Structural and functional studies on rabbit liver glycogenin

Glycogenin, the protein primer required for the biogenesis of muscle glycogen, has been isolated from rabbit liver glycogen. The protein comprised 0.0025% of liver glycogen by mass, 200-fold lower than the glycogenin content of muscle glycogen. Structural analyses, including determination of the amino acid sequence surrounding the glucosylated-tyrosine residue, showed identity with muscle glycogenin. Catalytically active liver glycogenin was partially purified and, like the skeletal muscle protein, catalysed an intramolecular, Mn2+- and UDP-Glc-dependent autoglucosylation reaction, forming a primer on which glycogen synthase could act. The results demonstrate that hepatic and muscle glycogenins are almost certainly identical proteins and that liver and skeletal muscle share a common mechanism for the biogenesis of glycogen molecules. The results also indicate that there is about one glycogenin molecule/liver glycogen alpha particle.     

No limiting role for glycogenin in determining maximal attainable glycogen levels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 micromol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles ( approximately 25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels ( approximately 170 micromol/g wet wt), approximately 30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.     

Caffeine ingestion does not impede the resynthesis of proglycogen and macroglycogen after prolonged exercise and carbohydrate supplementation in humans.

The purpose of this study was to examine the effects of caffeine (Caf) ingestion on pro- (PG) and macroglycogen (MG) resynthesis in 10 healthy men. Subjects completed two trials, consisting of a glycogen-depleting exercise, while ingesting either Caf or placebo capsules. Throughout recovery, biopsies were taken at 0 (exhaustion), 30, 120, and 300 min, and 75 g of carbohydrate were ingested at 0, 60, 120, 180, and 240 min. Whereas Caf ingestion resulted in a higher blood glucose concentration and decreased glycogen synthase fractional velocity (P 相似文献   

Glycogenin activity and mRNA expression in response to volitional exhaustion in human skeletal muscle.

Glycogenolysis results in the selective catabolism of individual glycogen granules by glycogen phosphorylase. However, once the carbohydrate portion of the granule is metabolized, the fate of glycogenin, the protein primer of granule formation, is not known. To examine this, male subjects (n = 6) exercised to volitional exhaustion (Exh) on a cycle ergometer at 75% maximal O2 uptake. Muscle biopsies were obtained at rest, 30 min, and Exh (99 +/- 10 min). At rest, total glycogen concentration was 497 +/- 41 and declined to 378 +/- 51 mmol glucosyl units/kg dry wt following 30 min of exercise (P < 0.05). There were no significant changes in proglycogen, macroglycogen, glycogenin activity, or mRNA in this period (P > or = 0.05). Exh resulted in decreases in total glycogen, proglycogen, and macroglycogen as well as glycogenin activity (P < 0.05). These decrements were associated with a 1.9 +/- 0.4-fold increase in glycogenin mRNA over resting values (P < 0.05). Glycogenolysis in the initial exercise period (0-30 min) was not adequate to induce changes in glycogenin; however, later in exercise when concentration and granule number decreased further, decrements in glycogenin activity and increases in glycogenin mRNA were demonstrated. Results show that glycogenin becomes inactivated with glycogen catabolism and that this event coincides with an increase in glycogenin gene expression as exercise and glycogenolysis progress.     

The occurrence of serine phosphate in glycogenin: a possible regulatory site

Glycogenin is the covalently bound protein found in muscle glycogen that is thought to be the primer for glycogen synthesis. We now report that glycogenin contains a phosphoserine residue. From a less than stoichiometric amount of phosphate in glycogenin as isolated, the content may be increased to one molecular proportion, using the catalytic subunit of cAMP-dependent protein kinase. The phosphoserine residue is present within a hitherto-undescribed amino acid sequence. In particular, the serine is not flanked by arginine, previously thought to be an essential adjunct for a serine residue to act as substrate for this kinase. We suggest that the serine phosphate may represent a means of regulating the ability of glycogenin to prime glycogen synthesis.     

