Crystal structure of the autocatalytic initiator of glycogen biosynthesis, glycogenin

Glycosyltransferase 8 domain-containing protein 1 (GLT8D1) is a UDP-dependent galactosyltransferase

Glycosyltransferases (GTs) are enzymes that catalyze the formation of glycosidic bonds and hundreds of GTs have been identified so far in humans. Glycosyltransferase 8 domain-containing protein 1 (GLT8D1) has been associated with central nervous system diseases and cancer. However, evidence on its enzymatic properties, including its substrates, has been scarcely described. In this paper, we have produced and purified recombinant secretory GLT8D1. The enzyme was found to be N-glycosylated. Differential scanning fluorimetry was employed to analyze the stabilization of GLT8D1 by Mn2+ and nucleotides, revealing UDP as the most stabilizing nucleotide scaffold. GLT8D1 displayed glycosyltransferase activity from UDP-galactose onto N-acetylgalactosamine but with a low efficiency. Modeling of the structure revealed similarities with other GT-A fold enzymes in CAZy family GT8 and glycosyltransferases in other families with galactosyl-, glucosyl-, and xylosyltransferase activities, each with retaining catalytic mechanisms. Our study provides novel structural and functional insights into the properties of GLT8D1 with implications in pathological processes.

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

Mechanism of glycogen synthase inactivation and interaction with glycogenin

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress against GS regulatory helices. This keeps GS in an inactive conformation mediated by phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided mutagenesis perturbing interactions with phosphorylated tails led to increased basal/unstimulated GS activity. We propose that multivalent phosphorylation supports GS autoinhibition through interactions from a dynamic “spike” region, allowing a tuneable rheostat for regulating GS activity. This work therefore provides insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.

Glycogen is a major energy reserve in eukaryotes and is synthesised in part by glycogenin (GN) and glycogen synthase (GS). Here, authors describe the structural basis of GS regulation, specifically the mechanism of inactivation by phosphorylation.

Identification and expression analysis of the PtGATL genes under different nitrogen and carbon dioxide treatments in Populus trichocarpa

Pectin is one of the most important components of the plant cell wall. Galacturonosyltransferase-like (GATL) is an important enzyme involved in forming pectin in Arabidopsis thaliana. In this study, 12 PtGATL genes were identified and characterized based on the Populus trichocarpa genome using bioinformatics methods. The results showed that the PtGATLs contained four typical motifs, including DXD, LPPF, GLG, and HXXGXXKPW. According to phylogenetic analysis, PtGATLs were divided into six groups. Chromosome distribution and genome synteny analysis showed that there were 11 segmental-duplicated gene pairs with repeated fragments on chromosomes 2, 5, 7, 8, 10, and 14. Tissue-specific expression profiles indicated that these PtGATLs had different expression patterns. The transcription level of PtGATLs was regulated by different carbon dioxide and nitrogen concentrations. In conclusion, the identification and analysis of PtGATL genes in poplar provide important information on the gene function.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-022-03129-y.

Ultrasound-Assisted Microencapsulation of Soybean Oil and Vitamin D Using Bare Glycogen Nanoparticles

Ultrasonically synthesized core-shell microcapsules can be made of synthetic polymers or natural biopolymers, such as proteins and polysaccharides, and have found applications in food, drug delivery and cosmetics. This study reports on the ultrasonic synthesis of microcapsules using unmodified (natural) and biodegradable glycogen nanoparticles derived from various sources, such as rabbit and bovine liver, oyster and sweet corn, for the encapsulation of soybean oil and vitamin D. Depending on their source, glycogen nanoparticles exhibited differences in size and 'bound' proteins. We optimized various synthetic parameters, such as ultrasonic power, time and concentration of glycogens and the oil phase to obtain stable core-shell microcapsules. Particularly, under ultrasound-induced emulsification conditions (sonication time 45 s and sonication power 160 W), native glycogens formed microcapsules with diameter between 0.3 μm and 8 μm. It was found that the size of glycogen as well as the protein component play an important role in stabilizing the Pickering emulsion and the microcapsules shell. This study highlights that native glycogen nanoparticles without any further tedious chemical modification steps can be successfully used for the encapsulation of nutrients.

Chlorovirus PBCV-1 Multidomain Protein A111/114R Has Three Glycosyltransferase Functions Involved in the Synthesis of Atypical N-Glycans

The structures of the four N-linked glycans from the prototype chlorovirus PBCV-1 major capsid protein do not resemble any other glycans in the three domains of life. All known chloroviruses and antigenic variants (or mutants) share a unique conserved central glycan core consisting of five sugars, except for antigenic mutant virus P1L6, which has four of the five sugars. A combination of genetic and structural analyses indicates that the protein coded by PBCV-1 gene a111/114r, conserved in all chloroviruses, is a glycosyltransferase with three putative domains of approximately 300 amino acids each. Here, in addition to in silico sequence analysis and protein modeling, we measured the hydrolytic activity of protein A111/114R. The results suggest that domain 1 is a galactosyltransferase, domain 2 is a xylosyltransferase and domain 3 is a fucosyltransferase. Thus, A111/114R is the protein likely responsible for the attachment of three of the five conserved residues of the core region of this complex glycan, and, if biochemically corroborated, it would be the second three-domain protein coded by PBCV-1 that is involved in glycan synthesis. Importantly, these findings provide additional support that the chloroviruses do not use the canonical host endoplasmic reticulum–Golgi glycosylation pathway to glycosylate their glycoproteins; instead, they perform glycosylation independent of cellular organelles using virus-encoded enzymes.

