Pentose phosphate pathway, arabinose

The six-carbon chain of ManNAc 6-P can be extended by three carbon atoms using an aldol-type condensation with a three-carbon fragment from PEP (Eq. 20-7, step c) to give N-acetylneuraminic acid (sialic acid).48 Tire nine-carbon chain of this molecule can cyclize to form a pair of anomers with 6-membered rings as shown in Eq. 20-7. In a similar manner, arabi-nose 5-P is converted to the 8-carbon 3-deoxy-D-manno-octulosonic acid (KDO) (Fig. 4-15), a component of the lipopolysaccharide of gram-negative bacteria (Fig. 8-30), and D-Erythrose 4-P is converted to 3-deoxy-D-arafrmo-heptulosonate 7-P, the first metabolite in the shikimate pathway of aromatic synthesis (Fig. 25-1).48a The arabinose-P used for KDO synthesis is formed by isomerization of D-ribulose 5-P from the pentose phosphate pathway, and erythrose 4-P arises from the same pathway. 

The five E. coli genes inserted in Z. mobilis allowed the entry of arabinose into the nonoxidative phase of the pentose phosphate pathway (Fig. 14-22), where it was converted to glucose 6-phosphate and fermented to ethanol. 

Occurrence and preparation. The tetrose (4) participates as D-erythrose 4-phosphate in the pentose phosphate pathway of carbohydrate metabolism.11 D-Erythrose can be prepared using a method of degradation from the pentose, D-arabinose,19 or from D-glucose.20 Synthesis of the 4-phosphate has been reported.21... 

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,... 

Figure 2. Redox cofactor requirement in L-arjabinose catabolism. L-Arabinose conversion to equimolar amounts of CO2 and ethanol is redox neutral, i- -anaerobic fermentation to ethanol should be possible. However, the conversion of L-arabinose to D-xylulose requires NADPH and NAD and produces NADH and NADP. NADPH is mainly regenerated in the oxidative part of the pentose phosphate pathway, where the reduction ofNADP is coupled to C02 production. The abbreviations are G6p, glucose 6-phosphate F6P, fructose 6-phosphate X5P, D-Xylulose 5-phosphate GAP, D-glyceraldehyde 3-phosphate. (Reproduced from Ref. 165 with permission from Elsevier Science)...

The fungal oxidoreductive catabolism of these two pentoses is unique for fungi and produces ultimately D-xylulose 5-phosphate, which is an intermediate of the canonical pentose phosphate pathway (Fig. 18.3). In contrast, prokaryotes use an isomerase step to convert D-xylose to D-xylulose, and L-arabinose to L-ribulose. o-xylulose 5-phosphate is then either formed by the action of xylulokinase (in the case of D-xylose) or a sequence of L-ribulokinase and L-ribulose-5-phosphate 4-epimerase (in the case of L-arabinose) (Mishra and Singh 1993). 

Okano K, Yoshida S, Tanaka T, Fukuda H, Kondo A (2009a) Homo D-LA fermentation from arabinose by redirection of phosphoketolase pathway to pentose phosphate pathway in L-lactate dehydrogenase gene-deficient Lactobacillus plantarum. Appl Environ Microbiol 75 5175-5178. doi 10.1128/AEM.00573-09... 

In comparison to the homofermentative processes, within the heterofermentative route, glucose is not converted by the glycolysis pathway but by the pentose phosphate pathway. Fructose-1,6-bisphosphate-aldolase and triosephosphate isom-erase are not involved. The microorganisms are adapted to the utilization of pentoses as carbon source and in addition convert hexoses like glucose via this route. These pentoses are found when degrading plant material and are mainly xylose, ribose, or arabinose. They are converted via xylulose-5-phosphate which is converted to acetyl-phosphate and glycerinaldehyde-3-phosphate. Afterward,... 

Different pathways are available in nature for metabolism of arabinose and xylose which are converted to xylulose 5-phosphate (intermediate com-poimd) to enter the pentose phosphate pathway as shown in Figure 10.5. In yeasts, xylose is first reduced by xylose reductase to xylitol, which in turn is oxidized to xylulose by xylitol dehydrogenase. In bacteria and some anaerobic fungi, xylose isomerase is responsible for direct conversion of xylose to xylulose. Xylulose is finally phosphorylated to xylulose-5-phos-phate by xylulokinase. In fungi, L-arabinose is reduced to L-arabitol (by arabinose reductase), L-xylulose (by arabitol dehydrogenase), xylitol (by L-xylulose reductase). Xylitol is finally converted to xylulose (by xylitol dehydrogenase), whose activity is also part of xylose utilization pathways. In bacteria, L-arabinose is converted to L-ribulose (by L-arabinose isomerase), L-ribulose-5-P (by L-ribulokinase) and finally D-xylulose-5-P (by L-ribulose-5-P 4-epimerase) (Bettiga et al., 2008). 

It was found that E. coli adapted to growth on glucuronic acid or galactu-ronic acid as sole carbon sources were unable to oxidize or ferment D-xylose or L-arabinose before an additional adaptation to these pentoses. If the metabolism had led through a decarboxylation of the free uronie acid, adaptation and utilization of the uronie acid as sole carbon source should have resulted in simultaneous adaptation to the homologous pentose. A pathway leading through the phosphates (Fig. 28), however, was not excluded by these experiments. 

Many LAB are able to ferment pentoses. They can only ferment heterofermen-tatively by entering the phosphogluconate pathway as either ribulose-5 phosphate or xylulose-5 phosphate (Kandler 1983) (Fig. 4). Pentoses (such as arabinose, ribose, xylose) are converted into lactate and acetate CO2 is not produced. The sum of the equation is... 

BleaUey PA, Arora KK, Williams JF. Evidence that aldolase and D-arabinose 5-phosphate are components of pentose pathway reactions in liver in vitro. Biochem. Int. 1984 8 491-500. Horecker BL, Mehler AH. Carbohydrate metabolism. Ann. Rev. Biochem. 1955 24 207-274. 

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