Manganese sulfate-dependent glycosylation of endogenous glycoproteins in human skeletal muscle is catalyzed by a nonglucose 6-P-dependent glycogen synthase and not glycogenin

Glycogenin, a Mn2+-dependent, self-glucosylating protein, is considered to catalyze the initial glucosyl transfer steps in glycogen biogenesis. To study the physiologic significance of this enzyme, measurements of glycogenin mediated glucose transfer to endogenous trichloroacetic acid precipitable material (protein-bound glycogen, i.e., glycoproteins) in human skeletal muscle were attempted. Although glycogenin protein was detected in muscle extracts, activity was not, even after exercise that resulted in marked glycogen depletion. Instead, a MnSO4-dependent glucose transfer to glycoproteins, inhibited by glycogen and UDP-pyridoxal (which do not affect glycogenin), and unaffected by CDP (a potent inhibitor of glycogenin), was consistently detected. MnSO4-dependent activity increased in concert with glycogen synthase fractional activity after prolonged exercise, and the MnSO4-dependent enzyme stimulated glucosylation of glycoproteins with molecular masses lower than those glucosylated by glucose 6-P-dependent glycogen synthase. Addition of purified glucose 6-P-dependent glycogen synthase to the muscle extract did not affect MnSO4-dependent glucose transfer, whereas glycogen synthase antibody completely abolished MnSO4-dependent activity. It is concluded that: (1) MnSO4-dependent glucose transfer to glycoproteins is catalyzed by a nonglucose 6-P-dependent form of glycogen synthase; (2) MnSO4-dependent glycogen synthase has a greater affinity for low molecular mass glycoproteins and may thus play a more important role than glucose 6-P-dependent glycogen synthase in the initial stages of glycogen biogenesis; and (3) glycogenin is generally inactive in human muscle in vivo.     

GNIP, a novel protein that binds and activates glycogenin, the self-glucosylating initiator of glycogen biosynthesis

Glycogenin is a self-glucosylating protein involved in the initiation of glycogen biosynthesis. Self-glucosylation leads to the formation of an oligosaccharide chain, which, when long enough, supports the action of glycogen synthase to elongate it and form a mature glycogen molecule. To identify possible regulators of glycogenin, the yeast two-hybrid strategy was employed. By using rabbit skeletal muscle glycogenin as a bait, cDNAs encoding three different proteins were isolated from the human skeletal muscle cDNA library. Two of the cDNAs encoded glycogenin and glycogen synthase, respectively, proteins known to be interactors. The third cDNA encoded a polypeptide of unknown function and was designated GNIP (glycogenin interacting protein). Northern blot analysis revealed that GNIP mRNA is highly expressed in skeletal muscle. The gene for GNIP generates at least four isoforms by alternative splicing. The largest isoform GNIP1 contains, from NH(2)- to COOH-terminal, a RING finger, a B box, a putative coiled-coil region, and a B30.2-like motif. The previously identified protein TRIM7 (tripartite motif containing protein 7) is also derived from the GNIP gene and is composed of the RING finger, B box, and coiled-coil regions. The GNIP2 and GNIP3 isoforms consist of the coiled-coil region and B30.2-like domain. Physical interaction between GNIP2 and glycogenin was confirmed by co-immunoprecipitation, and in addition GNIP2 was shown to stimulate glycogenin self-glucosylation 3-4-fold. GNIPs may represent a novel participant in the initiation of glycogen synthesis.     

Rabbit skeletal muscle glycogenin. Molecular cloning and production of fully functional protein in Escherichia coli.

