glycogen synthase inhibits glycogen synthesis
Mutations affecting the glycogen biosynthetic process in E. coli have been reported to occur that are accompanied by changes in cell size, growth, morphology, surface adherence, and synthesis of exopolysaccharides necessary for biofilm production (, ). Therefore, to explore whether or not aspP mutations may result in such pleiotropic changes, comparison of the cell growth patterns and biofilm formation capacities were performed by using both transformed and untransformed bacteria.
In order to be used for glycogen synthesis, ..
In contrast, the role of mal enzymes in glycogen synthesis and metabolism is not known, although it was suggested briefly in an early review by Preiss (). In this study we tested the amounts, the sizes, and the branch chain distributions of glycogen in isogenic mutants lacking MalP, MalQ, MalZ, and GlgA. We found that maltose/maltodextrin metabolism can lead to the formation of glycogen even in the absence of glycogen synthase. It is the activity of MalP (maltodextrin phosphorylase) that determines the strength of this pathway.
Our computer-assisted analyses using the PSORT algorithm () have revealed that ASPPase has characteristics of a typical cytosolic protein of Gram-negative bacteria (data not shown). It is therefore highly conceivable that, in contrast to other bacterial nucleotide-sugar pyrophosphatases that act as “ectoenzymes” involved in the scavenging of extracellular nucleotide-sugars (, ), ASPPase is in the same compartment of the cell as are the established processes of ADPG synthesis and utilization. Because it exhibits high affinity toward ADPG (Table ), thus competing with glycogen synthase for the same substrate, it can be readily predicted that, unless ASPPase is highly regulated, ADPG will be instantaneously hydrolyzed to produce glucose-1-phosphate and AMP, thus preventing glycogen biosynthesis. However ASPPase can be inhibited by nonsubstrate molecules. Moreover, results presented in Fig. showing a glucose-dependent reciprocal relationship between ASPPase activities and glycogen content in E. coli strongly indicate that aspP expression can be adjusted to the physiological status of the cell. Needless to say, further research testing possible mechanisms that regulate the expression of aspP is essential to clarify the contribution of ASPPase in the control of glycogen biosynthesis in bacteria.
Requires inorganic pyrophosphatase to push the reaction to the right.
Glycogen synthase can only add glucose to pre-existent glycogen chains,i.e, it is unable to start the synthesis of a new glycogen molecule. Glycogen synthesis is started by the addition oa a glucose molecule to a tyrosine residue present in the active site of a protein called glycogenin. After addition of around seven more glucose molecules, the new glycogen chain is ready to be acted upon by glycogen synthase
Glycogen synthase (UDP-glucose-glycogen ..
An adenosine diphosphate sugar pyrophosphatase (ASPPase, EC ) has been characterized by using Escherichia coli. This enzyme, whose activities in the cell are inversely correlated with the intracellular glycogen content and the glucose concentration in the culture medium, hydrolyzes ADP-glucose, the precursor molecule of glycogen biosynthesis. ASPPase was purified to apparent homogeneity (over 3,000-fold), and sequence analyses revealed that it is a member of the ubiquitously distributed group of nucleotide pyrophosphatases designated as “nudix” hydrolases. Insertional mutagenesis experiments leading to the inactivation of the ASPPase encoding gene, aspP, produced cells with marginally low enzymatic activities and higher glycogen content than wild-type bacteria. aspP was cloned into an expression vector and introduced into E. coli. Transformed cells were shown to contain a dramatically reduced amount of glycogen, as compared with the untransformed bacteria. No pleiotropic changes in the bacterial growth occurred in both the aspP-overexpressing and aspP-deficient strains. The overall results pinpoint the reaction catalyzed by ASPPase as a potential step of regulating glycogen biosynthesis in E. coli.
Glycogen Synthesis and Metabolism
In muscle it turns out that glycogen synthesis/breakdown is controlled by a very complex system enabling both rapid response to emergencies and exquisite overall control of the opposing activities to respond to a variety of situations. This is accomplished through the system. (The diagram shown is actually a simplified representation, especially of the synthase enzyme, which turns out to have 9 phosphorylatable sites which are phosphorylated by a number of different kinases responding to different complex physiological situations and with varying responses by the enzyme.)