The roles of MalQ enzymes from various microbes have been investigated. The amino acid sequence deduced from MalQ was 85% identical to the sequence of (Enterobacteriaceae) and 49% identical to . The disproportionation enzyme, D-enzyme (corresponding to MalQ in ), is considered to play an important role in starch metabolism in plants. The plant pathway of maltose metabolism is similar to that of bacteria, including Ruzanski successfully replaced the plant 4-α-glucanotransferase (DPE2; corresponding to MalQ) with the bacterial amylomaltase MalQ. Maltose metabolism was compared between and . Interestingly, the D-enzyme in may be equally involved in starch anabolism and catabolism. Similarly, given the main physiological role of MalQ in 2‒40, it has been suggested that MalQ may have roles not only in maltose utilization but may be involved in metabolism of maltodextrins formed during glycogen degradation in this bacterium. Therefore, enzymes involved in both the maltose system and biosynthesis of glycogen may participate in the degradation process in .
In a wild-type E. coli strain (containing GlgA), the contribution of the maltose system to the synthesis of glycogen when growth is on glucose, maltose, or maltodextrins can be seen only by the increase in the total amount of glycogen (from 0.03 to 0.05 and 0.07 mg/mg protein) (). This increase is indeed due to maltodextrin metabolism and not due to a stimulation of the GlgA-dependent pathway, as can be seen by analyzing the effect that deletions in the maltose enzymes have on glycogen synthesis.
This invention relates to glycogen biosynthesis enzymes in plants
Here we describe a novel mode of glycogen formation in E. coli that is based on the metabolism of maltose and maltodextrins and that is independent of the classical glycogen-synthesizing enzyme glycogen synthase (GlgA). There are four enzymes that participate in maltose/maltodextrin metabolism. Three are cytoplasmic: MalQ (amylomaltase), MalP (maltodextrin phosphorylase), and MalZ (maltodextrin glucosidase). The fourth is MalS, a periplasmic amylase. In contrast to the classical synthesis pathway involving GlgA, neither sugar phosphorylation nor ADP-dependent sugar activation is necessary for the polymerization of the α-1,4-linked glucosyl backbone. Instead, maltose and small maltodextrins (up to a chain length of seven glucosyl units) that are taken up by the binding protein-dependent ABC transporter are polymerized by MalQ to long dextrins that can function as substrates for the branching enzyme GlgB to form glycogen. Since glucose released by the action of MalQ is continually removed by glucokinase, it is no longer an acceptor for the transfer reaction of MalQ, nor does it inhibit MalQ activity. Thus, the chain lengths of the maltodextrins increase until they reach the minimal length to become substrates for the branching enzyme GlgB (). The molecular weight and side chain distribution of the product formed are very similar to those of the glycogen formed by the GlgA-dependent mechanism.
Biochemical Journal Nov 01, 1951, 50 ..
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.
Glycogen Synthesis (Glycogenesis) Pathway - YouTube
The present publication cannot make any contribution to the understanding of carbon or energy flux from glycogen into glycolysis or gluconeogenesis. We determined the steady-state concentration of glycogen under standardized growth conditions by varying the composition of the maltose enzymes with the implicit assumption that the degradation of glycogen remained unaltered. Clearly, the steady-state amount of glycogen represents a balance between synthesis and degradation. Continuous degradation of glycogen must occur. For instance, deletion of glgP, encoding glycogen phosphorylase, strongly increased the amount of glycogen (). Also, the phenomenon of endogenous induction of the maltose system by maltotriose formed by the degradation of glycogen, as discussed above, is evidence of continuous degradation of glycogen. Therefore, we conclude that the observed changes in the amount of glycogen upon changing maltose enzymes were due to changes in the rate of glycogen synthesis. Alterations in enzymes other than the maltose enzymes have been shown to affect the amount of glycogen. For instance, the overproduction of an ADP sugar pyrophosphatase dramatically reduced the amount of glycogen by degrading ADP-glucose, the donor substrate for GlgA ().
Glycogenesis is the process of glycogen synthesis, ..
The synthesis of glycogen in bacteria occurs when they are grown with limited nutrients but an abundance of a carbon source (, ). Escherichia coli accumulates glycogen at levels of more than half of its cell mass under optimal conditions. The glycogen gene cluster in E. coli consists of two operons oriented in tandem, glgBX and glgCAP, encoding enzymes that synthesize and degrade glycogen (). The encoded enzymes are a branching enzyme (glgB), a debranching enzyme (glgX), an ADP-glucose pyrophosphorylase (glgC), a glycogen synthase (glgA), and a glycogen phosphorylase (glgP). The polymerization of the α-1,4-linked glucosyl chain is mediated via the transfer of glucose from ADP-glucose by GlgA, the glycogen synthase, onto the nonreducing ends of linear dextrins that are subsequently branched (formation of α-1,6-glycosyl linkage) by GlgB, the branching enzyme. The expression of the glg gene cluster is complicated. It involves the global carbon storage regulator CsrA (, ), the cyclic AMP (cAMP)/catabolite gene activator protein (CAP) system (), and the stringent response (). In addition, the two-component regulatory system PhoP-PhoQ () connects the system to Mg2+ levels, and even the phosphotransferase system appears to affect the glycogen phosphorylase involved in the degradation of glycogen (, ). glgS, an additional gene involved in glycogen synthesis, is not part of the glg gene cluster. It is not essential for glycogen synthesis. glgS mutants show reduced glycogen contents and reveal RpoS dependency of glycogen synthesis (). The role of GlgS in glycogen synthesis remains unclear, but a role in protein-protein interaction has been suggested (). The mode of glycogen synthesis, in particular the initial priming reaction, is not well established in bacteria, but the mode of degradation is well established. GlgP, by forming glucose-1-phosphate (glucose-1-P), reduces the lengths of the α-1,6-branched chains to the sizes of maltotetraosyl and maltotriosyl residues, which then become substrates of GlgX, the debranching enzyme that releases maltotetrose and maltotriose (, ). It is the glycogen-dependent formation of maltotetraose and maltotriose that establishes the link to the maltose/maltodextrin-utilizing system.