Glycogenin is a self-glucosylating protein involved in the initiation reactions of glycogen synthesis. Initiation occurs in two stages, requiring first the covalent attachment of a glucose residue to Tyr-194 of glycogenin and then elongation to form an oligosaccharide chain. The latter reaction is known to be catalyzed by glycogenin itself. The glycogenin sequence determined from the protein by Campbell and Cohen (Campbell, D. G., and Cohen, P. (1989) Eur. J. Biochem. 185, 119-125) was used to design oligonucleotide probes to screen a rabbit muscle lambda gt11 library. A cDNA was isolated that predicted an amino acid sequence identical to that of Campbell and Cohen, except that Cys residues replaced Ser-88 and Leu-97. Northern analysis indicated a strongly hybridizing message of 1.8 kilobases, present in most tissues including skeletal muscle, but much weaker in kidney and scarcely detectable in liver. A much weaker 3-kilobase message was also detected in muscle. Polymerase chain reaction was used to isolate DNA fragments encoding a portion of glycogenin from rat and cow. The sequence of this segment was > 90% identical at the amino acid level across the three species, indicating that glycogenin is a highly conserved protein. Using the pET-8c vector, the glycogenin protein was expressed in Escherichia coli. Incubation of the recombinant glycogenin with UDP-[14C]glucose and Mn2+ resulted in labeling of the glycogenin protein, indicating that the recombinant glycogenin was enzymatically active and capable of self-glucosylation. Furthermore, after incubation with UDP-glucose, the recombinant glycogenin could serve as a substrate for glycogen synthase, leading to the production of high M(r) polysaccharide. Therefore, production of functional glycogenin did not require the intervention of any other mammalian protein.     

Glycogenin protein and mRNA expression in response to changing glycogen concentration in exercise and recovery

Glycogenin (GN-1) is essential for the formation of a glycogen granule; however, rarely has it been studied when glycogen concentration changes in exercise and recovery. It is unclear whether GN-1 is degraded or is liberated and exists as apoprotein (apo)-GN-1 (unglycosylated). To examine this, we measured GN-1 protein and mRNA level at rest, at exhaustion (EXH), and during 5 h of recovery in which the rate of glycogen restoration was influenced by carbohydrate (CHO) provision. Ten males cycled (65% VO2 max) to volitional EXH (117.8 +/- 4.2 min) on two separate occasions. Subjects were administered carbohydrate (CHO; 1 g.kg(-1).h(-1) Gatorlode) or water [placebo (PL)] during 5 h of recovery. Muscle biopsies were taken at rest, at EXH, and following 30, 60, 120, and 300 min of recovery. At EXH, total glycogen concentration was reduced (P < 0.05). However, GN-1 protein and mRNA content did not change. By 5 h of recovery, glycogen was resynthesized to approximately 60% of rest in the CHO trial and remained unchanged in the PL trial. GN-1 protein and mRNA level did not increase during recovery in either trial. We observed modest amounts of apo-GN-1 at EXH, suggesting complete degradation of some granules. These data suggest that GN-1 is conserved, possibly as very small, or nascent, granules when glycogen concentration is low. This would provide the ability to rapidly restore glycogen during early recovery.     

Mechanism of glycogen supercompensation in rat skeletal muscle cultures

A model to study glycogen supercompensation (the significant increase in glycogen content above basal level) in primary rat skeletal muscle culture was established. Glycogen was completely depleted in differentiated myotubes by 2 h of electrical stimulation or exposure to hypoxia during incubation in medium devoid of glucose. Thereafter, cells were incubated in medium containing glucose, and glycogen supercompensation was clearly observed in treated myotubes after 72 h. Peak glycogen levels were obtained after 120 h, averaging 2.5 and 4 fold above control values in the stimulated- and hypoxia-treated cells, respectively. Glycogen synthase activity increased and phosphorylase activity decreased continuously during 120 h of recovery in the treated cells. Rates of 2-deoxyglucose uptake were significantly elevated in the treated cells at 96 and 120 h, averaging 1.4–2 fold above control values. Glycogenin content increased slightly in the treated cells after 48 h (1.2 fold vs. control) and then increased considerably, achieving peak values after 120 h (2 fold vs. control). The results demonstrate two phases of glycogen supercompensation: the first phase depends primarily on activation of glycogen synthase and inactivation of phosphorylase; the second phase includes increases in glucose uptake and glycogenin level.     

