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Glycogen

In anaerobic yeast, polysaccharides are quantitatively bigger than the collective protein component. Of these, glucan, mannan and chitin perform a structural role in the cell wall, whereas glycogen is a reserve or storage polysaccharide that can be mobilized when required to provide metabolic energy. Glycogen is a high-molecular-weight branched polymer of a-D-glucose located in the cytosol (‘bodies') and vacuole (Wilson et al., 2010) with a ‘pool' associated with the cell wall (Gunja-Smith et al., 1977; Deshpande et al., 2011). Regulation of glycogen synthesis and dissimilation is complex and has been reviewed by Wilson et al. (2010). Its particular claim to fame has been somewhat dismissively described as ‘at the beginning of industrial fermentation procedure stored glycogen is rapidly degraded, while it accumulates once fermentation is complete (Feldman, 2012).

It has long been known that accumulated carbohydrate reserves in anaerobic yeast are rapidly broken down on exposure to oxygen (Chester, 1963). In a brewing context (Quain et al., 1981), glycogen is rapidly broken down after pitching into oxygenated wort, only to reaccumulate during active fermentation before the cycle repeats. The amount of glycogen at the end of fermentation is substantial and is about 30% of the dry biomass (less specific methods of analysis probably over report). On exposure to oxygen, glycogen accounts for < 5% of the dry weight. This rapid mobilization of glycogen has been linked to providing the metabolic energy for the synthesis of new sterols and unsaturated fatty acids. The key argument for this is that the plasma membrane in anaerobic yeast is lipid depleted with limited functionality for transport of wort nutrients (e.g. sugars). Detailed measurement of glucose, sucrose, fructose and specific gravity during the anaerobic-aerobic transition would appear to support this hypothesis (Quain and Tubb, 1982). Subsequent work (Quain et al., 1981) demonstrated a linear relationship between glycogen breakdown and sterol synthesis. The apparent fuelling of sterol synthesis by glycogen in turn triggered an awareness of the need to maintain glycogen levels in pitching yeast. Conditions such as a protracted storage in the fermenter cone or in storage vessel have been shown by many workers to result in glycogen breakdown although at a much slower rate than on exposure to oxygen (e.g. Quain and Tubb, 1982; McCaig and Bendiak, 1985; Powell et al., 2004: Somani et al., 2012).

The ‘glycogen story' makes intuitive sense but the experimental techniques and, in particular, analysis was of its time. To the author's knowledge, there has been no more sophisticated analysis to connect more literally glycogen breakdown to sterol synthesis. The expression of the sterol biosynthetic genes (ERG) has been reported to be up-regulated shortly after pitching (Higgins et al., 2003; Rautio et al., 2007). However, there are no reports as to the expression of glycogen phosphory- lase (GPH1) or debranching enzyme (GDB1) post-pitching. A more likely explanation is that these enzymes are already ‘present and able', as can be inferred from the slow dissimilation of glycogen during storage. Intriguingly, a recent paper (Gsell et al., 2015) has shown that deletion of GPH1, the gene for glycogen-mobilizing enzyme, results in decreased levels of sterol esters, triacylglycerols and other lipids. Clearly, this is work in progress and the authors conclude that ‘Gph1p may fulfil multiple independent functions which affect carbohydrate metabolism on the one hand and lipid metabolism on the other hand'.

 
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