Atomic Structure of Plant Glutamine Synthetase

Abstract

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...

Herbicide Phosphinothricin Causes Direct Stimulation Hormesis

“…PPT inhibits glutamine synthetase (GS; EC 6.3.1.2), a key enzyme in nitrogen metabolism that catalyses the formation of glutamine from glutamate and ammonia (Eisenberg et al 2000). PPT mimics the tetrahedral transition state in the enzyme's active site (Eisenberg et al 2000;Unno et al 2006), where it gets phosphorylated to phosphinothricin-phosphate (PPT-P) and remains bound irreversibly (Manderscheid and Wild 1986;Logusch et al 1991;Forlani et al 2006). Structurally related methionine sulfoximine is the first discovered GS inhibitor (Ronzio and Meister 1968).…”

Identification and Expression Analysis of Glutamine Synthetase Genes in Ramie (Boehmeria nivea L. Gaud)

“…The conserved cysteine residues at sites 303 and 368 of BnGS2 comprise redox response sites that are important for the activity of plastid GS [7,28]. Unno et al indicated that the presence of Ile at position 161 was required to produce a heat-stabile GS isozyme, respectively [29]. Therefore, the ramie BnGS genes may encode isoenzymes with high activity and heat stability with specific and conserved residues in protein sequences, subsequently exhibiting high nitrogen-utilization efficiency and high environmental adaptability.…”

“…In previous studies, cytosolic GS from wheat [7], barley [9], soybean [30], and maize [29] was classified into three sub-families, with GS2 forming an independent clade. Similar results were obtained in this work, in which two cytosolic BnGS isoforms formed independent branches, and BnGS2 formed one clade, suggesting that BnGS genes in ramie have a different phylogenetic relationship.…”

Aspectos do mecanismo de ação do amônio glufosinato: culturas resistentes e resistência de plantas daninhas

“…A glutamina sintetase é uma importante enzima na assimilação do nitrogênio presente nos estromas dos cloroplastos, fotorrespiração e balanço de carbono em plantas. A localização da glutamina sintetase pode ser classificada em dois grupos: a isoforma citossólica(GS1) e a isoforma plastídica (GS2) (Unno et al, 2006).…”