A terminal α3-galactose modification regulates an E3 ubiquitin ligase subunit in Toxoplasma gondii

Skp1, a subunit of E3 Skp1/Cullin-1/F-box protein ubiquitin ligases, is modified by a prolyl hydroxylase that mediates O2 regulation of the social amoeba Dictyostelium and the parasite Toxoplasma gondii The full effect of hydroxylation requires modification of the hydroxyproline by a pentasaccharide that, in Dictyostelium, influences Skp1 structure to favor assembly of Skp1/F-box protein subcomplexes. In Toxoplasma, the presence of a contrasting penultimate sugar assembled by a different glycosyltransferase enables testing of the conformational control model. To define the final sugar and its linkage, here we identified the glycosyltransferase that completes the glycan and found that it is closely related to glycogenin, an enzyme that may prime glycogen synthesis in yeast and animals. However, the Toxoplasma enzyme catalyzes formation of a Galα1,3Glcα linkage rather than the Glcα1,4Glcα linkage formed by glycogenin. Kinetic and crystallographic experiments showed that the glycosyltransferase Gat1 is specific for Skp1 in Toxoplasma and also in another protist, the crop pathogen Pythium ultimum The fifth sugar is important for glycan function as indicated by the slow-growth phenotype of gat1Δ parasites. Computational analyses indicated that, despite the sequence difference, the Toxoplasma glycan still assumes an ordered conformation that controls Skp1 structure and revealed the importance of nonpolar packing interactions of the fifth sugar. The substitution of glycosyltransferases in Toxoplasma and Pythium by an unrelated bifunctional enzyme that assembles a distinct but structurally compatible glycan in Dictyostelium is a remarkable case of convergent evolution, which emphasizes the importance of the terminal α-galactose and establishes the phylogenetic breadth of Skp1 glycoregulation.

Glycogen as a Building Block for Advanced Biological Materials

Biological nanoparticles found in living systems possess distinct molecular architectures and diverse functions. Glycogen is a unique biological polysaccharide nanoparticle fabricated by nature through a bottom-up approach. The biocatalytic synthesis of glycogen has evolved over time to form a nanometer-sized dendrimer-like structure (20-150 nm) with a highly branched surface and a dense core. This makes glycogen markedly different from other natural linear or branched polysaccharides and particularly attractive as a platform for biomedical applications. Glycogen is inherently biodegradable, nontoxic, and can be functionalized with diverse surface and internal motifs for enhanced biofunctional properties. Recently, there has been growing interest in glycogen as a natural alternative to synthetic polymers and nanoparticles in a range of applications. Herein, the recent literature on glycogen in the material-based sciences, including its use as a constituent in biodegradable hydrogels and fibers, drug delivery vectors, tumor targeting and penetrating nanoparticles, immunomodulators, vaccine adjuvants, and contrast agents, is reviewed. The various methods of chemical functionalization and physical assembly of glycogen nanoparticles into multicomponent nanodevices, which advance glycogen toward a functional therapeutic nanoparticle from nature and back again, are discussed in detail.

From the seminal discovery of proteoglycogen and glycogenin to emerging knowledge and research on glycogen biology

Although the discovery of glycogen in the liver, attributed to Claude Bernard, happened more than 160 years ago, the mechanism involved in the initiation of glucose polymerization remained unknown. The discovery of glycogenin at the core of glycogen's structure and the initiation of its glucopolymerization is among one of the most exciting and relatively recent findings in Biochemistry. This review focuses on the initial steps leading to the seminal discoveries of proteoglycogen and glycogenin at the beginning of the 1980s, which paved the way for subsequent foundational breakthroughs that propelled forward this new research field. We also explore the current, as well as potential, impact this research field is having on human health and disease from the perspective of glycogen storage diseases. Important new questions arising from recent studies, their links to basic mechanisms involved in the de novo glycogen biogenesis, and the pervading presence of glycogenin across the evolutionary scale, fueled by high throughput -omics technologies, are also addressed.

Structural basis of glycogen metabolism in bacteria

The evolution of metabolic pathways is a major force behind natural selection. In the spotlight of such process lies the structural evolution of the enzymatic machinery responsible for the central energy metabolism. Specifically, glycogen metabolism has emerged to allow organisms to save available environmental surplus of carbon and energy, using dedicated glucose polymers as a storage compartment that can be mobilized at future demand. The origins of such adaptive advantage rely on the acquisition of an enzymatic system for the biosynthesis and degradation of glycogen, along with mechanisms to balance the assembly and disassembly rate of this polysaccharide, in order to store and recover glucose according to cell energy needs. The first step in the classical bacterial glycogen biosynthetic pathway is carried out by the adenosine 5'-diphosphate (ADP)-glucose pyrophosphorylase. This allosteric enzyme synthesizes ADP-glucose and acts as a point of regulation. The second step is carried out by the glycogen synthase, an enzyme that generates linear α-(1→4)-linked glucose chains, whereas the third step catalyzed by the branching enzyme produces α-(1→6)-linked glucan branches in the polymer. Two enzymes facilitate glycogen degradation: glycogen phosphorylase, which functions as an α-(1→4)-depolymerizing enzyme, and the debranching enzyme that catalyzes the removal of α-(1→6)-linked ramifications. In this work, we rationalize the structural basis of glycogen metabolism in bacteria to the light of the current knowledge. We describe and discuss the remarkable progress made in the understanding of the molecular mechanisms of substrate recognition and product release, allosteric regulation and catalysis of all those enzymes.

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

Brain Glycogen Structure and Its Associated Proteins: Past, Present and Future

This chapter reviews the history of glycogen-related research and discusses in detail the structure, regulation, chemical properties and subcellular distribution of glycogen and its associated proteins, with particular focus on these aspects in brain tissue.

