Biochemical and molecular genetic aspects of pyruvate dehydrogenase complex from Saccharomyces cerevisiae

Bat2p is essential in Saccharomyces cerevisiae for fusel alcohol production on the non-fermentable carbon source ethanol

Branched-chain amino acids (BCAAs) are key substrates in the formation of fusel alcohols, important flavour components in fermented foods. The first step in the catabolic BCAA degradation is a transaminase step, catalyzed by a branched-chain amino acid transaminase (BCAAT). Saccharomyces cerevisiae possesses a mitochondrial and a cytosolic BCAAT, Bat1p and Bat2p, respectively. In order to study the impact of the BCAATs on fusel alcohol production derived from the BCAA metabolism, S. cerevisiae BCAAT-deletion mutants were constructed. The BCAA l-leucine was exogenously supplied during cultivations with mutants of S. cerevisiae. BAT1 deletion is not essential for fusel alcohol production, neither under glucose nor under ethanol growth conditions. The 3-methyl-1-butanol production rate of bat1Delta-cells on ethanol was decreased in comparison with that of wild-type cells, but the cells were still able to produce 3-methyl-1-butanol. However, drastic effects in fusel alcohol production were obtained in cells lacking BAT2. Although the constructed bat2Delta-single deletion strain and the bat1Deltabat2Delta-double deletion strain were still able to produce 3-methyl-1-butanol when grown on glucose, they were incapable of producing any 3-methyl-1-butanol when ethanol was the sole carbon source available. In the circumstances used, gene expression analysis revealed a strong upregulation of BAT2 gene activity in the wild type, when cells grew on ethanol as carbon source. Apparently, the carbon metabolism is able to influence the expression of BCAATs and interferes with the nitrogen metabolism. Furthermore, analysis of gene expression profiles shows that the expression of genes coding for other transaminases present in S. cerevisiae was influenced by the deletion of one or both BCAATs. Several transaminases were upregulated when a BCAAT was deleted. Strikingly, none of the known transaminases was significantly upregulated when BAT2 was deleted. Therefore we conclude that the expression of BAT2 is essential for 3-methyl-1-butanol formation on the non-fermentable carbon source, ethanol.

The pyruvate dehydrogenase complex of Mycoplasma hyopneumoniae contains a novel lipoyl domain arrangement

The genes encoding the pyruvate dehydrogenase (PDH) complex (pdhA, pdhB, pdhC and pdhD) from Mycoplasma hyopneumoniae have been cloned and sequenced. The genes are arranged into two operons, designated pdhAB and pdhCD, which are not found together in the chromosome. The pdhA, pdhB, pdhC and pdhD genes encode proteins of predicted molecular masses of 44.2 kDa (pyruvate dehydrogenase major subunit; E1alpha), 36.6 kDa (pyruvate dehydrogenase minor subunit; E1beta), 33.1 kDa (dihydrolipoyl acetyltransferase; E2) and 66.3 kDa (dihydrolipoyl dehydrogenase; E3), respectively. Sequence analysis of the pdhCD operon revealed the presence of a lipoyl-binding domain in pdhD but not in pdhC. The lipoyl domain is believed to act as a "swinging arm" that spans the gaps between the catalytic domains of each of the subunits. Portions of the N-terminal regions of pdhA and pdhD were expressed as 6xHis-tag fusion proteins in Escherichia coli and purified by nickel affinity chromatography. The purified proteins were used to raise antibodies in rabbits, and Western blot analysis was performed with the polyclonal rabbit antiserum. Both the pdhA and pdhD genes were expressed among various strains of M. hyopneumoniae as well as the porcine mycoplasmas, Mycoplasma hyorhinis and Mycoplasma flocculare. Southern hybridisation analysis using probes from pdhA and pdhD detected one copy of each gene in the chromosome of M. hyopneumoniae. Since previous studies have shown pyruvate dehydrogenase activity in M. hyopneumoniae [J. Gen. Microbiol. 134 (1988) 791], it appears likely that a functional lipoyl-binding domain in the N terminus of PdhC is not an absolute prerequisite for pyruvate dehydrogenase enzyme activity. We hypothesise that the lipoyl-binding domain of PdhD is performing the enzymatic function normally attributed to the PdhC lipoyl-binding domain in other organisms. Searches of pyruvate dehydrogenase gene sequences derived from other Mycoplasma species showed that a putative lipoyl domain was absent in the pdhC gene from Mycoplasma pulmonis. However, like other bacterial species, pdhC gene sequences from Mycoplasma capricolum, Mycoplasma genitalium and Mycoplasma pneumoniae contain a putative lipoyl domain.

Transcription profile of brewery yeast under fermentation conditions

AIMS

Yeast strains, used in the brewing industry, experience distinctive physiological conditions. During a brewing fermentation, yeast are exposed to anaerobic conditions, high pressure, high specific gravity and low temperatures. The purpose of this study was to examine the global gene expression profile of yeast subjected to brewing stress.

METHODS AND RESULTS

We have carried out a microarray analysis of a typical brewer's yeast during the course of an 8-day fermentation in 15 degrees P wort. We used the probes derived from Saccharomyces cerevisiae genomic DNA on the chip and RNA isolated from three stages of brewing. This analysis shows a high level of expression of genes involved in fatty acid and ergosterol biosynthesis early in fermentation. Furthermore, genes involved in respiration and mitochondrial protein synthesis also show higher levels of expression.

CONCLUSIONS

Surprisingly, we observed a complete repression of many stress response genes and genes involved in protein synthesis throughout the 8-day period compared with that at the start of fermentation.

SIGNIFICANCE AND IMPACT OF THE STUDY

This microarray data set provides an analysis of gene expression under brewing fermentation conditions. The data provide an insight into the various metabolic processes altered or activated by brewing conditions of growth. This study leads to future experiments whereby selective alterations in brewing conditions could be introduced to take advantage of the changing transcript profile to improve the quality of the brew.

Reduced Ca2+ uptake by mitochondria in pyruvate dehydrogenase-deficient human diploid fibroblasts

Physiological and pathological Ca2+ loads are thought to be taken up by mitochondria via a process dependent on aerobic metabolism. We sought to determine whether human diploid fibroblasts from a patient with an inherited defect in pyruvate dehydrogenase (PDH) exhibit a decreased ability to sequester cytosolic Ca2+ into mitochondria. Mobilization of Ca2+ stores with bradykinin (BK) increased the cytosolic Ca2+ concentration ([Ca2+]c) to comparable levels in control and PDH-deficient fibroblasts. In normal fibroblasts transfected with plasmid DNA encoding mitochondrion-targeted apoaequorin, BK elicited an increase in Ca2(+)-dependent aequorin luminescence corresponding to an increase in the mitochondrial Ca2+ concentration ([Ca2+]mt) of 2.0 +/- 0.2 microM. The mitochondrial uncoupling agent carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone blocked the BK-induced [Ca2+]mt increase, although it did not affect the [Ca2+]c transient. Basal [Ca2+]c and [Ca2+]mt in control and PDH-deficient cells were similar. However, confocal imaging of the potential-sensitive dye JC-1 indicated that the percentage of highly polarized mitochondria was reduced from 30 +/- 1% in normal cells to 19 +/- 2% in the PDH-deficient fibroblasts. BK-elicited [Ca2+]mt transients in PDH-deficient cells were reduced to 4% of control, indicating that PDH-deficient mitochondria have a decreased ability to take up cytosolic Ca2+. Thus cells with compromised aerobic metabolism have a reduced capacity to sequester Ca2+.