Elucidation of reaction mechanism ofthe enzymes involved in the syntbssis of photosynthetic pigments

光合作用的有机体利用植物胆蛋白进行光合作用和光传感。这类生物开发了捕光系统，以在其生活环境中完成高效的光合作用。红藻和蓝藻具有巨大的蛋白质-色素复合体，称为藻胆体作为捕光系统，其中藻胆蛋白(藻胆蛋白)被用于捕光色素。植物胆红素是由血红素加氧酶和铁还蛋白依赖的胆碱还原酶(FDBRs)生物合成的。PcyA是fdbr家族中的一员，它通过两个连续的步骤将胆绿素XIα(BV)还原为藻蓝胆素，其中电子由铁氧还蛋白提供。我们先前测定了蓝藻聚球藻PcyA的晶体结构。PCC6803与BV形成复合体。为了阐明PcyA反应的分子机理，我们合成了PcyA第一步还原产物色素，并测定了PcyA-色素络合物的晶体结构。γ-谷氨酰转肽酶催化谷胱甘肽等γ-谷氨酰基化合物的裂解，并将其γ-谷氨酰基转移到水或其他氨基酸和多肽中。氮杂丝氨酸和阿维菌素是GGT的经典不可逆抑制剂，但它们的结合部位和抑制机制尚不清楚。在1.65A分辨率下，我们测定了艾氏杆菌GGT与氮杂丝氨酸和阿维菌素的络合物的晶体结构。它们与Thr391的Oγ原子形成共价键，Thr391是GGT的催化残基。值得注意的是，在氮杂丝氨酸络合物中，氮杂丝氨酸的羰基被Thr391攻击形成四面体中间体。当Acivicin与GGT结合时，单键和双键在其二氢异恶唑环上发生迁移。

Photosynthetic organisms utilize phytobilins, linear tetrapyrrole pigments, for photosynthesis and light sensing. Such organisms develop light harvesting systems to accomplish efficient photosynthesis in their living environments. Red algae and cyanobacteria have giant protein-pigment complexes called phycobilisomes as a light-harvesting system, in which phycobilins (one of phytobilins) are utilized for light-harvesting pigments. Phytobilins are biosynthesized from heme by heme oxygenase and ferredoxin dependent bilin reductases (FDBRs). PcyA, a member of FDBR family, is unique in reducing biliverdin Xiα (BV) to phycocyanobilin by two sequential steps, in which electrons are supplied by ferredoxin. We previously determined the crystal structure of PcyA from cyanobacterium Synechocystis sp. PCC 6803 in complex with BV. To shed light on the molecular mechanism of PcyA reaction, we synthesized the pigment, the product of the first step of PcyA reduction, and determined the crystal structure of PcyA-pigment complex. On the basis of the structure site directed mutagenesis and functional analysis are under way.γ-Glutamyltranspeptidase (GGT) catalyzes the cleavage of such γ-glutamyl compounds as glutathione, and transfer of their γ-glutamyl group to water or to other amino acids and peptides. Azaserine and acivicin are classical and irreversible inhibitors of GGT, but their binding sites and the inhibition mechanisms remained to be defined. We have determined the crystal structures of GGT from Eacherichia coil in complex with azaserine and acivicin at 1.65 A resolution. They form a covalent bond with the Oγ atom of Thr391, the catalytic residue of GGT. Notably, in the azaserine complex the carbonyl of azaserine is attacked by Thr391 to form a tetrahedral intermediate. When acivicin is bound to GGT, a migration of the single and double bonds occurs in its dihydroisoxazole ring.