Critical Review of Plant Cell Wall Matrix Polysaccharide Glycosyltransferase Activities Verified by Heterologous Protein Expression

The life cycle and development of plants requires the biosynthesis, deposition, and degradation of cell wall matrix polysaccharides. The structures of the diverse cell wall matrix polysaccharides influence commercially important properties of plant cells, including growth, biomass recalcitrance, organ abscission, and the shelf life of fruits. This review is a comprehensive summary of the matrix polysaccharide glycosyltransferase (GT) activities that have been verified using in vitro assays following heterologous GT protein expression. Plant cell wall (PCW) biosynthetic GTs are primarily integral transmembrane proteins localized to the endoplasmic reticulum and Golgi of the plant secretory system. The low abundance of these enzymes in plant tissues makes them particularly difficult to purify from native plant membranes in quantities sufficient for enzymatic characterization, which is essential to study the functions of the different GTs. Numerous activities in the synthesis of the major cell wall matrix glycans, including pectins, xylans, xyloglucan, mannans, mixed-linkage glucans (MLGs), and arabinogalactan components of AGP proteoglycans have been mapped to specific genes and multi-gene families. Cell wall GTs include those that synthesize the polymer backbones, those that elongate side branches with extended glycosyl chains, and those that add single monosaccharide linkages onto polysaccharide backbones and/or side branches. Three main strategies have been used to identify genes encoding GTs that synthesize cell wall linkages: analysis of membrane fractions enriched for cell wall biosynthetic activities, mutational genetics approaches investigating cell wall compositional phenotypes, and omics-directed identification of putative GTs from sequenced plant genomes. Here we compare the heterologous expression systems used to produce, purify, and study the enzyme activities of PCW GTs, with an emphasis on the eukaryotic systems Nicotiana benthamiana, Pichia pastoris, and human embryonic kidney (HEK293) cells. We discuss the enzymatic properties of GTs including kinetic rates, the chain lengths of polysaccharide products, acceptor oligosaccharide preferences, elongation mechanisms for the synthesis of long-chain polymers, and the formation of GT complexes. Future directions in the study of matrix polysaccharide biosynthesis are proposed.

Palladium-mediated enzyme activation suggests multiphase initiation of glycogenesis

Biosynthesis of glycogen, the essential glucose (and hence energy) storage molecule in humans, animals and fungi¹, is initiated by the glycosyltransferase enzyme, glycogenin (GYG). Deficiencies in glycogen formation cause neurodegenerative and metabolic disease^2-4, and mouse knockout⁵ and inherited human mutations⁶ of GYG impair glycogen synthesis. GYG acts as a 'seed core' for the formation of the glycogen particle by catalysing its own stepwise autoglucosylation to form a covalently bound gluco-oligosaccharide chain at initiation site Tyr 195. Precise mechanistic studies have so far been prevented by an inability to access homogeneous glycoforms of this protein, which unusually acts as both catalyst and substrate. Here we show that unprecedented direct access to different, homogeneously glucosylated states of GYG can be accomplished through a palladium-mediated enzyme activation 'shunt' process using on-protein C-C bond formation. Careful mimicry of GYG intermediates recapitulates catalytic activity at distinct stages, which in turn allows discovery of triphasic kinetics and substrate plasticity in GYG's use of sugar substrates. This reveals a tolerant but 'proof-read' mechanism that underlies the precision of this metabolic process. The present demonstration of direct, chemically controlled access to intermediate states of active enzymes suggests that such ligation-dependent activation could be a powerful tool in the study of mechanism.

A two-phase model for the non-processive biosynthesis of homogalacturonan polysaccharides by the GAUT1:GAUT7 complex

Homogalacturonan (HG) is a pectic glycan in the plant cell wall that contributes to plant growth and development and cell wall structure and function, and interacts with other glycans and proteoglycans in the wall. HG is synthesized by the galacturonosyltransferase (GAUT) gene family. Two members of this family, GAUT1 and GAUT7, form a heteromeric enzyme complex in Arabidopsis thaliana Here, we established a heterologous GAUT expression system in HEK293 cells and show that co-expression of recombinant GAUT1 with GAUT7 results in the production of a soluble GAUT1:GAUT7 complex that catalyzes elongation of HG products in vitro The reaction rates, progress curves, and product distributions exhibited major differences dependent upon small changes in the degree of polymerization (DP) of the oligosaccharide acceptor. GAUT1:GAUT7 displayed >45-fold increased catalytic efficiency with DP11 acceptors relative to DP7 acceptors. Although GAUT1:GAUT7 synthesized high-molecular-weight polymeric HG (>100 kDa) in a substrate concentration-dependent manner typical of distributive (nonprocessive) glycosyltransferases with DP11 acceptors, reactions primed with short-chain acceptors resulted in a bimodal product distribution of glycan products that has previously been reported as evidence for a processive model of GT elongation. As an alternative to the processive glycosyltransfer model, a two-phase distributive elongation model is proposed in which a slow phase, which includes the de novo initiation of HG and elongation of short-chain acceptors, is distinguished from a phase of rapid elongation of intermediate- and long-chain acceptors. Upon reaching a critical chain length of DP11, GAUT1:GAUT7 elongates HG to high-molecular-weight products.

Proteomic Investigation of the Binding Agent between Liver Glycogen β Particles

Glycogen is a highly branched glucose polymer which plays an important role in glucose storage and the maintenance of blood sugar homeostasis. The dimeric protein glycogenin can self-glucosylate to act as a primer for glycogen synthesis, eventually resulting in small (∼20 nm diameter) glycogen β particles with a dimer of glycogenin at their core. In the liver, glycogen is also found in the form of α particles: large bound composites of many β particles. Here, we provide evidence using qualitative and quantitative proteomics and size-exclusion chromatography from healthy rat, mouse, and human liver glycogen that glycogenin is the binding agent linking β particles together into α particles.

Screening Arabidopsis thaliana mutants for low sensitivity to manganese identifies novel alleles of NRAMP1 and PGSIP6

Manganese (Mn) is an essential micronutrient; however, few genes required for growth under low-Mn conditions have been identified. In this study, we isolated Arabidopsis thaliana mutants sensitive to low-Mn conditions from ethyl methanesulfonate-mutagenized seeds. Among them, we identified the causal genes of two mutants. One mutant (35-34) exhibited a short root phenotype and low Mn concentration in the shoots. The other mutant (30-11) exhibited a small shoot phenotype with Mn concentrations similar to the control. Genetic mapping, allelism tests, and gene complementation tests identified the causal genes as At1g80830 (NRAMP1) for 35-34 and At5g18480 (PGSIP6) for 30-11. NRAMP1 was previously reported to be essential for Mn uptake under low-Mn conditions, thus validating our screening method. PGSIP6 encodes inositol phosphorylceramide glucuronosyltransferase, which is involved in glycosyl inositol phosphorylceramide sphingolipid glycosylation. PGSIP6-green fluorescent protein was localized to the Golgi apparatus, which is consistent with its function in the glycosylation of sphingolipids. Our screening identified a novel gene required for low-Mn tolerance, and we also provide new insights towards understanding the physiological function of PGSIP6.

