METABOLIC ENGINEERING OF PROPANEDIOL PATHWAYS

Microbial fermentation is an important technology for the conversion o f renewable resources to chemicals. In this paper, we describe the app lication of metabolic engineering for the development of two new ferme ntation processes: the microbial conversion of sugars to 1,3-propanedi ol (1,3-PD) and 1,2-propanediol (1,2-PD). A variety of naturally occur ring organisms ferment glycerol to 1,3-PD, but no natural organisms fe rment sugars directly to 1,3-PD. We first describe the fed-batch ferme ntation of glycerol to 1,3-PD by Klebsiella pneumoniae. We then presen t various approaches for the conversion of sugars to 1,3-PD, including mixed-culture fermentation, cofermentation of glycerol and glucose, a nd metabolic engineering of a ''sugars to 1,3-PD'' pathway in a single organism. Initial results are reported for the expression of genes fr om the K. pneumoniae 1,3-PD pathway in Saccharomyces cerevisiae. The b est naturally occurring organism for the fermentation:of sugars to 1,2 -PD is Thermoanaerobacterium thermosaccharolyticum. We describe the fe rmentation of several different sugars to 1,2-PD by this organism in b atch and continuous culture. We report that Escherichia coli strains e ngineered to express either aldose reductase or glycerol dehydrogenase convert glucose to (R)-1,2-PD. We then analyze the ultimate potential of fermentation processes for the production of propanediols. Linear optimization studies indicate that, under aerobic conditions, propaned iol yields that approach the theoretical maximum are possible and CO2 is the primary coproduct. Without the need to produce acetate, final p roduct titers in the range of 100 g/L should be possible; the high tit ers and low coproduct levels should make product recovery and purifica tion straightforward. The examples given in this paper illustrate the importance of metabolic engineering for fermentation process developme nt in general.