Effects Of Nitrogen Sources On The Metabolites Of Saccharomyces Cerevisiae

Nitrogen source is essential nutrient for Saccharomyces cerevisiae to carry out normal life activities.Nitrogen source can affect the content and proportion of metabolites by affecting the metabolic pathway of Saccharomyces cerevisiae.In order to investigate the relationship between nitrogen source and metabolites,three treatments were conducted according to 180 mg/L,280 mg/L and 380 mg/L nitrogen source.We discussed the changes of esters,higher alcohols and volatile organic acids as well as influence of nitrogen concentration on physicochemical properties of fermentation broth.The results are showed below:(1)Low nitrogen stress inhibited the propagation rate of yeast and reduced yeast concentration at stationary phase.The content of volatile acids in fermentation broth increased with the rising of nitrogen source concentration.(2)The content of esters increased with the rising of nitrogen concentration.The content of acetates produced by Saccharomyces cerevisiae was significantly higher than that of ethyl esters under 3 nitrogen sources.The concentration of synthetic grape must did not change the generation law of ester substances in fermentation broth,but it affected the content of esters at the end of fermentation.Under low nitrogen stress,the amount of ethyl phenethyl acetate and ethyl decanoate increased.The total amount of higher alcohols,the content of isobutanol,isoamyl alcohol and benzol gradually increased with the lower of nitrogen source concentration.But the content of propanol decreased under the same condition.The total amount of volatile organic acids decreased under lower nitrogen source in the fermentation broth.In order to investigate the increase of higher alcohols under low nitrogen stress.In this study,180 mg/L of nitrogen source concentration of 180 mg/L was designed as the low nitrogen stress group and 380 mg/L was the control group.Transcriptome sequencing was performed in the middle of logarithmic growth.And we studied the metabolic pathways of higher alcohols in Saccharomyces cerevisiae under low nitrogen stress.The results are showed below:(1)A total of 237 genes exhibited differential expression.Among all these genes,122 were up-regulated and 115 were down-regulated.(2)GO(Gene ontology)and KEGG Pathway(Kyoto Encyclopedia of genes and genomes)enrichment analysis found 44 differentially expressed genes associated withthe increase of higher alcohols under low nitrogen stress.Major of differential expression genes mapped to pathways of nitrogen catabolite repression,glucose metabolism,amino acid metabolism and nitrogen source translocation.(3)Under low nitrogen conditions,these differential genes can regulate Saccharomyces cerevisiae glucose metabolism pathway.Then more pyruvate enters the Erich pathway to produce aromatic amino acids(phenylalanine)and branched amino acids(valine and leucine).A part of the ?-ketoacid produced corresponding higher alcohols through decarboxylation and dehydrogenation,which resulted in the increase of phenethyl alcohol,isobutanol and isoamyl alcohol under low nitrogen stress.Under low nitrogen conditions,excessive pyruvate is used to synthesize branched and aromatic amino acids,which leads to a decrease in the metabolic flow of ?-ketobutyrate(precursors of 1-propanol)synthesized from pyruvate.In addition,alpha-ketobutyric acid can also be produced by threonine through the action of transammonia,and the content of threonine is relatively low under low nitrogen conditions,which reduces the metabolic flow from threonine to ?-ketobutyric acid and eventually leads to the reduction of propanol production under low nitrogen conditions.?-ketobutyric acid can also be produced by threonine through transamination.And the content of threonine is relatively low under low nitrogen conditions,which reduces the metabolic flow from threonine to ?-ketobutyric acid and eventually leads to the reduction of propanol production under low nitrogen conditions.