Crystal structures of eukaryote glycosyltransferases reveal biologically relevant enzyme homooligomers

Glycosyltransferases (GTases) transfer sugar moieties to proteins, lipids or existing glycan or polysaccharide molecules. GTases form an important group of enzymes in the Golgi, where the synthesis and modification of glycoproteins and glycolipids take place. Golgi GTases are almost invariably type II integral membrane proteins, with the C-terminal globular catalytic domain residing in the Golgi lumen. The enzymes themselves are divided into 103 families based on their sequence homology. There is an abundance of published crystal structures of GTase catalytic domains deposited in the Protein Data Bank (PDB). All of these represent either of the two main characteristic structural folds, GT-A or GT-B, or present a variation thereof. Since GTases can function as homomeric or heteromeric complexes in vivo, we have summarized the structural features of the dimerization interfaces in crystal structures of GTases, as well as considered the biochemical data available for these enzymes. For this review, we have considered all 898 GTase crystal structures in the Protein Data Bank and highlight the dimer formation characteristics of various GTases based on 24 selected structures.

Genome-Wide Identification, Evolutionary and Expression Analyses of the GALACTINOL SYNTHASE Gene Family in Rapeseed and Tobacco

Galactinol synthase (GolS) is a key enzyme in raffinose family oligosaccharide (RFO) biosynthesis. The finding that GolS accumulates in plants exposed to abiotic stresses indicates RFOs function in environmental adaptation. However, the evolutionary relationships and biological functions of GolS family in rapeseed (Brassica napus) and tobacco (Nicotiana tabacum) remain unclear. In this study, we identified 20 BnGolS and 9 NtGolS genes. Subcellular localization predictions showed that most of the proteins are localized to the cytoplasm. Phylogenetic analysis identified a lost event of an ancient GolS copy in the Solanaceae and an ancient duplication event leading to evolution of GolS4/7 in the Brassicaceae. The three-dimensional structures of two GolS proteins were conserved, with an important DxD motif for binding to UDP-galactose (uridine diphosphate-galactose) and inositol. Expression profile analysis indicated that BnGolS and NtGolS genes were expressed in most tissues and highly expressed in one or two specific tissues. Hormone treatments strongly induced the expression of most BnGolS genes and homologous genes in the same subfamilies exhibited divergent-induced expression. Our study provides a comprehensive evolutionary analysis of GolS genes among the Brassicaceae and Solanaceae as well as an insight into the biological function of GolS genes in hormone response in plants.

Glycosyltransferase MDR1 assembles a dividing ring for mitochondrial proliferation comprising polyglucan nanofilaments

Mitochondria, which evolved from a free-living bacterial ancestor, contain their own genomes and genetic systems and are produced from preexisting mitochondria by binary division. The mitochondrion-dividing (MD) ring is the main skeletal structure of the mitochondrial division machinery. However, the assembly mechanism and molecular identity of the MD ring are unknown. Multi-omics analysis of isolated mitochondrial division machinery from the unicellular alga Cyanidioschyzon merolae revealed an uncharacterized glycosyltransferase, MITOCHONDRION-DIVIDING RING1 (MDR1), which is specifically expressed during mitochondrial division and forms a single ring at the mitochondrial division site. Nanoscale imaging using immunoelectron microscopy and componential analysis demonstrated that MDR1 is involved in MD ring formation and that the MD ring filaments are composed of glycosylated MDR1 and polymeric glucose nanofilaments. Down-regulation of MDR1 strongly interrupted mitochondrial division and obstructed MD ring assembly. Taken together, our results suggest that MDR1 mediates the synthesis of polyglucan nanofilaments that assemble to form the MD ring. Given that a homolog of MDR1 performs similar functions in chloroplast division, the establishment of MDR1 family proteins appears to have been a singular, crucial event for the emergence of endosymbiotic organelles.

Sugar-peptidic bond interactions: spectroscopic characterization of a model system

Sugars are small carbohydrates which play numerous roles in living organisms such as storage of energy or as structural components. Modifications of specific sites within the glycan chain can modulate a carbohydrate's overall biological function as it happens with nucleic acids and proteins. Hence, identifying discrete carbohydrate modifications and understanding their biological effects is essential. A study of such processes requires of a deep knowledge of the interaction mechanism at the molecular level. Here, we use a combination of laser spectroscopy in jets and quantum mechanical calculations to characterize the interaction between phenyl-β-d-glucopyranoside and N-methylacetamide as a model to understand the interaction between a sugar and a peptide bond. The most stable structure of the molecular aggregate shows that the main interaction between the peptide fragment and the sugar proceeds via a C[double bond, length as m-dash]OH-O2 hydrogen bond. A second conformer was also found, in which the peptide establishes a C[double bond, length as m-dash]OH-O6 hydrogen bond with the hydroxymethyl substituent of the sugar unit. All the conformers present an additional interaction point with the aromatic ring. This particular preference of the peptide for the hydroxyl close to the aromatic ring could explain why glycogenin uses tyrosine in order to convert glucose into glycogen by exposing the O4H hydroxyl group for the other glucoses for the polymerization to take place.

