The Research Of Cell Wall Composition And Related Enzymatic Properties Of Saccharomyces Cerevisiae Aging Process

Saccharomyces cerevisiae is the most thoroughly studied eukaryotic single cellular microorganism and also a kind of representative model organism in the study. It is also of great value in production practice in the brewing industry, the pharmaceutical industry and food industry etc. In the practical production, strain aging and apoptosis has great adverse effect on the quality of the products, such as low fermentation degree, coarse taste, strong sour taste, turbidity of fermentation liquid and even failure of fermentation in beer production due to ageing of the strain for fermentation. So, Saccharomyces cerevisiae was used as the experimental object to study yeast ageing in this paper. Yeast cell walls are the outermost layer of yeast cells and directly contacted with outside growth environment. It is of crucial importance to maintain its own configuration and structure, osmotic pressure, material metabolism, immune recognition. Therefore the study was divided into several aspects such as configuration, composition, structure of yeast cell walls in the process of the yeast strain aging.The experimental results indicate:In the process of continuous liquid culture of yeast for a long time, metabolic capacity of yeast shows apparent downtrend. Degradation rate of reducing sugars decreased, the growth rate of yeast delayed and meanwhile the flocculating velocity increased. When yeast is cultured for20days, flocculent precipitate of yeast was noted in the culture medium within5min standing. To investigate the changes of yeast cell walls in ageing process, protein composition of yeast cell walls was analyzed by using two-dimensional electrophoresis. Great changes were found by analysis. The composition changes of yeast cell wall polysaccharides of different ageing levels were analyzed by high performance liquid chromatography. The results indicate that inner wall skeleton structural composition (glucan) proportion gradually increased and mannose content which has a modification to cell wall protein showed a decreased trend. It is considered that with the gradual increase of yeast colony ageing degree, great changes occurred in yeast cell walls, such as thickening of cell walls and low glycosylation of cell wall protein which is the main reason for yeast abnormal flocculation. To investigate why cell wall is thickening during the yeast ageing process, aggregation of yeast skeleton structure layer (glucan) was analyzed by and GC-MS technology in the study. Gel permeation chromatography results showed that molecular weight composition has an increasing trend with yeast cell ageing degree by analysis of glucan fragments hydrolyzed from yeast cells of different ageing degree. Combined with GC-MS analysis, β-1,6-glycosidic bond contents in glucan decreased, but proportion of β-1,3-glycosidic bond contents increased. Therefore it is concluded that once structure of yeast cell wall became loose during yeast ageing process, the yeast cell repaired itself through more glucan expression compensating for the damage to cells due to aggregation rate decrease to maintain its natural physiological metabolic activities, thus leading to the thickening of cell walls and slight changes of yeast shape from ellipse to round.