Evidence for the periplasmic location of glycogen in Saccharomyces

A fungal protein organizes both glycogen and cell wall glucans

Glycogen is a glucose storage molecule composed of branched α-1,4-glucan chains, best known as an energy reserve that can be broken down to fuel central metabolism. Because fungal cells have a specialized need for glucose in building cell wall glucans, we investigated whether glycogen is used for this process. For these studies, we focused on the pathogenic yeast Cryptococcus neoformans, which causes ~150,000 deaths per year worldwide. We identified two proteins that influence formation of both glycogen and the cell wall: glycogenin (Glg1), which initiates glycogen synthesis, and a protein that we call Glucan organizing enzyme 1 (Goe1). We found that cells missing Glg1 lack α-1,4-glucan in their walls, indicating that this material is derived from glycogen. Without Goe1, glycogen rosettes are mislocalized and β-1,3-glucan in the cell wall is reduced. Altogether, our results provide mechanisms for a close association between glycogen and cell wall.

Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.

A comprehensive review on α-D-Glucans: Structural and functional diversity, derivatization and bioapplications

Glucans are the most abundant natural polysaccharides across the living kingdom with tremendous biological activities. Now a days, α-D-glucans are gaining importance as a prebiotics, nutraceuticals, immunostimulants, antiproliferative agents and biodegradable polymers in pharmaceutical and cosmetic sectors. A wide variety of bioresources including bacteria, fungi, lichens, algae, plants and animals produce α-D-glucans either as an exopolysaccharide (EPS) or a cell wall component or an energy storage polymer. The α-D-glucans exhibit great structural and functional diversity as the type of linkage and percentage of branching dictate the functional properties of glucans. Among the different linkages, bioactivities are greatly confined to the α-D-(1 → 3) linkages whereas starch and other polymers consisting of α-D-(1 → 4) (1 → 6) linkages are specific for food and pharmaceutical applications. However, the bioactivities of the α-D-(1 → 3) glucans in native form is limited mainly due to their hydrophobic nature. Hence several derivatization techniques have been developed to improve the bioavailability as well as bioactive features such as antiviral, antimicrobial, anti-inflammatory, antioxidant, immunomodulatory and antitumor properties. Though, several reports have presented about α-D-glucans, still there is an ambiguity in terms of their structure among different natural sources and moreover no comprehensive information was available on their derivatization techniques and application potential. Therefore, the present review summarizes distinct description on diverse sources, type of linkages, derivatization techniques as well as the application potential of the native and modified α-D-glucans.

Mechanistic Study of Utilization of Water-Insoluble Saccharomyces cerevisiae Glucans by Bifidobacterium breve Strain JCM1192

Bifidobacteria exert beneficial effects on hosts and are extensively used as probiotics. However, due to the genetic inaccessibility of these bacteria, little is known about their mechanisms of carbohydrate utilization and regulation. Bifidobacterium breve strain JCM1192 can grow on water-insoluble yeast (Saccharomyces cerevisiae) cell wall glucans (YCWG), which were recently considered as potential prebiotics. According to the results of ¹H nuclear magnetic resonance (NMR) spectrometry, the YCWG were composed of highly branched (1→3,1→6)-β-glucans and (1→4,1→6)-α-glucans. Although the YCWG were composed of 78.3% β-glucans and 21.7% α-glucans, only α-glucans were consumed by the B. breve strain. The ABC transporter (malEFG1) and pullulanase (aapA) genes were transcriptionally upregulated in the metabolism of insoluble yeast glucans, suggesting their potential involvement in the process. A nonsense mutation identified in the gene encoding an ABC transporter ATP-binding protein (MalK) led to growth failure of an ethyl methanesulfonate-generated mutant with yeast glucans. Coculture of the wild-type strain and the mutant showed that this protein was responsible for the import of yeast glucans or their breakdown products, rather than the export of α-glucan-catabolizing enzymes. Further characterization of the carbohydrate utilization of the mutant and three of its revertants indicated that this mutation was pleiotropic: the mutant could not grow with maltose, glycogen, dextrin, raffinose, cellobiose, melibiose, or turanose. We propose that insoluble yeast α-glucans are hydrolyzed by extracellular pullulanase into maltose and/or maltooligosaccharides, which are then transported into the cell by the ABC transport system composed of MalEFG1 and MalK. The mechanism elucidated here will facilitate the development of B. breve and water-insoluble yeast glucans as novel synbiotics.IMPORTANCE In general, Bifidobacterium strains are genetically intractable. Coupling classic forward genetics with next-generation sequencing, here we identified an ABC transporter ATP-binding protein (MalK) responsible for the import of insoluble yeast glucan breakdown products by B. breve JCM1192. We demonstrated the pleiotropic effects of the ABC transporter ATP-binding protein in maltose/maltooligosaccharide, raffinose, cellobiose, melibiose, and turanose transport. With the addition of transcriptional analysis, we propose that insoluble yeast glucans are broken down by extracellular pullulanase into maltose and/or maltooligosaccharides, which are then transported into the cell by the ABC transport system composed of MalEFG1 and MalK. The mechanism elucidated here will facilitate the development of B. breve and water-insoluble yeast glucans as novel synbiotics.

