Pentose fermentation microorganisms

Fermentation of the lignoceliulosic hydrolyzates is more difficult than the well-established processes of ethanol production from e.g. molasses and starch. Hydrolyzates contain a broader range of inhibitory compounds, where the composition and the concentration of these compounds depend on the type of lignoceliulosic materials and the chemistry and nature of foe pretreatment and hydrolysis processes. Secondly, foe hydrolyzates of hemicelluloses contain not only hexoses but also pentoses, where xylose is foe dominant sugar in foe hydrolyzates from hardwood hemicelluloses. Therefore, foe fermenting microorganism should be able to produce efoanol from foe hydrolyzates with a... 

The process of fermentation of cellulose materials differs from that of starch or sugar materials (1) the hydrolysate of the lignocellulosic material is toxic to the fermentative microorganisms (2) there is more xylose in the hydrolysate. So, the utilization of pentose is one of the most important decisive factors for the process economics (Zaldivar et al. 2001). 

Table 4 Main approaches in finding pentose-fermenting ethanol-producing microorganism...

C. acetobutylicum is the model microorganism to study clostridial ABE fermentation. The respective biphasic metabolism is similar to other solventogenic clostridial species (Table 9.1, see Section 9.5). The fermentation starts with uptake of the substrate (e.g., hexoses and pentoses). Sugars are metabolized to... 

For the biosynthesis of cell components a microorganism must be supplied with appropriate low molecular weight compounds such as sugars, organic acids, amino acids etc. Many of 2-, 3-, 4- and 5-carbon compounds are formed in catabolic reactions. In propionic acid bacteria these reactions comprise the propionic acid fermentation, TCA cycle and hexose monophosphate shunt. The latter supplies the cell with erythrose-phosphate, ribose-5-phosphate and reducing equivalents (NADPH) needed for many syntheses. Erythrose-4-phosphate is used in the formation of aromatic amino acids phenylalanine, tryptophane, tyrosine. Ribose-5-phosphate is incorporated into nucleic acids. The pentose cycle and propionic acid fermentation, as mentioned before, have a number of common precursors and enzymes. The inclusion of common precursors into one or another pathway is regulated by the level of ATP (Labory, 1970), and this regulation in fact determines the ratio of catabolic and anabolic processes in the cell. 

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