GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

5. MICROBIAL CELL STRUCTURE

5.1. CELL WALLS OF MICROORGANISMS

5.1.3. Eukaryotic cell walls

Yeast Cell walls. Yeast cell walls consist mainly of Polysaccharides, with glucose and mannose as their structural units. For instance, the glucan of baker's yeast (Saccharomyces cerevisiae) cell walls consists of a backbone in which glucose residues are linked by β-1,6-bonds. The side chains contain β-1,3-bonds. The number of repeating units in the side chains can reach seven, while the number of structural units in the glucan molecule can range from several dozen to several thousand. The backbone of another polysaccharide, mannan, isolated from baker's yeast cell walls, contains mannose residues linked by α-1,6-bonds. Mannan is highly branched from its linear Structure, with mannose linked by α-1,2- and α-1,3-bonds in these branches.

In The Cell walls of the yeast Candida albicans, the backbone also consists of mannose residues linked by α-1,6-bonds. In the mannan molecules of some Yeasts, both α- and β-bonds occur simultaneously (Pichia pastoris). Some Mannans contain phosphorus.

In addition to homopolymers, yeast Cell walls contain heteropolymers composed of mannose and galactose. These polysaccharides are called galactomannans (genera Trichosporon, Torulopsis). The aminopolysaccharide Chitin has also been detected in yeast cell walls.

Yeast cell walls also contain Proteins. Their composition and localization within the walls are insufficiently studied. It is known that dicarboxylic Amino Acids predominate in the proteins of baker's yeast. Mannan-protein fractions, in which a chemical bond exists between polysaccharides and proteins, have been isolated from The Cell wall fractions of some yeasts.

Fungal cell walls. In fungal cell walls, the main and most common polysaccharide is the aminopolysaccharide chitin. The Structural elements of chitin are N-acetylglucosamine residues linked to each other by β-1,4-bonds. Chitin in the wall is bound to non-polysaccharide substances, mainly proteins and inorganic salts. Polysaccharides containing amino sugars or their derivatives are called mucopolysaccharides. In addition to chitin, the cell walls of some Fungi contain its deacetylated derivative, chitosan. Chitin and chitosan are also found in yeast cell walls.

Another crucial polysaccharide of the fungal wall is Cellulose, a polymer consisting of β-1,4-glucose. Cellulose is the most abundant organic substance. About 10¹¹ tons of cellulose are synthesized annually. In cell walls, cellulose is present in the form of microfibrils, each of which is up to several thousand nanometers long.

As in yeast Cells, fungal cell walls contain glucans and Melanins. When considering cell wall components as a criterion for classifying fungi into higher-rank taxa, it was found that many of them have characteristic pairs of polysaccharides at the core of their structure. Thus, fungi belonging to Hyphochytriomycetes contain Cellulose and Chitin, Chytridiomycetes contain chitin and β-glucan, and Zygomycetes contain chitin and chitosan.

In addition to carbohydrate components, fungal cell walls contain proteins that form polysaccharide-Protein Complexes.