Phosphoketolase Pathways

Figure 13.4 The phosphoketolase pathway used by heterofermentative lactic acid bacteria. (Adapted from Stanier et al. 1970 and Gottschalk 1979.)...

When screening a large number of Lactobacillus strains for lactate production using hemicellulose hydrolysate, strains L. pentosus CHCC2355 and L. brevis CHCC2097 were selected and evaluated further (Garde et al, 2002). Each strain produced lactate with 88% (for L. pentosus) and 61% of the theoretical yield (L. brevis), respectively, from wheat straw hydrolysate without visible inhibition. The operation of the phosphoketolase pathway in these... 

Perhaps the acid-tolerant, thermophilic Bacillus coagulans is the only known biocatalyst that naturally produces lactic acid from xylose via the pentose phosphate pathway, not the phosphoketolase pathway (Patel et al., 2006). Three strains, 17C5, P4-102B, and 36D1, can ferment both hexoses and pentoses to pure L(+)-lactic acid at 50 °C and pFI 5.0, an optimal environment... 

LAB are non-respiring microorganisms, principally generating ATP by fermentation of carbohydrates coupled to substrate-level phosphorylation. The two major pathways for the metabolism of hexoses are homofermentative or glycolysis (Embden-Meyerhof pathway), in which lactic acid is virtually the only end-product, and heterofermentative (phosphoketolase pathway), in which other end-products such as acetic acid, C02, and ethanol are produced in addition to lactic acid (Axelsson et al., 1989 Kandler, 1983 Zourari et al., 1992). 

Figure 4.2 Heterolactic fermentation. The fermentation of one mole of glucose yields one mole each of lactic acid, ethanol, and carbon dioxide, via the 6-phosphoglucanate/phosphoketolase pathway.

Okano, K., Yoshida, S., Tanaka, T., Ogino, C. et al (2009) Homo-d-lactic acid fermentation from arabinose by redirection of the phosphoketolase pathway to the pentose phosphate pathway in 1-lactate dehydrogenase gene-deficient Lactobacillus plan-tarum. Appl Environ. Microbiol, 75, 5175-5178,... 

The biochemistry of the lactic acid bacteria has received attention [4, 17-20]. Homofermentative strains such as the Pediococci use the glycolytic pathway for the dissimilation of carbohydrates, such as glucose, to yield pyruvic acid. Pyruvic acid acts as a hydrogen acceptor and is converted to lactic acid by means of an NADH-dependent lactic dehydrogenase. It is believed that the homofermentative strains use in addition the hexose monophosphate pathway and possibly a phosphoketolase pathway (Fig. 21.2) when pentoses are degraded. The heterofermentative strains on the other hand lack both aldolase and hexose isomerase, essential for the operation of the glycolytic pathway, while pyruvic acid will not readily function as a... 

Obligately heterofermentative Ferment both hexoses and pentoses by the 6-PG/PK (6-phosphogluconate/phosphoketolase pathway) and produce lactic acid, ethanol/acetic acid, and CO2 (this latter only from hexoses) in the ratio 1 1 1... 

Okano K, Yoshida S, Tanaka T, Ogino C, Fukuda H, Kondo A. (2009b). Homo D-lactic acid fermentation from xylose by introduction of xylose assimilation genes and redirection of the phosphoketolase pathway to the pentose phosphate pathway in L-lactate dehydrogenase gcnc-dchcicnt Lactobacillus plantarum. Appl Environ Microbiol, 75, 7858-7861. 

Fig. 1-4A). Heterofermenters, however, lack the enzyme fructose-diphosphate aldolase and must divert the flow of carbon through the 6-phospho-gluconate pathway (pentose phosphate or phosphoketolase pathway) as depicted in Fig. l-4b to yield lactic acid as well as ethanol, acetic acid (depending on redox potential), and CO2. Energetically, the consequence of only half of the carbon returning to the EMP is formation of 1 mole of ATP/glucose.

Pentose metaboKsm see Pentose phosphate cycle, Phosphoketolase pathway. 

Phosphoketolase pathway a pathway of carbohydrate degradation found in various microorgan-... 

The phosphoketolase pathway is a route where a is transformed to a C5 sugar (and CO2) and split into a C2 and a C3 molecule. The C3 molecule is then converted to lactic acid whereas the C2 molecule is converted to acetate or ethanol. In the same traditional view, C5 sugars were regarded as leading to this heterofermentative metabolism, which is less interesting from the point of view of industrial production as a lot of acetic acid or ethanol is produced simultaneously. Although some bacteria seem to fit well in this paradigm, more recent literature has shown that this view is oversimplified and somewhat obsolete for a number of reasons. 

Lactic acid producing organisms that do not have a phosphoketolase pathway can still produce acetate or ethanol, formed by the usual mixed acid fermentation, via pyruvate. This is the case for, for example, Lacto-coccus lactis [35]. 

Heterolactic fermentation pathway of glucose (6-phosphogluconate/phosphoketolase pathway]... 

Panagiotou G, Andersen MR, Grotkjaer T, et al. 2008. Systems analysis unfolds the relationship between the phosphoketolase pathway and growth in AipergiZ/Mi nidulans. PLoS One 3(12) e3847. 

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