Sunday, May 4, 2008

Microbial Synthesis of Phloroglucinol and Xylitol

Huimin Zhao, Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, Urbana, IL 61801

My group has been focused on the development and application of new protein engineering and metabolic engineering tools. Here I will present two specific examples. The first example concerns the development of a new bioprocess for synthesis of phloroglucinol, a specialty chemical that is currently produced at 140-200 tons/yr using chemical methods. Specifically, we discovered a novel type III polyketide synthase, PhlD, that enables direct biosynthesis of phloroglucinol from D-glucose.  Heterologous expression of PhlD in E. coli led to the production of phloroglucinol in vivo, with an estimated amount of 10 g/L by using a continuous fermentation.  To further improve the phloroglucinol yield, directed evolution was applied to enhance the activity of PhlD.  In addition, metabolic engineering was carried out to redirect the carbon flux inside E. coli to pathways responsible for the synthesis of phloroglucinol.  The second example concerns the development of a new bioprocess for synthesis of xylitol, one of DOE’s top 12 platform chemicals for biorefinery.  The current processes for xylitol manufacture, based on either chemical synthesis or fermentation, all rely on the use of pure D-xylose as a feedstock, resulting in relatively high cost of production.  To address this limitation, we used protein engineering to create a xylose reductase (XR) mutant with decreased specificity toward L-arabinose, while maintaining its high activity toward D-xylose.  Such engineered xylose-specific XR mutants will enable the direct use of inexpensive hemicellulose hydrolysates (mainly D-xylose and L-arabinose) as substrates in large-scale fermentation.

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