Evidence for glycogenin autoglucosylation cessation by inaccessibility of the acquired maltosaccharide

Glycogenin initiates the biosynthesis of proteoglycogen, the mammalian glycogenin-bound glycogen, by intramolecular autoglucosylation. The incubation of glycogenin with UDP-glucose results in formation of a tyrosine-bound maltosaccharide, reaching maximum polymerization degree of 13 glucose units at cessation of the reaction. No exhaustion of the substrate donor occurred at the autoglucosylation end and the full autoglucosylated enzyme continued catalytically active for transglucosylation of the alternative substrate dodecyl-maltose. Even the autoglucosylation cessation once glycogenin acquired a mature maltosaccharide moiety, proteoglycogen and glycogenin species ranging rM 47-200 kDa, derived from proteoglycogen, showed to be autoglucosylable. The results describe for the first time the ability of polysaccharide-bound glycogenin for intramolecular autoglucosylation, providing evidence for cessation of the glucose polymerization initiated into the tyrosine residue, by inaccessibility of the acquired maltosaccharide moiety to further autoglucosylation.     

Dodecyl-{beta}-D-maltoside as a substrate for glucosyl and xylosyl transfer by glycogenin

Glycogenin is the core protein of glycogen proteoglycan andis, at the same time, a self-glucosylating enzyme which catalysesearly glucosyl transfer steps in the biosynthesis of glycogen.In the course of this process, glycogenin is glucosylated progressivelyuntil an oligosaccharide containing 8–11 glucose residueshas been formed. Although glycogenin, under physiological conditions,is both enzyme and acceptor in the glucosyl transfer reactions,it is also capable of utilizing p-nitrophenyl-linked malto-oligosaccharidesas exogenous acceptors. In view of the potential usefulnessof exogenous acceptors in the study of the mechanism of theglycogenin reaction, we have expanded the search for such compoundsand report here that several alkyl glucosides and alkyl maltosidesmay serve as acceptors in glucosyl transfer by beef kidney glycogenin.Dodecyl-ß-D-maltoside (K_m {small tilde}100 µM)was the most effective acceptor among the compounds tested andyielded 30 times as much product as p-nitrophenyl-     

Interaction between glycogenin and glycogen synthase

Glycogen synthase plays a key role in regulating glycogen metabolism. In a search for regulators of glycogen synthase, a yeast two-hybrid study was performed. Two glycogen synthase-interacting proteins were identified in human skeletal muscle, glycogenin-1, and nebulin. The interaction with glycogenin was found to be mediated by the region of glycogenin which contains the 33 COOH-terminal amino acid residues. The regions in glycogen synthase containing both NH2- and COOH-terminal phosphorylation sites are not involved in the interaction. The core segment of glycogen synthase from Glu21 to Gly503 does not bind COOH-terminal fragment of glycogenin. However, this region of glycogen synthase binds full-length glycogenin indicating that glycogenin contains at least one additional interacting site for glycogen synthase besides the COOH-terminus. We demonstrate that the COOH-terminal fragment of glycogenin can be used as an effective high affinity reagent for the purification of glycogen synthase from skeletal muscle and liver.     

Self-glucosylation of glycogenin, the initiator of glycogen biosynthesis, involves an inter-subunit reaction

Glycogenin is a dimeric self-glucosylating protein involved in the initiation phase of glycogen biosynthesis. As an enzyme, glycogenin has the unusual property of transferring glucose residues from UDP-glucose to itself, forming an alpha-1,4-glycan of around 10 residues attached to Tyr194. Whether this self-glucosylation reaction is inter- or intramolecular has been debated. We used site-directed mutagenesis of recombinant rabbit muscle glycogenin-1 to address this question. Mutation of highly conserved Lys85 to Gln generated a glycogenin mutant (K85Q) that had only 1-2% of the self-glucosylating activity of wild-type enzyme. Consistent with previous work, mutation of Tyr194 to Phe in a GST-fusion protein yielded a mutant, Y194F, that was catalytically active but incapable of self-glucosylation. The Y194F mutant was able to glucosylate the K85Q mutant. However, there was an initial lag in the self-glucosylation reaction that was abolished by preincubation of the two mutant proteins. The interaction between glycogenin subunits was relatively weak, with a dissociation constant inferred from kinetic experiments of around 2 microM. We propose a model for the glucosylation of K85Q by Y194F in which mixing of the proteins is followed by rate-limiting formation of a species containing both subunit types. The results provide the most direct evidence to date that the self-glucosylation of glycogenin involves an inter-subunit reaction.     

