A complete molecular model of Taka-amylase A consisting of 478 amino acid residues was built with the aid of amino acid sequence data. Some typical structural features of the molecule are described. A model fitting of an amylose chain in the catalytic site of the enzyme showed a possible productive binding mode between substrate and enzyme. On the basis of the difference Fourier analysis and the model fitting study, glutamic acid (Glu230) and aspartic acid (Asp297), which are located at the bottom of the cleft, were concluded to be the catalytic residues, serving as the general acid and base, respectively.

X-ray absorption near-edge structure spectra of the manganese (Mn) cluster in physiologically native intermediate states of photosynthetic water oxidation induced by short laser flash were measured with a compact heat-insulated chamber equipped with an x-ray detector near the sample surface. The half-height energy of the Mn Kedge showed a period-four oscillation dependent on cycling of the Joliot-Kok's oxygen clock. The flash number-dependent shift in the Mn K-edge suggests that the Mn cluster is oxidized by one electron upon the S(0)-to-S(1), S(1)-to-S(2), and S(2)-to-S(3) transitions and then reduced upon the S(3)-to-S(0) transition that releases molecular oxygen.

The structures of isomaltase from Saccharomyces cerevisiae and in complex with maltose were determined at resolutions of 1.30 and 1.60 Å, respectively. Isomaltase contains three domains, namely, A, B, and C. Domain A consists of the (β/α)8‐barrel common to glycoside hydrolase family 13. However, the folding of domain C is rarely seen in other glycoside hydrolase family 13 enzymes. An electron density corresponding to a nonreducing end glucose residue was observed in the active site of isomaltase in complex with maltose; however, only incomplete density was observed for the reducing end. The active site pocket contains two water chains. One water chain is a water path from the bottom of the pocket to the surface of the protein, and may act as a water drain during substrate binding. The other water chain, which consists of six water molecules, is located near the catalytic residues Glu277 and Asp352. These water molecules may act as a reservoir that provides water for subsequent hydrolytic events. The best substrate for oligo‐1,6‐glucosidase is isomaltotriose; other, longer‐chain, oligosaccharides are also good substrates. However, isomaltase shows the highest activity towards isomaltose and very little activity towards longer oligosaccharides. This is because the entrance to the active site pocket of isomaltose is severely narrowed by Tyr158, His280, and loop 310–315, and because the isomaltase pocket is shallower than that of other oligo‐1,6‐glucosidases. These features of the isomaltase active site pocket prevent isomalto‐oligosaccharides from binding to the active site effectively.

When expressed in Escherichia coli, cytosolic poplar glutaredoxin C1 (CGYC active site) exists as a dimeric iron-sulfur-containing holoprotein or as a monomeric apoprotein in solution. Analytical and spectroscopic studies of wild-type protein and site-directed variants and structural characterization of the holoprotein by using x-ray crystallography indicate that the holoprotein contains a subunit-bridging [2Fe-2S] cluster that is ligated by the catalytic cysteines of two glutaredoxins and the cysteines of two glutathiones. Mutagenesis data on a variety of poplar glutaredoxins suggest that the incorporation of an iron-sulfur cluster could be a general feature of plant glutaredoxins possessing a glycine adjacent to the catalytic cysteine. In light of these results, the possible involvement of plant glutaredoxins in oxidative stress sensing or ironsulfur biosynthesis is discussed with respect to their intracellular localization.iron-sulfur protein ͉ plant

Plants provide nourishment for animals and other heterotrophs as the sole primary producer in the food chain. Glutamine synthetase (GS), one of the essential enzymes for plant autotrophy catalyzes the incorporation of ammonia into glutamate to generate glutamine with concomitant hydrolysis of ATP, and plays a crucial role in the assimilation and re-assimilation of ammonia derived from a wide variety of metabolic processes during plant growth and development. Elucidation of the atomic structure of higher plant GS is important to understand its detailed reaction mechanism and to obtain further insight into plant productivity and agronomical utility. Here we report the first crystal structures of maize (Zea mays L.) GS. The structure reveals a unique decameric structure that differs significantly from the bacterial GS structure. Higher plants have several isoenzymes of GS differing in heat stability and catalytic properties for efficient responses to variation in the environment and nutrition. A key residue responsible for the heat stability was found to be Ile-161 in GS1a. The three structures in complex with substrate analogues, including phosphinothricin, a widely used herbicide, lead us to propose a mechanism for the transfer of phosphate from ATP to glutamate and to interpret the inhibitory action of phosphinothricin as a guide for the development of new potential herbicides.Inorganic nitrogen is an essential, often limiting nutrient for plant growth and development. In most natural soils, nitrate is the major form of inorganic nitrogen. After uptake of nitrate, plants first reduce it to ammonia, and then assimilate it into an organic compound as an amide moiety of glutamine. Because glutamine synthetase (GS) 7 catalyzes the very step of assimilation of inorganic nitrogen and because the amide moiety of glutamine is utilized as the donor of amino residue to synthesize a number of essential metabolites such as amino acids, nucleic acids, and amino sugars, glutamine synthesis by plant GS is the cornerstone of plant productivity and thus nitrogen nourishment of all animals on the Earth. For this reason, the importance of plant GS is comparable with that of ribulose-1,5-bisphosphate carboxylase/oxygenase, the carbon dioxide assimilating enzyme (1).Comparison of the primary structures of GSs from prokaryotes and eukaryotes, results in plant GS being categorized as type II, this type commonly occurring in eukaryotes including animals (2). In contrast, type I GS is widely found in prokaryotes (2). The regulatory mechanisms of type I GS activity such as adenylylation and metabolite feedback have been thoroughly characterized (3). The crystal structures of GS from Mycobacterium tuberculosis (4) and Salmonella typhimurium (5) have been determined and the proteins shown to be dodecameric, with each dodecamer being composed of one identical subunit with a molecular mass of about 52 kDa. Types I and II GSs are thought to share a common ancestor but to have diverged into the two types at a very early stage during molecula...