Propionic acid utilisation

Utilising Candida cylindracea lipase (CCL) a chiral propionic acid was resolved by DSM [34, 46]. Only the undesired enantiomer of the ester was hydrolysed and at a conversion of 64% the remaining desired ester had an ee of 98% (Scheme 6.12). Although this means that the yield of the enantiopure ester is below 40% it did enable a new access to enantiopure captopril. 

Product quality of PHAs is very much dependent on the polyester composition (see 3.2.2.1). In 1987, Byrom found that poly-(3HB-co-3HV) can be produced in large-scale fed-batch culture by supplementing the nutritional medium of a glucose-utilising mutant of Alcaligenes eutrophus (today known as Cupriavidus necator) with propionic acid (precursor for 3HV formation) [50]. Later it was shown that the utilisation of valeric acid instead of propionic acid results in a higher proportion of 3HV units [51]. The improvements in product quality of co- and terpolyesters, however, results in an increase of the production costs of the polymer because of the high price of the precursors. 

It has been suggested that through its effects on (1) rumen fermentation (some experiments have shown increased microbial growth and increased propionic acid production) and (2) cell metabolism (increased utilisation of carbohydrate and reduced lipid mobilisation), nicotinic acid may be a useful supplement to dairy cows, particularly in situations of subclinical ketosis. However, the experimental evidence is not consistent. Nicotinic acid does not always give positive responses in the rumen and increases in blood concentrations were not observed in all experiments. Current recommendations do not advocate the supplementation of dairy cow diets in order to increase milk yield and composition. 

The bacteria number 10 -10 ° per millilitre of rumen contents. Over 200 species have been identified, and for descriptions of them the reader is referred to the works listed at the end of this chapter. Most of these bacteria are non-spore-forming anaerobes. Table 8.3 lists a number of the more important species and indicates the substrate they utilise and the products of the fermentation. This information is based on studies of isolated species in vitro and is not completely applicable in vivo. For example, it appears from Table 8.3 that succinic acid is an important end product, but in practice this is converted into propionic acid by other bacteria such as Selenomonas ruminantium (see Fig. 8.6) such interactions between microorganisms are an important feature of rumen fermentation. A further point is that the activities of a given species of bacteria may vary from one strain of that species to another. The total... 

Table 3. Comparison of ATP synthesis and utilisation in response to increasing protein supply or increasing ruminal propionic acid supply in one experimentJ... 

Methanogenic bacteria utilise available hydrogen, affecting bacterial growth in a way which favours production of acetate rather than higher acids (e.g. propionate, butyrate). The net effea is to ultimately raise the pH. As long as methanogenic activity continues, this will act to maintain the pH near neutrality. 

Another highly potential production species is the gram-positive bacteria Clostridium propionicum. This microorganism is able to utilise lactate, glycerol and alanine as substrate. Propionate, acetate, formate, n-propanol and succinate are produced. The optimal pH value is 6.8 and the best temperature is 30 °C. C. propionicum uses the acrylic acid pathway to produce the desired product... 

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