| X-ray crystallographic studies have been performed for the structure determination of two proteins, branching enzyme and angiostatin. In these studies isomorphous replacement, anomalous dispersion and molecular replacement methods were used to calculate the electron density maps of the aforementioned proteins.;Branching enzyme is one of three enzymes involved in the biosynthesis of starch in plants and glycogen in animals and bacteria. It has an important role in the determination of the final structures of starch and glycogen. This enzyme catalyzes the cleavage of alpha-1,4 glucosidic bonds and subsequently transfers this chain into the alpha-1,6 position. The conversion of this linear polysaccharide into a branched network not only makes starch and glycogen more reactive to both synthesis and digestion, it also assures its solubility in the cell. Insoluble glycogen caused by mutations in the branching enzyme gene (Glycogen Storage Disease type IV) is a lethal genetic disease for which no clinical treatment is known.;Escherichia coli branching enzyme was crystallized and high-resolution data to 2.3 A resolution was collected. Phasing information was obtained using isomorphous replacement and anomalous dispersion methods. This, in addition to four fold averaging, led to the calculation of an electron density map. The structure shows that branching enzyme presents the central (alpha/beta) barrel catalytic domain that is conserved among members of the alpha-amylase family of enzymes, to which branching enzyme belongs. In addition, a mechanism for branching enzyme has been proposed based on sugar substrate modeling and comparison of the branching enzyme structure with other members of the alpha-amylase family of enzymes.;The structure of human angiostatin was determined by molecular replacement at 1.75 A resolution. The structure revealed that all three kringle lysine-binding sites contain a bound bicine molecule, while those of kringle 2 and kringle 3 are cofacial. Moreover, the separation of the kringle 2 and kringle 3 lysine binding sites is sufficient to accommodate the alpha-helix of the 30 residue peptide VEK-30 found in the kringle 2/VEK-30 complex. Together the three kringles produce a central cavity suggestive of a unique domain where they may function in concert. (Abstract shortened by UMI.). |
