ABSTRACT
The second group is made of β-glucans. In fungi, these are mostly made of glucosyl units bound by β-1,3-and/or β-1,6-linkages. β-1,3-glucans are the most abundant polysaccharides present in the fungal cell walls, with the above-indicated exception of Zygomycota, which contain the polysaccharide only in the cell wall of the sporangiospores. The structure of β-1,3-glucans was determined by the electron microscopic observation of S. cerevisiae protoplasts synthesizing the unbranched polysaccharide in the form of micro…brils (Kreger and Kopecka 1975). Similar micro…brils measuring about 0.5 mm in length were synthesized by incubation of cell-free extracts from S. cerevisiae with uridine diphosphate glucose (UDPGlc) (Larriba, Morales, and Ruiz-Herrera 1981). This product was completely solubilized by an exo β-1,3-glucanase giving rise to glucose as the only product. The absence of gentobiose (a disaccharide made of glucose units bound by β-1,6linkages) in the product of hydrolysis, revealed the absence of β-1,6-branches. All these β-1,3-glucans are poorly crystalline and similar to the so-called hydroglucan obtained by boiling the branched β-glucan from yeast with mineral acids, a treatment that eliminates the β-1,6-bound branches. It has been suggested that the micro…brils of the β-1,3-glucans synthesized by yeast protoplasts, or cell-free extracts, or the β-1,3-glucan curdlan from Alcaligenes faecalis var. myxogenes (Harada et al. 1979) are similar to lentinan, a β-1,3-glucan from L. edodes (Bluhm and Sarko 1977). It was suggested that the structure of lentian was composed of three intertwined helical chains whose association is stabilized by extensive hydrogen bonding at the C-2 hydroxyl residue and which contains six glucose moieties per turn (Kopecka and Kreger 1986). Nevertheless, small changes in the number of glucose units per turn or the hydrogen bonding that keeps the chains associated may occur (see Ruiz-Herrera 1992 for discussion).
