L-Arabinose pathways

D-Arabinose is metabolized by a D-arabinose-inducible system through D-ribulose, D-ribulose 5-phosphate, and D-xylulose 5-phosphate. D-Arabinose is not an inducer of the L-arabinose pathway. However, D-ribulose is a substrate for the L-ribulokinase [28]. Therefore, if one streaks a D-ribulokinaseless mutant on a mineral D-arabinose plate, the parental strain is unable to grow, and some mutants that arise (besides those mutants possessing D-ribulokinase activity) are constitutive for... 

Constitutive mutants (C) for the L-arabinose pathway have been isolated from the wild type as mutants resistant to the D-fucose inhibition of growth on mineral L-arabinose D-fucose medium and from D-ribulokinaseless mutants dK ) as double mutants dK O) able to grow on mineral D-arabinose medium (see Sections II,B,1 II,B,2). The former C mutants were mapped by recombination frequencies [6]. With the availability of deletions ending at various positions within the araC gene, it became possible to map the C mutant sites using deletion mapping. All the C mutations isolated as dK C were mapped by this procedure [54] (Fig. 5). 

Figure I. The fungal and bacterial pathways for D-xylose and L-arabinose catabolism. All pathways have in common that D-xylulose 5-phosphate is produced. The enzymes in the bacterial pathways are xylose isomerase and xylulokinase for the D-xylose pathway and L-arabinose isomerase, ribulokinase and L-ribulosephosphate 4-epimerase for the L-arabinose pathway. The fungal D-xylose pathway has the enzymes aldose reductase, xylitol dehydrogenase and xylulokinase. The enzymes in the L-arabinose pathways ofmold and yeast are aldose reductase, L-arabinitol 4-dehydrogenase, L-xylulose reductase, xylitol dehydrogenase and xylulokinase. The differences between the mold and yeast pathway are in the cofactor requirements.

Information about the L-arabinose pathway in yeast is rare. It is probably similar to the mold pathway. It requires a xylitol dehydrogenase as shown by Shi et al. 24). In a mutant of Pichia stipitis, which was unable to grow on L-arabinose, overexpression of a xylitol dehydrogenase could restore growth on L-arabinose. In a study of Dien et al. (5) more than 100 yeast species were tested for L-arabinose fermentation. Most of them produced arabinitol and xylitol indicating that the yeast pathway is indeed similar to the pathway of molds and not to the pathway of bacteria. There is only little knowledge of the enzymes in the yeast pathway. Aldose reductases which are active with L-arabinose and D-xylose were described e.g. for the yeasts S. cerevisiae 25) and P. stipitis 17). The enzymes have similar affinity toward D-xylose and L-arabinose and convert both sugars with a similar rate. The S. cerevisiae enzyme however is strictly NADPH-dependent, while the P. stipitis enzyme can use both NADH and... 

The bacterial L-arabinose pathway has been successfully expressed in S. cerevisiae by using genes from E.coli and B. subtilis (30). The resulting strain was able to grow on and ferment L-arabinose at high rates. 

The role of L-arabinose in these biosynthetic pathways is also of interest. It seems probable that, unlike D-xylose itself, this sugar may be readily... 

There are three known metabolic pathways to 3-deoxyulosonic acids. In the first, aldonic acids, formed by oxidation of aldopyranoses to the corresponding lactones, are dehydrated to the ulosonic acids (see Fig. 6). Thus, D-arabinose is converted by way of D-arabinonic acid to 3-deoxy-o-ffZj/cero-pentulosonic acid. The latter is then subjected to oxidative cleavage, yielding pyruvic and glycolic acids. L-Arabinose is oxidized and dehydrated to the 3-deoxy-L-g Z2/cero-pentulosonic acid, which is further oxidized to... 

Pathways in the formation of NDP-sugars. A seiection of monosaccharide conversions occurring at the phosphate ester and nucieotide-monosaccharide ievei in animais, piants and bacteria are shown all pathways radiate from fructose-6-phosphate, indicating the central role of this metabolite. The dTDP, GDP and UDP-yV-acetylmu-ramic pathways are peculiar to bacteria, whereas ADP-o-glucose, UDP-o-apiose and UDP-L-arabinose are found in plants. For reasons of simplicity, other pathways, e. g., to UDP-L-rhamnose in plants, GDP-o-rhamnose in Pseudomonas aeroginosa and GDP-o-arabinose in trypanosomatids are not included. The figure is reproduced from Chap. 6.4 of the first edition of this book... 

Xylitol is the probable connecting point between the D-xylose and L-arabi-nose metabolic pathways (Fig. 5). L-arabinose is the form found most abundantly in nature. Early work by Chaing and Knight showed that cell-free extracts of Penicillium chrysogenum convert L-arabinose to both L-ribose and L-xylulose through the intermediate, L-arabinitol (= L-arabitol) [80]. Only one enzyme, aldose reductase, appears to be responsible for the conversion of L-arabinose to L-arabinitol. Aldose reductase also acts on D-arabinose to produce D-arabitol. Witterveen et al. obtained a mutant of Aspergillus niger deficient in... 

Related pathways implicating uridine 5-(D-xylopyranosyl pyrophosphate) have been elucidated by a combination of physiological and enzymic studies. For instance, the incorporation of L-arabinose into xylan, with no rearrangement of its carbon skeleton,can be explained by the following sequence of enzymic reactions, of which all but the last have been demonstrated in extracts of plant tissues. - ... 

Kawaguchi, H., Sasaki, M., Vertes, A.A, Inui, M., and Yukawa, H. (2008) Engineering of an L-arabinose metabolic pathway in Corynebacterium glutamicum. Appl. Microbiol Biotechnol,... 

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)...

Isoaltholactone (119) has been obtained from G. malayanus, G. montanus and G. tapis and its relative stereochemistry assigned from NMR data and X-ray analysis. A synthesis from L-arabinose provided the absolute stereochemistry of (119). The biosynthetic pathway in Scheme 2 was proposed to account for the formation of altholactone (114) and isoaltholactone (119) from the same postulated precursor, 5-hydroxygoniothalamin (105) (124). 

C strains isolated by either method have different, but coordinate, constitutive levels of the enzymes and transport system in the L-ara-binose pathway. Those strains with constitutive levels below the wild-type inducible level are further inducible by L-arabinose [6]. The constitutive enzyme levels of some of the C strains, which are near to the wild-type inducible level, are decreased (maximum, twofold) by L-arabinose. This is probably due to the restoration of self catabolite repression (see Section VIII). 

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