Mechanistic insights into the allosteric regulation of bacterial ADP-glucose pyrophosphorylases

ADP-glucose pyrophosphorylase (AGPase) controls bacterial glycogen and plant starch biosynthetic pathways, the most common carbon storage polysaccharides in nature. AGPase activity is allosterically regulated by a series of metabolites in the energetic flux within the cell. Very recently, we reported the first crystal structures of the paradigmatic AGPase from Escherichia coli (EcAGPase) in complex with its preferred physiological negative and positive allosteric regulators, adenosine 5'-monophosphate (AMP) and fructose 1,6-bisphosphate (FBP), respectively. However, understanding the molecular mechanism by which AMP and FBP allosterically modulates EcAGPase enzymatic activity still remains enigmatic. Here we found that single point mutations of key residues in the AMP-binding site decrease its inhibitory effect but also clearly abolish the overall AMP-mediated stabilization effect in wild-type EcAGPase. Single point mutations of key residues for FBP binding did not revert the AMP-mediated stabilization. Strikingly, an EcAGPase-R130A mutant displayed a dramatic increase in activity when compared with wild-type EcAGPase, and this increase correlated with a significant increment of glycogen content in vivo The crystal structure of EcAGPase-R130A revealed unprecedented conformational changes in structural elements involved in the allosteric signal transmission. Altogether, we propose a model in which the positive and negative energy reporters regulate AGPase catalytic activity via intra- and interprotomer cross-talk, with a "sensory motif" and two loops, RL1 and RL2, flanking the ATP-binding site playing a significant role. The information reported herein provides exciting possibilities for industrial/biotechnological applications.

Understanding the role of tyrosine in glycogenin

α-/β-Glucose shows a particular affinity for the O3H and O4H moieties of β-PhGlc in the synthesis of glucogen.

Cardiomyopathy as presenting sign of glycogenin-1 deficiency-report of three cases and review of the literature

We describe a new type of cardiomyopathy caused by a mutation in the glycogenin-1 gene (GYG1). Three unrelated male patients aged 34 to 52 years with cardiomyopathy and abnormal glycogen storage on endomyocardial biopsy were homozygous for the missense mutation p.Asp102His in GYG1. The mutated glycogenin-1 protein was expressed in cardiac tissue but had lost its ability to autoglucosylate as demonstrated by an in vitro assay and western blot analysis. It was therefore unable to form the primer for normal glycogen synthesis. Two of the patients showed similar patterns of heart dilatation, reduced ejection fraction and extensive late gadolinium enhancement on cardiac magnetic resonance imaging. These two patients were severely affected, necessitating cardiac transplantation. The cardiomyocyte storage material was characterized by large inclusions of periodic acid and Schiff positive material that was partly resistant to alpha-amylase treatment consistent with polyglucosan. The storage material had, unlike normal glycogen, a partly fibrillar structure by electron microscopy. None of the patients showed signs or symptoms of muscle weakness but a skeletal muscle biopsy in one case revealed muscle fibres with abnormal glycogen storage. Glycogenin-1 deficiency is known as a rare cause of skeletal muscle glycogen storage disease, usually without cardiomyopathy. We demonstrate that it may also be the cause of severe cardiomyopathy and cardiac failure without skeletal muscle weakness. GYG1 should be included in cardiomyopathy gene panels.

Quantum Mechanics/Molecular Mechanics Study of the Sialyltransferase Reaction Mechanism

The sialyltransferase is an enzyme that transfers the sialic acid moiety from cytidine 5'-monophospho-N-acetyl-neuraminic acid (CMP-NeuAc) to the terminal position of glycans. To elucidate the catalytic mechanism of sialyltransferase, we explored the potential energy surface along the sialic acid transfer reaction coordinates by the hybrid quantum mechanics/molecular mechanics method on the basis of the crystal structure of sialyltransferase CstII. Our calculation demonstrated that CstII employed an S_N1-like reaction mechanism via the formation of a short-lived oxocarbenium ion intermediate. The computational barrier height was 19.5 kcal/mol, which reasonably corresponded with the experimental reaction rate. We also found that two tyrosine residues (Tyr156 and Tyr162) played a vital role in stabilizing the intermediate and the transition states by quantum mechanical interaction with CMP.

Giant mimivirus R707 encodes a glycogenin paralogue polymerizing glucose through α- and β-glycosidic linkages

Acanthamoeba polyphaga mimivirus is a giant virus encoding 1262 genes among which many were previously thought to be exclusive to cellular life. For example, mimivirus genes encode enzymes involved in the biosynthesis of nucleotide sugars and putative glycosyltransferases. We identified in mimivirus a glycogenin-1 homologous gene encoded by the open reading frame R707. The R707 protein was found to be active as a polymerizing glucosyltransferase enzyme. Like glycogenin-1, R707 activity was divalent-metal-ion-dependent and relied on an intact DXD motif. In contrast with glycogenin-1, R707 was, however, not self-glucosylating. Interestingly, the product of R707 catalysis featured α1-6, β1-6 and α1-4 glycosidic linkages. Mimivirus R707 is the first reported glycosyltransferase able to catalyse the formation of both α and β linkages. Mimivirus-encoded glycans play a role in the infection of host amoebae. Co-infection of Acanthamoeba with mimivirus and amylose and chitin hydrolysate reduced the number of infected amoebae, thus supporting the importance of polysaccharide chains in the uptake of mimivirus by amoebae. The identification of a glycosyltransferase capable of forming α and β linkages underlines the peculiarity of mimivirus and enforces the concept of a host-independent glycosylation machinery in mimivirus.

Glycogen metabolism in humans

In the human body, glycogen is a branched polymer of glucose stored mainly in the liver and the skeletal muscle that supplies glucose to the blood stream during fasting periods and to the muscle cells during muscle contraction. Glycogen has been identified in other tissues such as brain, heart, kidney, adipose tissue, and erythrocytes, but glycogen function in these tissues is mostly unknown. Glycogen synthesis requires a series of reactions that include glucose entrance into the cell through transporters, phosphorylation of glucose to glucose 6-phosphate, isomerization to glucose 1-phosphate, and formation of uridine 5'-diphosphate-glucose, which is the direct glucose donor for glycogen synthesis. Glycogenin catalyzes the formation of a short glucose polymer that is extended by the action of glycogen synthase. Glycogen branching enzyme introduces branch points in the glycogen particle at even intervals. Laforin and malin are proteins involved in glycogen assembly but their specific function remains elusive in humans. Glycogen is accumulated in the liver primarily during the postprandial period and in the skeletal muscle predominantly after exercise. In the cytosol, glycogen breakdown or glycogenolysis is carried out by two enzymes, glycogen phosphorylase which releases glucose 1-phosphate from the linear chains of glycogen, and glycogen debranching enzyme which untangles the branch points. In the lysosomes, glycogen degradation is catalyzed by α-glucosidase. The glucose 6-phosphatase system catalyzes the dephosphorylation of glucose 6-phosphate to glucose, a necessary step for free glucose to leave the cell. Mutations in the genes encoding the enzymes involved in glycogen metabolism cause glycogen storage diseases.

