1.3- Propanediol is a valuable feedstock chemical that is commonly used as a monomer for the synthesis of polyethers, polyesters, and polyurethanes. It can also be used for the produc­tion of cosmetics, lubricants, foods, medicines, composites, adhesives, laminates, powder and UV- cured coatings, moldings, and anti — freeze. Glycerol is metabolized via oxidative and reductive pathways in Klebsiella, Citrobacter, Clostridium, and Enterobacter species. In the oxidative pathway, the nicotinamide adenine dinucleotide (NAD)-dependent glycerol dehy­drogenase (GldA) catalyzes the conversion of glycerol to dihydroxyacetone (DHA) with subsequent phosphorylation by dihydroxyacetone phosphate (DHAP) kinase to produce DHAP (Daniel et al. 1995) after which DHAP enters the glycolytic pathway. The reductive pathway is catalyzed by coenzyme B 12-dependent glycerol dehydratase, which converts glycerol to 3-hydroxypropionaldehyde (3-HPA). 3-HPA is reduced to 1,3-PDO by NADH — dependent enzyme 1,3-propanediol dehydrogenase (1,3-PDODH). As the conversion of glyc­erol to 1,3-PDO results in the net consumption of reducing equivalents, this pathway provides a mechanism for achieving redox balance in the absence of electron acceptors such as oxygen, nitrate, and fumarate.

Glycerol fermentation by Klebsiella results in the accumulation of two main products,

1.3- PDO and acetate, whereas Clostridium species produce butyrate and 1,3-PDO. Although high levels of 1,3-PDO are produced by Klebsiella species, their use on an industrial scale is limited because they are opportunistic pathogens capable of causing urinary tract and abdominal infections, and pneumonia. Therefore, Clostridia are preferred because many Clostridium species of biotechnological importance are not pathogenic and certain species do not require the presence of vitamin B12.

When batch fermentations with C. butyricum were performed using crude glycerol at a concentration of 112 g/L, the maximum amount of 1,3-PDO obtained was about 63.4 g/L with a specific yield of 0.69 mol/mol glycerol (Table 6.1; Barbirato et al. 1998). A newly isolated strain of C. butyricum was found to produce up to 46 g/L of 1,3-PDO a with a high volumetric productivity of 3.4 g/L/h (Papanikolaou et al. 2000). In another report, continuous fermenta­tion with K. pneumoniae yielded 35.2-48.5 g/L of 1,3-PDO. In that report, a low concentra­tion of glycerol was used initially and the concentration was gradually increased to prevent accumulation of substrate in the bioreactor (Menzel et al. 1997). The final volumetric pro­ductivity was in the range of 4.9-8.8 g/L/h. The addition of fumarate to the cultures of K. pneumoniae enhanced the utilization of glycerol and led to increased 1,3-PDO formation. Although the net 1,3-PDO yield of 0.57mol/mol glycerol was obtained with or without the addition of fumarate, the volumetric productivity increased up to 17mmol/L/h, an increase of almost 36% when compared to the corresponding control where glucose was used as substrate (Lin et al. 2005). This increase was attributed to two main factors. First, the addition of fumarate increased the activities of major enzymes involved in glycerol utilization and

1.3- PDO synthesis, that is, GldA, glycerol dehydratase, and 1,3-PDODH, which increased the metabolic flux toward 1,3-PDO production. Second, higher levels of fumarate led to a decrease in the NAD+/NADH ratio, which resulted in a greater concentration of reducing equivalents and hence a greater conversion of 3-HPA to 1,3-PDO.