits of similar structure related by a non-crystallographic dyad. The secondary structure of the protein comprises nine helical segments per subunit. The enzyme has been shown to be catalytically active in the crystal form. Removal of the coenzyme from the crystals made it possible to derive from the difference Fourier map the position of the active site in the enzyme molecule. Significant conformational changes have been observed which accompany the interconversion of intermediates of the enzymic reaction.
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