The discovery of glycogenin and the priming mechanism for glycogen biogenesis

The biogenesis of glycogen in skeletal muscle requires a priming mechanism that has recently been elucidated. The first step is catalysed by a protein tyrosine glucosyltransferase and involves the formation of a novel glycosidic linkage, namely the covalent attachment of glucose to a single tyrosine residue (Tyr194) on a priming protein, termed glycogenin. The next stage is the extension of the glucan chain from Tyr194 and involves the sequential addition of up to seven further glucosyl residues. This reaction is brought about autocatalytically by glycogenin itself, which is a Mn2+/Mg(2+)-dependent UDP-Glc-requiring glucosyltransferase. The glucan primer is elongated by glycogen synthase, but only when glycogenin and glycogen synthase are complexed together. Glycogen synthase dissociates from glycogenin during the synthesis of a glycogen molecule, enabling glycogen molecules to reach their maximum theoretical size. Each mature glycogen beta particle in muscle contains one molecule of glycogenin attached covalently, and an average one glycogen synthase catalytic subunit bound non-covalently. As evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general, the molecular details of mammalian glycogen biogenesis may serve as a useful model for other systems.     

Muscle glycogen resynthesis during recovery from cycle exercise: no effect of additional protein ingestion.

In the present study, we have investigated the effect of carbohydrate and protein hydrolysate ingestion on muscle glycogen resynthesis during 4 h of recovery from intense cycle exercise. Five volunteers were studied during recovery while they ingested, immediately after exercise, a 600-ml bolus and then every 15 min a 150-ml bolus containing 1) 1.67 g. kg body wt(-1). l(-1) of sucrose and 0.5 g. kg body wt(-1). l(-1) of a whey protein hydrolysate (CHO/protein), 2) 1.67 g. kg body wt(-1). l(-1) of sucrose (CHO), and 3) water. CHO/protein and CHO ingestion caused an increased arterial glucose concentration compared with water ingestion during 4 h of recovery. With CHO ingestion, glucose concentration was 1-1.5 mmol/l higher during the first hour of recovery compared with CHO/protein ingestion. Leg glucose uptake was initially 0.7 mmol/min with water ingestion and decreased gradually with no measurable glucose uptake observed at 3 h of recovery. Leg glucose uptake was rather constant at 0.9 mmol/min with CHO/protein and CHO ingestion, and insulin levels were stable at 70, 45, and 5 mU/l for CHO/protein, CHO, and water ingestion, respectively. Glycogen resynthesis rates were 52 +/- 7, 48 +/- 5, and 18 +/- 6 for the first 1.5 h of recovery and decreased to 30 +/- 6, 36 +/- 3, and 8 +/- 6 mmol. kg dry muscle(-1). h(-1) between 1.5 and 4 h for CHO/protein, CHO, and water ingestion, respectively. No differences could be observed between CHO/protein and CHO ingestion ingestion. It is concluded that coingestion of carbohydrate and protein, compared with ingestion of carbohydrate alone, did not increase leg glucose uptake or glycogen resynthesis rate further when carbohydrate was ingested in sufficient amounts every 15 min to induce an optimal rate of glycogen resynthesis.     

Direct detection of glycogenin reaction products during glycogen initiation

Glycogenin initiates glycogen synthesis in an autocatalytic reaction in which individual glucose residues are covalently linked to Tyrosine 194 in order to form a short priming chain of glucose residues that is a substrate for glycogen synthase which, combined with the branching enzyme, catalyzes the bulk synthesis of glycogen. We sought to develop a new enzymatic assay to better characterize both the chemical and enzymatic characteristics of this unusual reaction. By directly detecting the reaction products using electrospray mass spectrometry this procedure permits both the visualization of the intact individual reaction species produced as a function of time and quantitation of the levels of each of species. The quantitation of the reaction agrees well with previous measurements of both catalytic rate and the change in rate as a function of average glucosylation. The results from this assay provide new insight into the mechanism by which glycogenin catalyzes the initiation reaction.     

Postexercise muscle glycogen resynthesis in obese insulin-resistant Zucker rats.

