The enzymatic production of R-phenylacetylcarbinol (R-PAC), with either whole cells or partially purified pyruvate decarboxylase (PDC) as the biocatalyst, requires high PDC activity and an inexpensive source of pyruvate for an economical feasible biotransformation process. Microbial pyruvate produced by a vitamin auxotrophic strain of Candida glabrata was selected as a potential substrate for biotransformation. With an optimal thiamine concentration of 60 ??g/l, a pyruvic acid concentration of 43 g/l and yield of 0.42 g/g glucose consumed were obtained. Using microbially-produced unpurified pyruvate resulted in similar PAC concentrations to those with commercial pure substrate confirming its potential for enzymatic PAC production. To obtain high activity yeast PDC, Candida utilis was cultivated in a controlled bioreactor. Optimal conditions for PDC production were identified as: fermentative cell growth at initial pH at 6.0 followed by pH downshift to 3.0. Average specific PDC carboligase activity of 392 ?? 20 U/g DCW was achieved representing a 2.7-fold increase when compared to a constant pH process. A mechanism was proposed in which the cells adapted to the pH decrease by increasing PDC activity to convert the accumulated internal pyruvic acid via acetaldehyde to ethanol thereby reducing intracellular acidification. The effect of pH shift on specific PDC activity of Saccharomyces cerevisiae achieved a comparable increase of specific PDC carboligase activity to 335 U/g DCW. The effect of pyruvic acid at pH 3.0 on induction of PDC activity was confirmed by cultivation at pH 3 with added pyruvic acid. Using microarray techniques, genome-wide transcriptional analyses of the effect of pH shift on S. cerevisiae revealed a transient increased expression of PDC1 after pH shift, which corresponded to the increase in specific PDC activity (although the latter was sustained for a longer period). The results showed significant gene responses to the pH shift with approximately 39 % of the yeast genome involved. The induced transcriptional responses to the pH shift were distinctive and showed only limited resemblance to gene responses reported for other environmental stress conditions, namely increased temperature, oxidative conditions, reduced pH (succinic acid), alkaline pH and increased osmolarity.
| Identifer | oai:union.ndltd.org:ADTP/257644 |
| Date | January 2006 |
| Creators | Chen, Allen Kuan-Liang, Biotechnology & Biomolecular Sciences, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Biotechnology and Biomolecular Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Allen Kuan-Liang Chen, http://unsworks.unsw.edu.au/copyright |
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