Reserve carbohydrates metabolism in the yeast Saccharomyces cerevisiae

Abstract

Glycogen and trehalose are the two glucose stores of yeast cells. The large variations in the cell content of these two compounds in response to different environmental changes indicate that their metabolism is controlled by complex regulatory systems. In this review we present information on the regulation of the activity of the enzymes implicated in the pathways of synthesis and degradation of glycogen and trehalose as well as on the transcriptional control of the genes encoding them. cAMP and the protein kinases Snf1 and Pho85 appear as major actors in this regulation. From a metabolic point of view, glucose-6-phosphate seems the major effector in the net synthesis of glycogen and trehalose. We discuss also the implication of the recently elucidated TOR-dependent nutrient signalling pathway in the control of the yeast glucose stores and its integration in growth and cell division. The unexpected roles of glycogen and trehalose found in the control of glycolytic flux, stress responses and energy stores for the budding process, demonstrate that their presence confers survival and reproductive advantages to the cell. The findings discussed provide for the first time a teleonomic value for the presence of two different glucose stores in the yeast cell.

Carbohydrate utilization and the lager yeast transcriptome during brewery fermentation

“…The disaccharide trehalose, while historically viewed as a carbon and energy reserve (Fales, 1951), is now considered to function principally as a stress protectant and is known to have a role in protecting against the effects of a number of stresses (Francois and Parrou, 2001), including those associated with brewery-handling of yeast (Gibson et al, 2007). Of the genes involved in trehalose synthesis, only TPS1 demonstrated an increase in transcription (at 60 h), while TPS2 demonstrated a clear reduction in transcript levels throughout the sampling period.…”

“…In this investigation four genes involved in glycogen nucleation, synthesis and branching showed an increase in transcription level at or after 60 h, which corresponds to the time when cellular glycogen would typically increase during fermentation (O'ConnorCox et al, 1996;Quain, 1982). Three genes are involved in glycogen catabolism, of which two, GPH1 and GDB1, are typically associated with glycogen breakdown (Francois and Parrou, 2001). These genes showed no change in expression throughout the incubation period.…”

“…These genes showed no change in expression throughout the incubation period. However, transcription of SGA1, which encodes a glucoamylase associated with glycogen catabolism during sporulation (Francois and Parrou, 2001), was shown to increase throughout the fermentation. This enzyme is, however, also associated with the breakdown of maltose and maltotriose (Francois and Parrou, 2001) and the increase in transcription may therefore be related to the predominance of these sugars in the wort towards the end of the fermentation period.…”

“…However, transcription of SGA1, which encodes a glucoamylase associated with glycogen catabolism during sporulation (Francois and Parrou, 2001), was shown to increase throughout the fermentation. This enzyme is, however, also associated with the breakdown of maltose and maltotriose (Francois and Parrou, 2001) and the increase in transcription may therefore be related to the predominance of these sugars in the wort towards the end of the fermentation period. Glycogen catabolism is typically associated with the first and last hours of fermentation (Quain, 1982) and it may be that the sampling period did not include the times when transcriptional change in glycogen catabolism-related genes occurred.…”

HXT5 expression is under control of STRE and HAP elements in the HXT5 promoter

“…However, a subset of genes that is characteristic for this phase was already induced when glucose and all other nutrients were still available in the medium, and expression was induced far in advance of other so-called diauxic and stationary phase events (Herman, 2002;Werner-Washburne et al, 1993, 1996. Examples include genes involved in the synthesis of reserve carbohydrates (for review, see Francois and Parrou, 2001) and the general stress-response gene CTT1 (Marchler et al, 1993). Expression of the hexose transporter HXT5 was also induced when still ample glucose was present in the medium (Verwaal et al, 2002).…”