Pyruvate decarboxylase encoded by the PDC1 gene contributes, at least partially, to the decarboxylation of α-ketoisocaproate for isoamyl alcohol formation in Saccharomyces cerevisiae

Mechanisms of production and control of acetate esters in yeasts

Acetate esters, such as isoamyl acetate and ethyl acetate, are major aroma components of alcoholic beverages. They are produced through synthesis from acetyl CoA and the corresponding alcohol by alcohol acetyltransferase (AATase) with specific control of reaction factors, including unsaturated fatty acids and precursors, the percentage of nitrogen, and oxygen. However, the mechanisms by which these specific reaction factors affect acetate ester production remain largely unknown. The cellular mechanisms underlying the effects of these factors on acetate ester production were examined by purifying AATase from yeast, characterizing it, and cloning the ATF gene encoding AATase from sake yeast and bottom-fermenting yeast. Genetic and biochemical studies suggested that the decrease in acetate production with the addition of oxygen and unsaturated fatty acids was due to a decrease in enzyme synthesis resulting from transcriptional repression of the ATF1 gene, which is responsible for most of the AATase activity. Furthermore, these results suggest that expression of the ATF1 gene is intricately regulated by a number of transcriptional regulatory genes such as ROX1 and RAP1. Based on these results, the mechanism of ester regulation by oxygen, unsaturated fatty acids and precursors, and ratio of nitrogen source are becoming clearer from a molecular biological point of view. The physiological significance of ester production by yeast is then discussed. In this review, we summarize the studies on AATase, ATF gene, regulation of ester production, and physiological significance of acetate ester.

Characterization and Correlation of Microbiota and Higher Alcohols Based on Metagenomic and Metabolite Profiling during Rice-Flavor Baijiu Fermentation

Higher alcohol, as an inevitable product of fermentation, plays an important role in the flavor and quality of Baijiu. However, the relationship between the complex microbial metabolism and the formation of higher alcohols in rice-flavor Baijiu was not clear. To investigate the relationship between microorganisms and higher alcohol production, two fermentation mashes inoculated with starters from Heyuan Jinhuangtian Liquor Co., Ltd. (Heyuan, China) as JM and Guangdong Changleshao Co., Ltd. (Meizhou, China) as CM, respectively, with significant differences in higher alcohol profiles during rice-flavor Baijiu fermentation were selected. In general, higher alcohols presented a rapid accumulation during the early fermentation stages, especially in JM, with higher and faster increases than those in CM. As for their precursors including amino acids, pyruvic acid and ketoacids, complex variations were observed during the fermentation. Metagenomic results indicated that Saccharomyces cerevisiae and Rhizopus microsporus were the microorganisms present throughout the brewing process in JM and CM, and the relative abundance of R. microsporus in JM was significantly higher than that in CM. The results of higher alcohol metabolism in JM may contribute to the regulation of higher alcohols in rice-flavor Baijiu.

Identification of Core Regulatory Genes and Metabolic Pathways for the n-Propanol Synthesis in Saccharomyces cerevisiae

The n-propanol produced by Saccharomyces cerevisiae has a remarkable effect on the taste and flavor of Chinese Baijiu. The n-propanol metabolism-related genes were deleted to evaluate the role in the synthesis of n-propanol to ascertain the key genes and pathways for the production of n-propanol by S. cerevisiae. The results showed that CYS3, GLY1, ALD6, PDC1, ADH5, and YML082W were the key genes affecting the n-propanol metabolism in yeast. The n-propanol concentrations of α5ΔGLY1, α5ΔCYS3, and α5ΔALD6 increased by 121.75, 22.75, and 17.78%, respectively, compared with α5. The n-propanol content of α5ΔPDC1, α5ΔADH5, and α5ΔYML082W decreased by 24.98, 8.35, and 8.44%, respectively, compared with α5. The contents of intermediate metabolites were measured, and results showed that the mutual transformation of glycine and threonine in the threonine pathway and the formation of propanal from 2-ketobutyrate were the core pathways for the formation of n-propanol. Additionally, YML082W played important role in the synthesis of n-propanol by directly producing 2-ketobutyric acid through l-homoserine. This study provided valuable insights into the n-propanol synthesis in S. cerevisiae and the theoretical basis for future optimization of yeast strains in Baijiu making.

iTRAQ-based proteomic analysis reveals the molecule mechanism of reducing higher alcohols in Chinese rice wine by nitrogen compensation

Purpose

Higher alcohol is a by-product of the fermentation of wine, and its content is one of the most important parameters that affect and are used to appraise the final quality of Chinese rice wine. Ammonium compensation is an efficient and convenient method to reduce the content of higher alcohols, but the molecule mechanism is poorly understood. Therefore, an iTRAQ-based proteomic analysis was designed to reveal the proteomic changes of Saccharomyces cerevisiae to elucidate the molecular mechanism of ammonium compensation in reducing the content of higher alcohols.

Methods

The iTRAQ proteomic analysis method was used to analyze a blank group and an experimental group with an exogenous addition of 200 mg/L (NH4)2HPO4 during inoculation. The extracted intracellular proteins were processed by liquid chromatography-mass spectrometry and identified using bioinformatics tools. Real-time quantitative polymerase chain reaction was used to verify the gene expression of differentially expressed proteins.

Results

About 4062 proteins, including 123 upregulated and 88 downregulated proteins, were identified by iTRAQ-based proteomic analysis. GO and KEGG analysis uncovered that significant proteins were concentrated during carbohydrate metabolism, such as carbon metabolism, glyoxylate, and dicarboxylate metabolism, pyruvate metabolism, and the nitrogen metabolism, such as amino acid synthesis and catabolism pathway. In accordance with the trend of differential protein regulation in the central carbon metabolism pathway and the analysis of carbon metabolic flux, a possible regulatory model was proposed and verified, in which ammonium compensation facilitated glucose consumption, regulated metabolic flow direction into tricarboxylic acid, and further led to a decrease in higher alcohols. The results of RT-qPCR confirmed the authenticity of the proteomic analysis results at the level of gene.

Conclusion

Ammonium assimilation promoted by ammonium compensation regulated the intracellular carbon metabolism of S. cerevisiae and affected the distribution of metabolic flux. The carbon flow that should have gone to the synthesis pathway of higher alcohols was reversed to the TCA cycle, thereby decreasing the content of higher alcohols. These findings may contribute to an improved understanding of the molecular mechanism for the decrease in higher alcohol content through ammonium compensation.

Comparative transcriptome analysis reveals the key regulatory genes for higher alcohol formation by yeast at different α-amino nitrogen concentrations

Higher alcohols are important flavor substance in alcoholic beverages. The content of α-amino nitrogen (α-AN) in the fermentation system affects the formation of higher alcohols by Saccharomyces cerevisiae. In this study, the effect of α-AN concentration on the higher alcohol productivity of yeast was explored, and the mechanism of this effect was investigated through metabolite and transcription sequence analyses. We screened 12 most likely genes and constructed the recombinant strain to evaluate the effect of each gene on high alcohol formation. Results showed that the AGP1, GDH1, and THR6 genes were important regulators of higher alcohol metabolism in S. cerevisiae. This study provided knowledge about the metabolic pathways of higher alcohols and gave an important reference for the breeding of S. cerevisiae with low-yield higher alcohols to deal with the fermentation system with different α-AN concentrations in the brewing industry.