Pentose Metabolism

Onkenhout W, Groener JE, Verhoeven NM, Yin C, Laan LA (2002) L-Arabinosuria a new defect in human pentose metabolism. Mol Genet Metab 77 80-85... 

Besides biomass and C02, no significant amount of extracellular products was detected. Xylitol and arabitol were only accumulated in small amounts, with a maximum of 2.0 and 3.8 g/L, respectively. Other metabolic products usually associated with the pentose metabolism of D. hansenii, such as ethanol and glycerol (46,47), were only found in trace amounts, less than 0.5 and 0.3 g/L, respectively. 

Zhang, M., Deanda, K., Finkelstein, M., and Picataggion, S., Metabolic engineering of a pentose metabolism pathway in ethanologenic zymomonas mobilis. Science 1995,267, 240-243. 

B. L. Horecker, Pentose Metabolism in Bacteria, Wiley, New York, 1962. 

Keywords. Xylose, Fermentation, Respiration, Metabolic engineering. Regulation, Regulatory mechanisms. Pentose metabolism. Oxygen regulation. Glucose regulation... 

Horecker BL (1963) Pentose Metabolism in Bacteria, New York John WUey, New York, p 100 Chiang C, Knight SG (1960) Nature 188 79... 

A wide range of bacterial species utilizes D-xylose and L-arabinose as carbon and energy sources. In most cases, the direct isomerization of aldopentoses to their corresponding ketoses is the first step in pentose metabolism. For example, D-xylose is converted into D-xylulose and L-arabinose is converted into L-ribulose L-ribulose can be converted into D-xylulose by epimerase. D-Xylulose is the key intermediate for further metabolism. For this reason, most bacterial species can also readily utilize L-arabinose. 

Figure 1. Proposed pentose metabolism and Entner-Doudoroffpathways in engineered Zymomonas mobilis. (Reproduced with ...

The position of phosphoketolase in pentose metabolism. P = phosphate. TPP = thiamin pyrophosphate. 

Similar reactions have recently been found in plants to be involved in pentose metabolism.D-Xylose-l-phosphate was found to react with UTP to form UDPXy. A second enzyme, which may be different from the epimerase of glucose-galactose interconversions, epimerizes carbon 4 of the pentose to produce the UDP derivative of L-arabinose. The number of specific pyrophosphorylases for sugar phosphates in plant tissue is not known at this time. 

The hexose monophosphate shunt pathway ties together hexose and pentose metabolism and provides reactions for the interconversion of these two groups of sugars. In addition this pathway also provides for the metabolism of the three-, four-, and seven-carbon sugars (108). 

In the dark, it is assumed that stored starch or other glucan is hydrolyzed to sugar-phosphates which are metabolized to PEP via glycolysis and perhaps to some extent by pentose metabolism. Carboxylation of PEP results largely from atmospheric CO2 to give oxalacetate, which is immediately reduced to malate. Some oxalacetate will ultimately go into aspartate synthesis. Some of the malate will be decarboxylated to pyruvate (or perhaps PEP), where the 3-carbon frag-... 

The first step of xylose catabolism is its conversion to xylulose. In bacteria, it takes place by the direct isomerization catalysed by xylose isomerase. In PeniciUium chrysogenum, a sequence of enzymes in the initial steps of pentose metabolism was observed that differs from xylose isomerization in bacteria [91, 92]. These enzymes were common in yeast and filamentous fungi. In this oxido-reductive pathway, xylose is first reduced to the xyhtol in the presence of NAD(P)-linked xylose reductase, which is then reoxidized by NAD(P)-hnked dehydrogenase to give xylulose (Fig. 1). It has been assumed that this oxido-reductive pathway is common among fungi [93]. Both the enzymes involved, xylose reductase and xylitol dehydrogenase, were found to be inducible and relatively specific for the D-xylose and xyhtol in F. oxysporum, whereas D-xylose isomerase was not detected. 

Feldmaim, S.D., Sahm, H., and Sprenger, G.A. (1992) Pentose metabolism in Zymomonas mobilis wild-type and recombinant strains. Appl. Microbiol Biotechnol, 38, 354-361. 

Several investigators had observed the formation of hexose phosphates during nucleoside and pentose metabolism by cell extracts. It was considered until recently that these hexose phosphates arose from a reversal of the Embden-Meyerhof scheme operative on triose phosphate derived from pentose. It has now been found by Dische that more hexose may be generated from adenosine in hemolysates than would be expected from the proportion of 3-carbon fragments in the pentose molecule, i.e., 0.75 mole hexose per mole pentose instead of a maximal 0.6. Furthermore, much of the carbon of the pentose was in hexose monophosphate produced under conditions in which hexose-6-phosphate and hexose-1,6-diphosphate were not interconvertible. 

Zanin GM, Santana CC, Bon EPS, Giordano RCL, de Moraes FF, Andrietta SR, de Carvalho Neto CC, Macedo IC, Fo DL, Ramos LP, Fontana JD (2000) Brazilian bioethanol program. Appl Biochem Biotechnol 84-86 1147-1161 Zhang M, Eddy C, Deanda K, Finkelstein M, Picataggio S (1995) Metabolic engineering of a pentose metabolism pathway in ethanologenic Zymomonas mobilis. Science 267 240-243... 

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