Getting a handle on glycogen synthase - Its interaction with glycogenin

Glycogen is a polymer of glucose that serves as a major energy reserve in eukaryotes. It is synthesized through the cooperative action of glycogen synthase (GS), glycogenin (GN) and glycogen branching enzyme. GN initiates the first enzymatic step in the glycogen synthesis process by self glucosylation of a short 8-12 glucose residue primer. After interacting with GN, GS then extends this sugar primer to form glycogen particles of different sizes. We discuss recent developments in the structural biology characterization of GS and GN enzymes, which have contributed to a better understanding of how the two proteins interact and how they collaborate to synthesize glycogen particles.

Muscle pathology and whole-body MRI in a polyglucosan myopathy associated with a novel glycogenin-1 mutation

We report a 46-year-old female with late-onset skeletal myopathy affecting proximal limb muscles. Muscle biopsy revealed a polyglucosan myopathy with PAS-positive inclusions predominantly in glycogen-depleted fibers, which were demonstrated as type I fibers by ATPase staining. Whole-body magnetic imaging disclosed that the paravertebral, scapular, and pelvic girdle muscles, the anterior compartment of the arms, and the posterior compartment of the thighs were preferentially involved. Genetic analysis revealed a homozygous novel mutation in exon 6 of the glycogenin-1 gene (GYG1) (c.634C>T, p.His212Tyr). Protein analysis revealed normal levels of glycogenin-1 even before alpha-amylase digestion indicating preserved protein expression but impaired glucosylation. In vitro functional assay demonstrated that this variant impaired the autoglucosylating ability resulting in a non-functional protein. We report a glycogenin-1 related myopathy with a distinct histopathology and unique muscle imaging pattern.

Structural basis for the recruitment of glycogen synthase by glycogenin

Glycogen is a primary form of energy storage in eukaryotes that is essential for glucose homeostasis. The glycogen polymer is synthesized from glucose through the cooperative action of glycogen synthase (GS), glycogenin (GN), and glycogen branching enzyme and forms particles that range in size from 10 to 290 nm. GS is regulated by allosteric activation upon glucose-6-phosphate binding and inactivation by phosphorylation on its N- and C-terminal regulatory tails. GS alone is incapable of starting synthesis of a glycogen particle de novo, but instead it extends preexisting chains initiated by glycogenin. The molecular determinants by which GS recognizes self-glucosylated GN, the first step in glycogenesis, are unknown. We describe the crystal structure of Caenorhabditis elegans GS in complex with a minimal GS targeting sequence in GN and show that a 34-residue region of GN binds to a conserved surface on GS that is distinct from previously characterized allosteric and binding surfaces on the enzyme. The interaction identified in the GS-GN costructure is required for GS-GN interaction and for glycogen synthesis in a cell-free system and in intact cells. The interaction of full-length GS-GN proteins is enhanced by an avidity effect imparted by a dimeric state of GN and a tetrameric state of GS. Finally, the structure of the N- and C-terminal regulatory tails of GS provide a basis for understanding phosphoregulation of glycogen synthesis. These results uncover a central molecular mechanism that governs glycogen metabolism.

Evolving views of pectin biosynthesis

Recent progress in the identification and characterization of pectin biosynthetic proteins and the discovery of pectin domain-containing proteoglycans are changing our view of how pectin, the most complex family of plant cell wall polysaccharides, is synthesized. The functional confirmation of four types of pectin biosynthetic glycosyltransferases, the identification of multiple putative pectin glycosyl- and methyltransferases, and the characteristics of the GAUT1:GAUT7 homogalacturonan biosynthetic complex with its novel mechanism for retaining catalytic subunits in the Golgi apparatus and its 12 putative interacting proteins are beginning to provide a framework for the pectin biosynthetic process. We propose two partially overlapping hypothetical and testable models for pectin synthesis: the consecutive glycosyltransferase model and the domain synthesis model.

Molecular insights into glycogen α-particle formation

Glycogen, a hyperbranched complex glucose polymer, is an intracellular glucose store that provides energy for cellular functions, with liver glycogen involved in blood-glucose regulation. Liver glycogen comprises complex α particles made up of smaller β particles. The recent discovery that these α particles are smaller and fewer in diabetic, compared with healthy, mice highlights the need to elucidate the nature of α-particle formation; this paper tests various possibilities for binding within α particles. Acid hydrolysis effects, examined using dynamic light scattering and size exclusion chromatography, showed that the binding is not simple α-(1→4) or α-(1→6) glycosidic linkages. There was no significant change in α particle size after the addition of various reagents, which disrupt disulfide, protein, and hydrogen bonds and hydrophobic interactions. The results are consistent with proteinaceous binding between β particles in α particles, with the inability of protease to break apart particles being attributed to steric hindrance.

Three members of the Arabidopsis glycosyltransferase family 8 are xylan glucuronosyltransferases

Xylan is a major component of the plant cell wall and the most abundant noncellulosic component in the secondary cell walls that constitute the largest part of plant biomass. Dicot glucuronoxylan consists of a linear backbone of β(1,4)-linked xylose residues substituted with α(1,2)-linked glucuronic acid (GlcA). Although several genes have been implicated in xylan synthesis through mutant analyses, the biochemical mechanisms responsible for synthesizing xylan are largely unknown. Here, we show evidence for biochemical activity of GUX1 (for GlcA substitution of xylan 1), a member of Glycosyltransferase Family 8 in Arabidopsis (Arabidopsis thaliana) that is responsible for adding the glucuronosyl substitutions onto the xylan backbone. GUX1 has characteristics typical of Golgi-localized glycosyltransferases and a K(m) for UDP-GlcA of 165 μm. GUX1 strongly favors xylohexaose as an acceptor over shorter xylooligosaccharides, and with xylohexaose as an acceptor, GlcA is almost exclusively added to the fifth xylose residue from the nonreducing end. We also show that several related proteins, GUX2 to GUX5 and Plant Glycogenin-like Starch Initiation Protein6, are Golgi localized and that only two of these proteins, GUX2 and GUX4, have activity as xylan α-glucuronosyltransferases.

