Glycerol carbonate propionate

PHAs ate produced from a wide variety of srrbstrates such as renewable tesotrrces (srrcrose, starch, cellulose, triacylglycerols), fossil tesoirrces (methane, mineral oil, hgnite, hard coal), byproducts (molasses, whey, glycerol), chemicals (propionic add, 4-ltydroxybutyric add) and carbon dioxide [10]. 

To avoid the cosynthesis of acetate by P. thoenii, it was suggested to replace conventional carbon sources with glycerol, thus obtaining a theoretical yield of 0.804 g of propionic acid per g of glycerol and no acetic acid (Boyaval and Corre, 1995 Himmi et al., 2000). 

At least two unusual features characterize this biosynthesis. One is the origin of the methyl branches in the polyketide chain from the methyl group of methionine. This is in contrast to the biosynthesis of most macrolide antibiotics in which chain branches are formed by utilization of the appropriate homologs in place of acetate chain extension units, i.e., propionate units. The second unusual feature of this biosynthesis is the mode of incorporation of glycerol and particularly the fact that it is a specific precursor of the three-carbon polyketide chain starter unit [50 ]. 

The main products of glucose fermentation are propionic and acetic acids and carbon dioxide. All the strains of the genus can ferment pyruvate, dioxyacetate and glycerol with the same end products as in glucose fermentation. 

One of these is by heterotrophic CO2 fixation, first discovered in propionic acid bacteria by Wood and Workman (1936, 1938). The CO2 fixation is especially high when the bacteria are grown on glycerol. Both the fixation of carbon dioxide and formation of succinate by propionibacteria are inhibited by NaF (Wood and Workman, 1940). Another pathway of the condensation of C3- and Ci-compounds was discovered by Swick and Wood (1960), who showed that lliese bacteria contain a transcarboxylase that has a role in producing propionate firom methylmalonyl-CoA. This was preceded by an observation (Wood and Leaver, 1953) fiiat propionic acid bacteria have two mechanisms for CO2 fixation and only one of these is inhibited by NaF. The enzyme, discovered by Swick and Wood, catalyzed a new type of biochemical reactions— transcarboxylation between a carboxyl donor and an acceptor ... 

Typical for the propionic acid fermentation is the formation of identical fermentation products from Ce-, C5-, C4- and Cs-compounds. The ratio of products may differ, depending to a large extent on the degree of oxidation of the utilized carbon source. The ratio of propionic to acetic acid in glycerol medium was found to be 2 1, in lactate medium it was 1 1.5, and in pyruvate medium it was 1 2 (Vorobjeva, 1958a). Utilization of pyruvate resulted in the production of acetic acid with a constant rate (Vorobjeva, 1958b). The yield of propionic acid was twofold lower, and after four days it was almost unchanged (Fig. 3.6). No equimolarity between the acetate and CO2... 

Figure 3.6. Growth curve of P. shermanii in synthetic media. A with sodium lactate B with sodium pyruvate C on glycerol supplemented with 0.2% aspartic acid. Curve 1 - acetic acid 2 - propionic acid 3 - cellular carbon. From Vorobjeva (1958a).

Zhu Y, Li J, Tan M, Liu L, Jiang L, Sun J, Lee P, Du G, Chen J (2010) Optimization and scale-up of propionic acid production by propionic acid-tolerant Propionibacterium acidipropionici with glycerol as the carbon source. Bioresour Technol 101 8902-8906... 

Stjernholm, R. and Wood, H. G. (1960) Glycerol dissimilation and the occurrence of a symmetrical three-carbon intermediate in the propionic acid fermentation. J. Biol. Chem. 235,2757-2761. 

In this case, the balance does not demand any formation of CO2 or acetic acid. Further, six ATPs are predicted when three glycerol molecules are fermented. It is interesting that Van Niel reported in his thesis (Van Niel 1928) that seven strains of propionic acid bacteria produced 90-100 % propionic acid when grown with glycerol as major carbon source. Research efforts are focusing on production of propionic acid from glycerol see section Research and Development in this chapter. 

Various bacteria own the ability to produce propionic acid within their metabolic pathways. Present-day research is focused on strains of Propionibacteriaceae and Clostridiaceae. Propionibacteria are using the dicarboxylic acid pathway (methylmalonyl coenzyme A-pathway) to produce the desired product. These gram-positive, anaerobic bacteria are able to use glucose, sucrose, lactate, lactose and glycerol as carbon source. The metabolic end products are propionate, succinate, carbon dioxide and acetate. Professionals acknowledge Propionibacterium... 

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