Asparagine synthetase

Asparagine synthetase (or aspartate-ammonia ligase) is a chiefly cytoplasmic enzyme that generates asparagine from aspartate. This amidation reaction is similar to that promoted by glutamine synthetase. The enzyme is ubiquitous in its distribution in mammalian organs, but basal expression is relatively low in tissues other than the exocrine pancreas.(See Template:Leucine metabolism in humans – this diagram does not include the pathway for β-leucine synthesis via leucine 2,3-aminomutase) Asparagine synthetase (or aspartate-ammonia ligase) is a chiefly cytoplasmic enzyme that generates asparagine from aspartate. This amidation reaction is similar to that promoted by glutamine synthetase. The enzyme is ubiquitous in its distribution in mammalian organs, but basal expression is relatively low in tissues other than the exocrine pancreas. Above average presence of asparagine synthetase in certain leukemia strains has been linked to be a significant contributing factor of chemotherapy resistance, particularly to the chemotherapy drug, L-asparaginase. Escherichia coli derived asparagine synthetase is a dimeric protein with each subunit folding into two distinct domains. The N-terminal region consists of two layers of six-stranded antiparallel β-sheets between which is the active site responsible for the hydrolysis of glutamine. The C-terminal domain consists of a five-stranded parallel β-sheet flanked on either side by α-helices. This domain is responsible for the binding of both Mg2+ATP and aspartate. These two active sites are connected by a tunnel lined primarily with backbone atoms and hydrophobic, nonpolar amino acid residues. Structural characterization of asparagine synthetase from mammalian sources have been difficult due to the low abundance and instability of the enzyme during purification procedures. Using information from Escherichia coli derived asparagine synthetase, some basic mechanisms of the enzyme have been understood. The N-terminal active site catalyzes glutamine hydrolysis to yield glutamate and ammonia. The C-terminal active site catalyzes activation of the side-chain carboxylate of aspartate to form an electrophilic intermediate, β-aspartyl-AMP (βAspAMP) 1, and inorganic pyrophosphate (PPi). The tunnel that links the two active sites allows for the passage of an ammonia molecule to act as a common intermediate to couple the two half-reactions carried out in the independent active sites of the enzyme. Thus, after being released in, and channeled from, the glutaminase site, the ammonia molecule attacks the bound βAspAMP 1 to give asparagine and AMP via a tetrahedral intermediate. In plants, inorganic nitrogen is taken up from the environment in forms of nitrate or ammonium. Assimilation of this nitrogen into asparagine for use in nitrogen recycling, transport, and storage is an essential process for plant development, making asparagine synthetase vital to maintaining these asparagine reserves. Specific events in development which depend on asparagine synthetase are nitrogen mobilization in germinating seeds, nitrogen recycling and flow in vegetative cells in response to biotic and abiotic stresses, and nitrogen remobilization from source to sink organs. In mammals, asparagine synthetase expression has been found to be linked to cell growth, and its mRNA content is linked to changes in the cell cycle. Hamster BHK ts11 cells produce an inactive asparagine synthetase enzyme, and this loss of asparagine synthetase activity directly led to cell cycle arrest in the cells as a consequence of a depletion of cellular asparagine. Upregulation of asparagine synthetase mRNA was observed as well in these hamster cells. Other experiments demonstrated that quiescent rat thyroid cells entering S phase as a result of thyroid-stimulating hormone treatment was matched with a concurrent increase in asparagine synthetase mRNA content. There seem to be two major groups of asparagine synthetase: Cancerous cells exhibit rapid growth and cell division and subsequently have an increased nutritional need. The particularly low-level expression of asparagine synthetase in primary acute lymphoblastic leukemia (ALL) and numerous ALL cell lines, as compared to that of normal cells, makes asparagine depletion an effective method of treatment due to the cells' unusual dependency on circulating serum asparagine as a necessary nutrition for growth. As a result, L-asparaginase is a common chemotherapy drug utilized in the treatment of ALL and may have applications in other asparagine synthetase negative cancers, such as lymphomas, due to its aspariginase activity to deplete serum asparagine. This depletion of serum asparagine leads to a subsequent rapid efflux of cellular asparagine, which is immediately acted upon and destroyed by the L-asparaginase as well. Due to the transient response from these susceptible cancers in reaction to the asparagine depletion, tumor growth is significantly inhibited due to nutritional deficiency.