Aldose 1-epimerase

D-Glucose O2 Consumption Glucose Oxidase and Catalase Aldose-1 -epimerase"... 

BOROHYDRIDE REDUCTION ALDOL CONDENSATION ALDOSE 1-EPIMERASE ALDOSE REDUCTASE ALGINATE LYASE ALKALINE PHOSPHATASE,... 

Other Properties of a Mutarotase (Aldose- 1-Epimerase) from Hog Kidney, Proc. VHth Int. Congr. Biochem. (1967) F, 141. 

Lapedes, S. M., Chase, A. M., Aldose- 1-Epimerase from Hog Kidney. 

At equilibrium, D-galactopyranose is present in aqueous solution as about 27% a anomer and 73% /3 anomer,3123 and transport into cells has been reported as not specific for either form for Sac-charomyces cerevisiae and Kluyveromyces (Saccharomyces) fragilis, grown on complex media.312 1 Within the cells, aldose 1-epimerase (EC 5.1.3.3) converts /3-D-galactose into the a anomer312 1 for which galactokinase activity is highly specific.325... 

Aldose-1-epimerase), quite widespread in animal tissue and bacteria, which catalyses mutarotation. The Escherichia coli enzyme has a maximum activity close to neutral pH. The activation energy 4G =11.9 kcal mol" is greatly lowered, as usual with respect to that of the non-enzymically catalysed reaction, close to 17 kcal mol". o-Glucose, D-galactose, and D-fucose are substrates but not D-mannose (Hucho and Wallenfels 1971). 

While this paper deals with the chemistry and reaction characteristics of glucose isomerase, it should be noted that glucose oxidase, glucoamylase, aldose-1-epimerase, a-glucosidase, alcohol dehydrogenase, pullulanase, and fungal a-amylase have been successfully immobilized onto this support. 

Tphis review outlines the current knowledge of the enzyme mutarotase " (aldose-l-epimerase) and evaluates the evidence that it may have evolved from an origin in primitive bacteria into an important transport system for sugars in higher organisms. 

Interconverions. The mutual transformations of monosaccharides involve the following 3 types of reactions a) epimerization, in which the steric arrangement on a C-atom is reversed by epimerases (e.g. in galactose metabolism) b) isomerization, in which aldoses and ketoses are reversibly interconverted by... 

Now, finally, sedoheptulose 7-phosphate undergoes a transketolase-catalyzed (EC 2.2.1.2) process (as in Scheme 11.7) to remove two carbon atoms using the enzyme cofactor thiamine diphosphate to yield ribose 5-phosphate and a two-carbon fragment that has remained attached to the thiamine cofactor of transketo-lase (EC 2.2.1.1, sedoheptulose 7-phosphate) (Scheme 11.11). When the two-carbon fragment is added to glyceraldehyde 3-phosphate, the material of Scheme 11.8 again is applied and xylulose 5-phosphate results. The xylulose 5-phosphate isomerizes to ribulose 5-phosphate as in Scheme 11.9 (with intervention of ribulose phosphate 3-epimerase (EC 5.1.3.1). And, the ribose 5-phosphate, an aldose, isomerizes (an aldose-ketose isomerase, EC 5.3.1.6, ribose 5-phosphate isomerase) to ribulose 5-phosphate. 

Epimerase. Another rearrangement of the pentose phosphate molecule was recently discovered. This involves an isomerization at carbon 3, to form xylulose-5-phosphate (IV). The enzyme responsible has been named phosphoketopentose epimerase. The equilibrium of the epimerase favors slightly the formation of xylulose. The isomerization of the aldose and ketose forms may be presumed to proceed via an ene-diol, as was considered for the isomerization of hexoses. Such a mechanism may also be involved in the synthesis and utilization of arabinose. A comparable mechanism may be invoked for the inversion at carbon 3. Evidence for... 

In addition, a range of aldose phosphate isomerases, for example, o-manno-se 6-phosphate isomerase, as well as a good selection of epimerases and oxidoreductases, are available. 

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.

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