The glycogen degradation processes in eukaryotes have been studied extensively. During this process, debranching enzymes exhibit both hydrolysis and transferase activity. The glycogen degradation pathway is less well understood in bacteria and archaea. Several recent studies have linked glycogen to pathogen colonization and virulence. The breakdown of glycogen may play an important role in the interactions between the host and pathogenic bacteria, such as , , , and , which accumulate glycogen throughout their life cycles. However, the mechanisms and roles of glycogen during infection by microbes such as are still not fully understood. Elucidation of the mechanisms underlying glycogen metabolism and degradation may reduce the impact of pathogens within the human body. This review focuses on the biochemical properties and functions of enzymes important in the glycogen degradation pathway in
Several reviews have compared bacterial glycogen and starch metabolism. Cenci proposed the switch of glycogen to starch metabolism in Archaeplastida. Both bacteria and plants synthesize storage polysaccharides by an ADP-glucose-based pathway. The bacterial debranching enzyme, GlgX, which appears to be involved in glycogen degradation, shares substrate specificity for Glc4-branch chains with Isa3, a GlgX-derived debranching enzyme in plants. Therefore, comparison of bacterial glycogen with starch in plants may yield a better understanding of glycogen metabolism.
According to the CAZy classification, both GlgP and MalP can be grouped into the GT35 family based on an α-glucosidic linkage. In addition, β-1,4- and β-1,3- glucoside-specific phosphorylases have been reported. The biochemical properties and physiological functions of bacterial α-glucan phosphorylases, including GlgP and MalP, have been reviewed. In , the amino acid sequence of MalP shows 45% similarity to that of GlgP. The catalytic domains are highly conserved, while regulatory sites are poorly conserved, and the overall structure of MalP appears to be similar to that of GlgP. Thus, bacterial phosphorylases GlgP and MalP follow the same catalytic mechanisms, but differ in substrate specificity and regulation. MalP has a preference for linear maltodextrins with DP = 7, whereas activity for glycogen is approximately 1/10 lower. (Enterobacteriaceae) GlgP shows similar substrate preferences to those of MalP.
Phosphorylases catalyze the phosphorolytic cleavage of glycogen or dextrin to release α-glc1-P via the sequential removal of glycosyl residues from the nonreducing ends of the glycogen molecule. α-glc-1-P released from glycogen can be readily converted into glucose 6-phosphate.
The amino acid sequence of MalP is 87% identical to that of . (Enterobacteriaceae) and 55% identical to that of Similarly, comparison of the amino acid sequence of MalP from with that of revealed 41% identity. This protein also showed 40% identity to GlgP. MalP of shows a preference for maltodextrin with at least five glucose residues. In contrast, the specificity of GlgP for glycogen is four times greater than that for maltodextrin. Thus, GlgP involved in the degradation of glycogen cannot replace MalP in utilization of maltose and maltodextrins. MalP recognizes maltodextrins with DP > 4‒5 and catalyzes the sequential removal of glucosyl residues from the nonreducing ends of the dextrins by phosphorolysis, forming α-glc1-P.
The physiological role of MalP has been investigated in several bacteria. Mutants lacking MalP accumulate large amounts of long-chain maltodextrins when grown on maltose or maltodextrin, suggesting that MalP is involved in both maltose utilization and glycogen synthesis pathways. The deletion mutant of , , showed reduced intracellular glycogen degradation, confirming the pathway for glycogen degradation via GlgP, GlgX, and MalP. also forms maltodextrin in the course of glycogen degradation, indicating that glycogen is degraded in this bacterium by a pathway similar to that in . GlgP catalyzes the phosphorolysis of glucose residues at least five units from the branch point of glycogen, accumulating the glycogen phosphorylase-limit glycogen.
Sequence alignment between GlgX, TreX from , and isoamylase from showed common features, including four conserved regions among amylases in the GH13. The three-dimensional structure of an archaeal glycogen debranching enzyme, TreX, shows similarity to the structure of GlgX. However, TreX has both glucosidase and transferase activities. The preference for branch chain length varies among bacteria. TreX is unique in exhibiting higher activity on branched substrates with longer maltooligosaccharides, while the debranching enzyme from (NPDE) shows high specificity for long branch chains (DP > 8). These differences in specificity for branch chain length between GlgX, TreX, and NPDE suggest that glycogen degradation in other bacteria may differ from that in .
The three-dimensional structure of GlgX from K12 was determined at 2.25 Å resolution. The structural features observed at the substrate binding groove provided a molecular explanation for the unique substrate specificity of the Glc4 branch chain () suitable only for Glc4 of GlgP-limit dextrin. This strict specificity ensures that the debranching enzyme will not interfere with the normal process of branching during glycogen biosynthesis in Consequently, GlgX does not hydrolyze native glycogens, starches, or linear maltooligosaccharides. As Glc4 released from GlgP-limit glycogen by GlgX must be further metabolized, it has been postulated that the product, Glc4, may be elongated through the action of MalQ and further processed by GlgP. However, a better understanding of the mechanisms and kinetics is needed due to the complex enzyme system that includes MalQ, GlgX, GlgP, and MalP.
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 .
Glycogen is one of the key carbon sources for infectious microbes, such as , , and , that are classified as clinically important human pathogens. Thus, microbes must obtain nutrients from their host environment during infection. The interaction of bacteria with plants is solely extracellular, while interaction with animal hosts can be intracellular or extracellular. They contain a minimal set of enzymes, with activities sufficient to use glycogen as a source of carbon energy. Several bacterial genomes, such as those of , , and , appear to have the minimal set of glycogen-active enzymes GT5 (GlgA), GT35 (GlgP and MalP), and GH13 (GlgX and GlgB), but no GH15 (glucoamylase). Therefore, investigation of the microbial complex carbohydrate utilization pathways may generate novel insights into host-pathogen interactions.
A study on mutants with deletions in the glycogen and gene clusters indicated that the action of MalQ on maltose or maltodextrin can lead to glycogen formation. Park proposed a model of maltodextrin utilization for the formation of glycogen in the absence of GlgA. As shown in , MalQ plays an important role in the interconnection between GlgA-dependent glycogen synthesis and the maltose utilization system, as glucose formed from maltose by MalQ may also enter the glycogen synthesis pathway in via GlgA. Moreover, can form the primer required for the elongation process by the disproportionation reaction of MalQ. This was consistent with the lack of glycogenin analogs in bacteria and the genomes of bacteria known to accumulate glycogen.