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Central role of the vacuole in nutrient modulation of volatile sulfur compound formation during fermentation by Saccharomyces cerevisiae
| Content Provider | Semantic Scholar |
|---|---|
| Author | Winter, Gal |
| Copyright Year | 2012 |
| Abstract | Volatile sulfur compounds (VSCs) are a key component of wine aroma, contributing both attractive (‘fruity’) and repulsive (‘rotten eggs’) odours. These compounds largely develop in wine by yeast metabolic activity, as yeast consume the sulfur nutrients available in the grape juice and release a range of VSCs. This thesis studies the formation of VSCs by the wine yeast Saccharomyces cerevisiae and its modulation through nutrient supplementation, focusing on organic sulfur nutrient sourcescysteine, glutathione (GSH) and their polyfunctional thiol conjugates. The thesis first objective was to characterise the effect of an organic nutrient preparation on the resultant sulfur aroma composition. Using metabolite profiling we demonstrated two distinct effects; c on entration of the ‘fruity’ polyfunctional thiols 3-mercaptohexan-1-ol (3MH) and 3mercaptohexyl acetate (3MHA), was increased while the concentration of the ‘rotten egg’ aroma compound, hydrogen sulfide (H2S), was decreased. Nutrient supplementation also changed the kinetics of H 2S production during fermentation by advancing its onset. Transcriptomic analysis revealed that this was not due to expression changes within the sulfate assimilation pathway, which is known to be a major contributor to H 2S production. Further investigations suggested an organic sulfur component of the nutrient mix, GSH, as an activator of an alternative pathway for H 2S formation. GSH was previously shown to contribute to H 2S formation. Results here infer that this contribution is through GSH sulfur containing constituentcysteine. Although cysteine catabolism to release H 2S was demonstrated in yeast, the cellular mechanisms regulating this pathway are not yet understood. As S. cerevisiae is a model organism of the eukaryotic cell, the study of cysteine catabolism to release H 2S hold a broader relevance, rising from H 2S increasing prominence as an essential modulator of many physiological processes, particularly considering that cysteine is the main source for H 2S formation in mammalian cells. These research interests motivated the next objective of this thesisa comprehensive genome-wide screen for cysteine catabolism to release H 2S, to shed light on the cellular mechanisms involved in this pathway. A technical barrier in meeting this goal was the lack of available methodology for high throughput detection of cysteine-generated H 2S. To overcome this difficulty a novel, patent-pending method was developed, suitable for high throughput detection of cysteine-generated H 2S. The method utilizes a redox reaction in which sulfide ion reduces methylene blue, leading to its decolourisation. Incorporation of methylene blue into the fermentation medium allows real-time, in situ, detection of H2S and the generation of an accurate H 2S production profile. Following the method validation, its principles were implemented in a genome-wide screen for cysteine catabolism to release H 2S. Results of the screen revealed a surprising set of cellular factors affecting this process. The yeast vacuole, not previously associated with cysteine catabolism, emerged as a major compartment for cysteine degradation with deletants impaired in vacuole biogenesis unable to catabolise cysteine to release H 2S. The mechanism of vacuole acidification was identified as prominent in cysteine catabolism to release H 2S; deletants for each of the eight subunits of a vacuole acidification sub-complex (V 1 of the yeast V-ATPase) were independently classified as essential for cysteine degradation to H 2S. This is the first genome-wide study aimed to elucidate the cellular factors affecting cysteine catabolism. Foundations laid here support the use of S. cerevisiae as a model organism to study cysteine catabolism and may provide insights into the underlying cause of cysteine accumulation and H 2S generation in eukaryotes. In conclusion, this thesis demonstrates the effective use of nutrients supplementation as a means for wine aroma management. Results and techniques obtained here significantly contribute to our understanding of volatile sulfur compounds formation and provide tools that can be implemented in winemaking for better modulation of sulfur aroma compounds in wine. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://researchdirect.westernsydney.edu.au/islandora/object/uws:17452/datastream/PDF/view |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