Glycogen and its metabolism: some new developments and old themes

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

Structural and biochemical insight into glycogenin inactivation by the glycogenosis-causing T82M mutation

The X-ray structure of rabbit glycogenin containing the T82M (T83M according to previous authors amino acid numbering) mutation causing glycogenosis showed the loss of Thr82 hydrogen bond to Asp162, the residue involved in the activation step of the glucose transfer reaction mechanism. Autoglucosylation, maltoside transglucosylation and UDP-glucose hydrolyzing activities were abolished even though affinity and interactions with UDP-glucose and positioning of Tyr194 acceptor were conserved. Substitution of Thr82 for serine but not for valine restored the maximum extent of autoglucosylation as well as transglucosylation and UDP-glucose hydrolysis rate. Results provided evidence sustaining the essential role of the lost single hydrogen bond for UDP-glucose activation leading to glycogenin-bound glycogen primer synthesis.

Conformational plasticity of glycogenin and its maltosaccharide substrate during glycogen biogenesis

Glycogenin initiates the synthesis of a maltosaccharide chain covalently attached to itself on Tyr195 via a stepwise glucosylation reaction, priming glycogen synthesis. We have captured crystallographic snapshots of human glycogenin during its reaction cycle, revealing a dynamic conformational switch between ground and active states mediated by the sugar donor UDP-glucose. This switch includes the ordering of a polypeptide stretch containing Tyr195, and major movement of an approximately 30-residue "lid" segment covering the active site. The rearranged lid guides the nascent maltosaccharide chain into the active site in either an intra- or intersubunit mode dependent upon chain length and steric factors and positions the donor and acceptor sugar groups for catalysis. The Thr83Met mutation, which causes glycogen storage disease XV, is conformationally locked in the ground state and catalytically inactive. Our data highlight the conformational plasticity of glycogenin and coexistence of two modes of glucosylation as integral to its catalytic mechanism.

Molecular pathogenesis of a new glycogenosis caused by a glycogenin-1 mutation

Glycogenin-1 initiates the glycogen synthesis in skeletal muscle by the autocatalytic formation of a short oligosaccharide at tyrosine 195. Glycogenin-1 catalyzes both the glucose-O-tyrosine linkage and the α1,4 glucosidic bonds linking the glucose molecules in the oligosaccharide. We recently described a patient with glycogen depletion in skeletal muscle as a result of a non-functional glycogenin-1. The patient carried a Thr83Met substitution in glycogenin-1. In this study we have investigated the importance of threonine 83 for the catalytic activity of glycogenin-1. Non-glucosylated glycogenin-1 constructs, with various amino acid substitutions in position 83 and 195, were expressed in a cell-free expression system and autoglucosylated in vitro. The autoglucosylation was analyzed by gel-shift on western blot, incorporation of radiolabeled UDP-(14)C-glucose and nano-liquid chromatography with tandem mass spectrometry (LC/MS/MS). We demonstrate that glycogenin-1 with the Thr83Met substitution is unable to form the glucose-O-tyrosine linkage at tyrosine 195 unless co-expressed with the catalytically active Tyr195Phe glycogenin-1. Our results explain the glycogen depletion in the patient expressing only Thr83Met glycogenin-1 and why heterozygous carriers without clinical symptoms show a small proportion of unglucosylated glycogenin-1.

Mechanisms of monomeric and dimeric glycogenin autoglucosylation

Initiation of glucose polymerization by glycogenin autoglucosylation at Tyr-194 is required to prime de novo biosynthesis of glycogen. It has been proposed that the synthesis of the primer proceeds by intersubunit glucosylation of dimeric glycogenin, even though it has not been demonstrated that this mechanism is responsible for the described polymerization extent of 12 glucoses produced by the dimer. We reported previously the intramonomer glucosylation capability of glycogenin without determining the extent of autoglucopolymerization. Here, we show that the maximum specific autoglucosylation extent (MSAE) produced by the non-glucosylated glycogenin monomer is 13.3 ± 1.9 glucose units, similar to the 12.5 ± 1.4 glucose units measured for the dimer. The mechanism and capacity of the dimeric enzyme to carry out full glucopolymerization were also evaluated by construction of heterodimers able to glucosylate exclusively by intrasubunit or intersubunit reaction mechanisms. The MSAE of non-glucosylated glycogenin produced by dimer intrasubunit glucosylation was 16% of that produced by the monomer. However, partially glucosylated glycogenin was able to almost complete its autoglucosylation by the dimer intrasubunit mechanism. The MSAE produced by heterodimer intersubunit glucosylation was 60% of that produced by the wild-type dimer. We conclude that both intrasubunit and intersubunit reaction mechanisms are necessary for the dimeric enzyme to acquire maximum autoglucosylation. The full glucopolymerization capacity of monomeric glycogenin indicates that the enzyme is able to synthesize the glycogen primer without the need for prior dimerization.

Polymer phosphorylases: clues to the emergence of non-replicative and replicative polymers

Polymer formation is arguably one of the essential factors that allowed the emergence, stabilisation and spread of life on Earth. Consequently, studies concerning biopolymers could shed light on the origins of life itself. Of particular interest are RNA and polysaccharide polymers, the archetypes of the contrasting proposed evolutionary scenarios and their respective polymerases. Nucleic acid polymerases were hypothesised, before their discovery, to have a functional similarity with glycogen phosphorylase. Further identification and characterisation of nucleic acid polymerases; particularly of polynucleotide phosphorylase (PNPase), provided experimental evidence for the initial premise. Once discovered, frequent similarities were found between PNPase and glycogen phosphorylase, in terms of catalytic features and biochemical properties. As a result, PNPase was seen as a model of primitive polymerase and used in laboratory precellular systems. Paradoxically, however, these similarities were not sufficient as an argument in favour of an ancestral common polymerisation mechanism prior to polysaccharides and polyribonucleotides. Here we present an overview of the common features shared by polymer phosphorylases, with new proposals for the emergence of polysaccharide and RNA polymers.