Glycogen--a covalently linked component of the cell wall in Saccharomyces cerevisiae

Glycogen in Saccharomyces cerevisiae is present in two pools, one soluble and intracellular, the other present in the cell wall and rendered water-insoluble owing to its covalent linkage to cell wall beta-glucan. The insoluble glycogen fraction was solubilized using beta-1,3-glucanase. The alpha beta-glucan complex obtained showed intense red staining with iodine and was isolated from free beta-glucans by affinity chromatography using concanavalin A sepharose 4B. Further use of molecular sieving has confirmed that glycogen is linked to beta-glucan as the non-retained fraction on Biogel P2 split into two peaks on treatment with amyloglucosidase. Partial acid hydrolysis and subsequent paper chromatography of the alpha beta-glucan complex isolated revealed the presence of gentiobiose and other higher oligosaccharides, indicating that glycogen is linked to beta-1,3-glucan through a beta-1,6 branch. The insoluble glycogen can be extracted in a soluble form by acetic acid treatment and is known as acid-soluble glycogen. The presence of glycogen in the cell wall is confirmed by controlled enzymatic release of alpha beta-glucan complex using lyticase from Arthobacter luteus without disruption of the plasma membrane, as can be visualized using electron microscopy.

Regulation of energy metabolism in yeast. Inheritance of a pleiotropic mutation causing defects in metabolism of energy reserves, ethanol utilization and formation of cytochrome a.a3

The recessive, nuclear gene mutation glc1, which causes glycogen deficiency in Saccharomyces cerevisiae, is highly pleiotropic. Studies of the inheritance of glc1 revealed two classes of phenotypic characteristics: I. Traits invariably associated with the mutant gene and II. Traits whose expressions require the presence of glc1 and one or more additional genes. Class I traits include glycogen deficiency and the loss of capacity to accumulate trehalose in nonproliferating conditions. Traits in the second class include a decreased rate of growth on ethanol medium, a deficiency in cytochrome a.a3 and an enhanced accumulation of pigment, probably a metalloporphyrin. Constructed strains containing both glc1 and the constitutive maltose fermentation gene MAL4c can accumulate trehalose but not glycogen during growth on glucose. However, accumulated trehalose is degraded when cells are exposed to nonproliferating conditions. It is proposed that the glc1 mutation affects a regulatory system, probably involving a protein kinase and/or protein phosphatase, which regulates glycogen synthase and trehalase. Independent regulation of trehalose synthesis by a system controlled by MAL4c is indicated.

Two pools of glycogen in Saccharomyces

The effect of different extraction procedures on the yields of water-soluble and water-insoluble glycogen fractions from a number of Saccharomyces strains was studied by using a specific method for glycogen determination. The similarity of the yields obtained by the different procedures showed that neither form of glycogen is an artifact, and variations in the relative amounts of glycogen in the two fractions during cell growth and in different yeast strains suggest that they represent different pools of storage material with specific roles in cell development and differentiation. A proportion of the water-insoluble glycogen fraction, solubilized by mechanical agitation, was shown to be strongly associated with a beta-glucan-like polysaccharide that may be a cell wall component.

Interaction of concanavalin A with external mannan-proteins of Saccharomyces cerevisiae. Glycoprotein nature of beta-glucanases

beta-Glucanases secreted into culture fluid by protoplasts or intact cells of the yeast Saccharomyces cerevisiae were investigated for the presence of covalently linked carbohydrates. Gel filtration of the enzymes on Biogel A-1.5m showed that endo-beta-1,3-glucanase is a polydisperse enzyme of high-molecular weight which elutes in about the same volume as external yeast invertase. Exo-beta-glucanase was eluted from the gel as a much lighter enzyme. Endo-beta-1,3-glucanase added to a mixture of extracellular mannoproteins was precipitated by concanavalin A to a similar extent to mannan, invertase and acid phosphatase. Under the same conditions exo-beta-glucanase did not interact with the lectin, but was partially precipitated from the solution in the absence of foreign mannan or mannan-proteins. The results show that endo-beta-1,3-glucanase of S. cerevisiae is a mannoprotein of a similar nature to external invertase and acid phosphatase. However, exo-beta-glucanase appears to be a glycoprotein which does not contain the highly branched mannan polymer in its molecule.