No abstract

From the similarities between the three presented structures, it is concluded that these represent the enzymatically active R state conformer. A mechanism for the deaminase reaction is proposed. It comprises steps to open the pyranose ring of the substrate and a sequence of general base-catalyzed reactions to bring about isomerization and deamination, with Asp72 playing a key role as a proton exchanger.

The intracellular concentration of the enzyme glucosamine-6-phosphate synthase, encoded by the gene gimS in Escherichia coli, is repressed about threefold by growth on the amino sugars glucosamine and N-acetylglucosamine. This regulation occurs at the level of gimS transcription. It is not due just to the presence of intracellular amino sugar phosphates, because mutations which derepress the genes of the nag regulon (coding for proteins involved in the uptake and metabolism ofN-acetylglucosamine) also repress the expression ofglmS in the absence of exogenous amino sugars.

The differences in active-site geometry are related to alterations in the substrate-binding properties associated with the allosteric transition. The rigid nature of the two mobile structural units of each monomer seems to be essential in order to explain the observed kinetics of the deaminase hexamer. The triggers for both the homotropic and heterotropic allosteric transitions are discussed and particular residues are assigned to these functions. A structural basis for an entropic term in the allosteric transition is an interesting new feature that emerges from this study.

Wild-type Escherichia coli grows more slowly on glucosamine (GlcN) than on N-acetylglucosamine (GlcNAc) as a sole source of carbon. Both sugars are transported by the phosphotransferase system, and their 6-phospho derivatives are produced. The subsequent catabolism of the sugars requires the allosteric enzyme glucosamine-6-phosphate (GlcN6P) deaminase, which is encoded by nagB, and degradation of GlcNAc also requires the nagA-encoded enzyme, N-acetylglucosamine-6-phosphate (GlcNAc6P) deacetylase. We investigated various factors which could affect growth on GlcN and GlcNAc, including the rate of GlcN uptake, the level of induction of the nag operon, and differential allosteric activation of GlcN6P deaminase. We found that for strains carrying a wild-type deaminase (nagB) gene, increasing the level of the NagB protein or the rate of GlcN uptake increased the growth rate, which showed that both enzyme induction and sugar transport were limiting. A set of point mutations in nagB that are known to affect the allosteric behavior of GlcN6P deaminase in vitro were transferred to the nagB gene on the Escherichia coli chromosome, and their effects on the growth rates were measured. Mutants in which the substrate-induced positive cooperativity of NagB was reduced or abolished grew even more slowly on GlcN than on GlcNAc or did not grow at all on GlcN. Increasing the amount of the deaminase by using a nagC or nagA mutation to derepress the nag operon improved growth. For some mutants, a nagA mutation, which caused the accumulation of the allosteric activator GlcNAc6P and permitted allosteric activation, had a stronger effect than nagC. The effects of the mutations on growth in vivo are discussed in light of their in vitro kinetics.Escherichia coli is renowned for its ability to use a diverse array of organic compounds as sources of carbon and energy. However, different carbohydrates do not produce the same growth yield. One of the best carbon sources, after glucose, is N-acetylglucosamine (GlcNAc), an amino sugar. The other common amino sugar, glucosamine (GlcN), is also a source of carbon and nitrogen for E. coli, but use of this sugar results in lower growth rates than use of GlcNAc. Both GlcNAc and GlcN are phosphotransferase system (PTS) sugars (38) in E. coli so that their uptake occurs concomitantly with their phosphorylation, which produces intracellular N-acetylglucosamine 6-phosphate (GlcNAc6P) and glucosamine 6-phosphate (GlcN6P). GlcNAc6P is first deacetylated by the nagA gene product, GlcNAc6P deacetylase (NagA; EC 3.5.1.25); this produces GlcN6P, which is then subject to deamination and isomerization by the nagB-encoded GlcN6P deaminase (NagB; EC 3.5.99.6; formerly GlcN6P isomerase), resulting in fructose 6-phosphate, which enters the glycolytic pathway, and ammonia. Thus, the last amino sugar-specific step in the catabolism of both GlcN and GlcNAc depends on the nagB-encoded enzyme (Fig. 1A).GlcNAc6P has many functions during the metabolism of the amino sugars, both as a metabolite and as a regulator. As well ...