The cellular basis of chitin synthesis in fungi and insects: common principles and differences

Chitin is a polymer of N-acetylglucosamine, which assembles into microfibrils of about 20 sugar chains. These microfibrils serve as a structural component of natural biocomposites found in cell walls and specialized extracellular matrices such as cuticles and peritrophic membranes. Chitin synthesis is performed by a wide range of organisms including fungi and insects. The underlying biosynthetic machinery is highly conserved and involves several enzymes, of which the chitin synthase is the key enzyme. This membrane integral glycosyltransferase catalyzes the polymerization reaction. Most of what we know about chitin synthesis derives from studies of fungal and insect systems. In this review, common principles and differences will be worked out at the levels of gene organization, enzymatic properties, cellular localization and regulation.

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

Evolution and function of the plant cell wall synthesis-related glycosyltransferase family 8

Carbohydrate-active enzyme glycosyltransferase family 8 (GT8) includes the plant galacturonosyltransferase1-related gene family of proven and putative alpha-galacturonosyltransferase (GAUT) and GAUT-like (GATL) genes. We computationally identified and investigated this family in 15 fully sequenced plant and green algal genomes and in the National Center for Biotechnology Information nonredundant protein database to determine the phylogenetic relatedness of the GAUTs and GATLs to other GT8 family members. The GT8 proteins fall into three well-delineated major classes. In addition to GAUTs and GATLs, known or predicted to be involved in plant cell wall biosynthesis, class I also includes a lower plant-specific GAUT and GATL-related (GATR) subfamily, two metazoan subfamilies, and proteins from other eukaryotes and cyanobacteria. Class II includes galactinol synthases and plant glycogenin-like starch initiation proteins that are not known to be directly involved in cell wall synthesis, as well as proteins from fungi, metazoans, viruses, and bacteria. Class III consists almost entirely of bacterial proteins that are lipooligo/polysaccharide alpha-galactosyltransferases and alpha-glucosyltransferases. Sequence motifs conserved across all GT8 subfamilies and those specific to plant cell wall-related GT8 subfamilies were identified and mapped onto a predicted GAUT1 protein structure. The tertiary structure prediction identified sequence motifs likely to represent key amino acids involved in catalysis, substrate binding, protein-protein interactions, and structural elements required for GAUT1 function. The results show that the GAUTs, GATLs, and GATRs have a different evolutionary origin than other plant GT8 genes, were likely acquired from an ancient cyanobacterium (Synechococcus) progenitor, and separate into unique subclades that may indicate functional specialization.

The regulation of muscle glycogen: the granule and its proteins

Despite decades of studying muscle glycogen in many metabolic situations, surprisingly little is known regarding its regulation. Glycogen is a dynamic and vital metabolic fuel that has very limited energetic capacity. Thus its regulation is highly complex and multifaceted. The stores in muscle are not homogeneous and there appear to be various metabolic pools. Each granule is capable of independent regulation and fundamental aspects of the regulation appear to be associated with a complex set of proteins (some are enzymes and others serve scaffolding roles) that associate both with the granule and with each other in a dynamic fashion. The regulation includes altered phosphorylation status and often translocation as well. The understanding of the roles and the regulation of glycogenin, protein phosphatase 1, glycogen targeting proteins, laforin and malin are in their infancy. These various processes appear to be the mechanisms that give the glycogen granule precise, yet dynamic regulation.

Structural analysis of Thermus thermophilus HB27 mannosyl-3-phosphoglycerate synthase provides evidence for a second catalytic metal ion and new insight into the retaining mechanism of glycosyltransferases

Mannosyl-3-phosphoglycerate synthase is a glycosyltransferase involved in the two-step synthetic pathway of mannosylglycerate, a compatible solute that accumulates in response to salt and/or heat stresses in many microorganisms thriving in hot environments. The three-dimensional structure of mannosyl-3-phosphoglycerate synthase from Thermus thermophilus HB27 in its binary complex form, with GDP-alpha-D-mannose and Mg(2+), shows a second metal binding site, about 6 A away from the mannose moiety. Kinetic and mutagenesis studies have shown that this metal site plays a role in catalysis. Additionally, Asp(167) in the DXD motif is found within van der Waals contact distance of the C1' atom in the mannopyranose ring, suggesting its action as a catalytic nucleophile, either in the formation of a glycosyl-enzyme intermediate according to the double-displacement S(N)2 reaction mechanism or in the stabilization of the oxocarbenium ion-like intermediate according to the D(N)*A(Nss) (S(N)i-like) reaction mechanism. We propose that either mechanism may occur in retaining glycosyltransferases with a GT-A fold, and, based on the gathered structural information, we identified an extended structural signature toward a common scaffold between the inverting and retaining glycosyltransferases.

Glycogenin-1 deficiency and inactivated priming of glycogen synthesis

Glycogen, which serves as a major energy reserve in cells, is a large, branched polymer of glucose molecules. We describe a patient who had muscle weakness, associated with the depletion of glycogen in skeletal muscle, and cardiac arrhythmia, associated with the accumulation of abnormal storage material in the heart. The skeletal muscle showed a marked predominance of slow-twitch, oxidative muscle fibers and mitochondrial proliferation. Western blotting showed the presence of unglucosylated glycogenin-1 in the muscle and heart. Sequencing of the glycogenin-1 gene, GYG1, revealed a nonsense mutation in one allele and a missense mutation, Thr83Met, in the other. The missense mutation resulted in inactivation of the autoglucosylation of glycogenin-1 that is necessary for the priming of glycogen synthesis in muscle.