ATP: glutamine synthetase adenylyltransferase from Escherichia coli B. Purification and properties

Characterization and improved properties of Glutamine synthetase from Providencia vermicola by site-directed mutagenesis

In this study, a novel gene for Glutamine synthetase was cloned and characterized for its activities and stabilities from a marine bacterium Providencia vermicola (PveGS). A mutant S54A was generated by site directed mutagenesis, which showed significant increase in the activity and stabilities at a wide range of temperatures. The K_m values of PveGS against hydroxylamine, ADP-Na₂ and L-Glutamine were 15.7 ± 1.1, (25.2 ± 1.5) × 10^-5 and 32.6 ± 1.7 mM, and the k_cat were 17.0 ± 0.6, 9.14 ± 0.12 and 30.5 ± 1.0 s^-1 respectively. In-silico-analysis revealed that the replacement of Ser at 54th position with Ala increased the catalytic activity of PveGS. Therefore, catalytic efficiency of mutant S54A had increased by 3.1, 0.89 and 2.9-folds towards hydroxylamine, ADP-Na₂ and L-Glutamine respectively as compared to wild type. The structure prediction data indicated that the negatively charged pocket becomes enlarged and hydrogen bonding in Ser54 steadily promotes the product release. Interestingly, the residual activity of S54A mutant was increased by 10.7, 3.8 and 3.8 folds at 0, 10 and 50 °C as compared to WT. Structural analysis showed that S54A located on the loop near to the active site improved its flexibility due to the breaking of hydrogen bonds between product and enzyme. This also facilitated the enzyme to increase its cold adaptability as indicated by higher residual activity shown at 0 °C. Thus, replacement of Ala to Ser54 played a pivotal role to enhance the activities and stabilities at a wide range of temperatures.

Nitrogen assimilation in Escherichia coli: putting molecular data into a systems perspective

We present a comprehensive overview of the hierarchical network of intracellular processes revolving around central nitrogen metabolism in Escherichia coli. The hierarchy intertwines transport, metabolism, signaling leading to posttranslational modification, and transcription. The protein components of the network include an ammonium transporter (AmtB), a glutamine transporter (GlnHPQ), two ammonium assimilation pathways (glutamine synthetase [GS]-glutamate synthase [glutamine 2-oxoglutarate amidotransferase {GOGAT}] and glutamate dehydrogenase [GDH]), the two bifunctional enzymes adenylyl transferase/adenylyl-removing enzyme (ATase) and uridylyl transferase/uridylyl-removing enzyme (UTase), the two trimeric signal transduction proteins (GlnB and GlnK), the two-component regulatory system composed of the histidine protein kinase nitrogen regulator II (NRII) and the response nitrogen regulator I (NRI), three global transcriptional regulators called nitrogen assimilation control (Nac) protein, leucine-responsive regulatory protein (Lrp), and cyclic AMP (cAMP) receptor protein (Crp), the glutaminases, and the nitrogen-phosphotransferase system. First, the structural and molecular knowledge on these proteins is reviewed. Thereafter, the activities of the components as they engage together in transport, metabolism, signal transduction, and transcription and their regulation are discussed. Next, old and new molecular data and physiological data are put into a common perspective on integral cellular functioning, especially with the aim of resolving counterintuitive or paradoxical processes featured in nitrogen assimilation. Finally, we articulate what still remains to be discovered and what general lessons can be learned from the vast amounts of data that are available now.

In vitro studies of the uridylylation of the three PII protein paralogs from Rhodospirillum rubrum: the transferase activity of R. rubrum GlnD is regulated by alpha-ketoglutarate and divalent cations but not by glutamine

P(II) proteins have been shown to be key players in the regulation of nitrogen fixation and ammonia assimilation in bacteria. The mode by which these proteins act as signals is by being in either a form modified by UMP or the unmodified form. The modification, as well as demodification, is catalyzed by a bifunctional enzyme encoded by the glnD gene. The regulation of this enzyme is thus of central importance. In Rhodospirillum rubrum, three P(II) paralogs have been identified. In this study, we have used purified GlnD and P(II) proteins from R. rubrum, and we show that for the uridylylation activity of R. rubrum GlnD, alpha-ketoglutarate is the main signal, whereas glutamine has no effect. This is in contrast to, e.g., the Escherichia coli system. Furthermore, we show that all three P(II) proteins are uridylylated, although the efficiency is dependent on the cation present. This difference may be of importance in understanding the effects of the P(II) proteins on the different target enzymes. Furthermore, we show that the deuridylylation reaction is greatly stimulated by glutamine and that Mn(2+) is required.

Fructose-1,6-bisphosphate, a regulator of metabolism

Fructose-1,6-bisphosphate affects the rate of a large variety of enzyme reactions. In some instances its role as a physiologic effector is well documented. In many cases the effects of fructose bishosphate on particular enzymes have been demonstrated in vitro but the link to physiologic conditions has not yet been established. It is the purpose of this paper to summarize the scattered findings in fructose bisphosphate as an effector of enzyme reactions and to draw some conclusions about the role of the compound in metabolic regulation.

Glutamine-stimulated modification and degradation of glutamine synthetase in hepatoma tissue culture cells

Effects of glutamine on glutamine synthetase (GS) activity of hepatoma tissue culture (HTC) cells were studied with the aid of a specific goat anti-rat GS serum. Immunodiffusion and immunoelectrophoretic tests show that rat liver GS and HTC cell GS are immunologically similar but not identical. Immunotitrations of HTC cell extracts demonstrate that in cells incubated in high concentrations (5 mM) of glutamine, a cross-reacting form of GS with a decreased enzyme-specific activity accumulates. On prolonged incubation of cells in high glutamine, there is net degradation of GS to form immunologically inactive products. Radio-immunoprecipitation experiments show that glutamine acts by accelerating the degradation of preformed GS.

Adenosine triphosphate: glutamine synthetase adenylyltransferase of Escherichia coli: two active molecular forms

Two active forms of purified ATP:glutamine synthetase adenylyl-transferase from Escherichia coli are apparent on polyacrylamide gel electrophoresis at pH 8. The slower migrating component, which is identical to the P(I)-protein fraction of the glutamine synthetase deadenylylating enzyme system, has S(20.w) congruent with 5.1 S and a molecular weight of about 130,000. The more rapidly migrating adenylyltransferase component has S(20.w) congruent with 4.0 S and a molecular weight of about 70,000. During storage at 4 degrees C, the larger adenylyltransferase component (P(I)) converts to the smaller active unit with a concomitant loss of both P(I) deadenylylating activity and soluble protein. It is concluded that the low-molecular weight form of the adenylyltransferase is a subunit of the deadenylylating P(I)-protein.