We determined the effect of an acute bout of swimming (8 x 30 min) followed by either carbohydrate administration (0.5 mg/g glucose ip and ad libitum access to chow; CHO) or fasting (Fast) on postexercise glycogen resynthesis in soleus muscle and liver from female lean (ZL) and obese insulin-resistant (ZO) Zucker rats. Resting soleus muscle glycogen concentration ([glycogen]) was similar between genotypes and was reduced by 73 (ZL) and 63% (ZO) after exercise (P < 0.05). Liver [glycogen] at rest was greater in ZO than ZL (334 +/- 31 vs. 247 +/- 16 micromol/g wet wt; P < 0.01) and fell by 44 and 94% after exercise (P < 0.05). The fractional activity of glycogen synthase (active/total) increased immediately after exercise (from 0.22 +/- 0.05 and 0.32 +/- 0.04 to 0.63 +/- 0.08 vs. 0.57 +/- 0.05; P < 0.01 for ZL and ZO rats, respectively) and remained elevated above resting values after 30 min of recovery. During this time, muscle [glycogen] in ZO increased 68% with CHO (P < 0.05) but did not change in Fast. Muscle [glycogen] was unchanged in ZL from postexercise values after both treatments. After 6 h recovery, GLUT-4 protein concentration was increased above resting levels by a similar extent for both genotypes in both fasted (approximately 45%) and CHO-supplemented (approximately 115%) rats. Accordingly, during this time CHO refeeding resulted in supercompensation in both genotypes (68% vs. 44% for ZL and ZO). With CHO, liver [glycogen] was restored to resting levels in ZL but remained at postexercise values for ZO after both treatments. We conclude that the increased glucose availability with carbohydrate refeeding after glycogen-depleting exercise resulted in glycogen supercompensation, even in the face of muscle insulin-resistance.     

A 42,000-Da protein in rabbit tissues and in a glycogen synthase preparation cross-reacts with antibodies to glycogenin

Rabbit skeletal muscle glycogen previously has been shown to be covalently bound to a 40,000-Da protein ("glycogenin") via a novel glucosyl-tyrosine linkage [I.R. Rodriguez and W.J. Whelan (1985) Biochem. Biophys. Res. Commun. 132, 829-836]. Antibodies raised against rabbit skeletal muscle glycogenin cross-react with a similar protein present in rabbit heart and liver glycogens, as well as with a 42,000-Da "acceptor protein" present in high-speed supernatants of rabbit muscle, heart, retina, and liver. This 42,000-Da protein incorporates [U-14C]Glc when an ammonium sulfate fraction prepared from the tissue supernatants is incubated with UDP-[U-14C]Glc. The [U-14C]Glc incorporated can be removed quantitatively by treatment with amylolytic enzymes, indicating that the [U-14C]Glc incorporation represents elongation of a preexisting glucan attached to the acceptor protein. Furthermore, a commercial preparation of rabbit skeletal muscle glycogen synthase contains this 42,000-Da protein. We propose that the 42,000-Da protein represents the free form of glycogenin in tissues, with its covalently attached glucan chain(s) providing a "primed" elongation site for glycogen synthesis.     

The intramolecular autoglucosylation of monomeric glycogenin

The ability of monomeric glycogenin to autoglucosylate by an intramolecular mechanism of reaction is described using non-glucosylated and partially glucosylated recombinant glycogenin. We determined that monomer glycogenin exists in solution at concentration below 0.60-0.85 μM. The specific autoglucosylation rate of non-glucosylated and glucosylated monomeric glycogenin represented 50 and 70% of the specific rate of the corresponding dimeric glycogenin species. The incorporation of a unique sugar unit into the tyrosine hydroxyl group of non-glucosylated glycogenin, analyzed by autoxylosylation, occurred at a lower rate than the incorporation into the glucose hydroxyl group of the glucosylated enzyme. The intramonomer autoglucosylation mechanism here described for the first time, confers to a just synthesized glycogenin molecule the capacity to produce maltosaccharide primer for glycogen synthase, without the need to reach the concentration required for association into the more efficient autoglucosylating dimer. The monomeric and dimeric interconversion determining the different autoglucosylation rate, might serve as a modulation mechanism for the de novo biosynthesis of glycogen at the initial glucose polymerization step.