Epimerase and

Cokesa Z, H-J Knackmuss, P-G Rieger (2004) Biodegradation of all stereoisomers of the EDAT substitute iminodisuccinate by Agrobacterium tumefaciens BY6 requires an epimerase and a stereoselective C-N lyase. Appl Environ Microbiol 70 3941-3947. 

Ribulose 5-phosphate is capable of a reversible isomerization to other pentose phosphates-xylulose 5-phosphate and ribose 5-phosphate. These reactions are catalyzed by two respective enzymes, viz., pentose-phosphate epimerase and pentose-phosphate isomerase, according to the scheme below ... 

Tabata, K., Koizumi, S., Endo, T. and Ozaki, A., Production of A-acetyl-D-neuraminic acid by coupling bacteria expressing N-acetyl-o-glucosamine 2-epimerase and N-acetyl-o-neuraminic acid synthetase. Enzyme Microb. Technol. 2002, 30, 327-333. 

We propose that the first step in NDP-kasugamine biosynthesis is 2-epimeriza-tion of the postulated UDP-A -acetyl-D-glucosamine precursor, which is suggested by the similarity of the KasQ protein with known UDP-(Af-acetyl-)D-glucosamine 2-epimerases and catalyzes the conversion to UDP-N-acetyl-D-mannosamine. To... 

This enzyme [EC 5.1.3.1] (also known as phosphoribu-lose epimerase, erythrose-4-phosphate epimerase, and pentose-5-phosphate 3-epimerase) catalyzes the interconversion of D-ribulose 5-phosphate and D-xylulose 5-phosphate. The enzyme can also act on D-erythrose 4-phosphate. 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

Elevated in deficiencies of galactokinase, galactose-1-phosphate uridyltransferase (classical galactosemia), UDP galactose-4-epimerase, and Fanconi-Bickel-syn drome, portocaval shunt, and cirrhosis, depending on time after lactose consumption. 

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. 

Subsequent Reactions Catalyzing 6-Deoxyhexose Formation 3,5-Epimerase and Reductase. After the formation of the 4-keto intermediate, a step common to all deoxyhexose formations, at least two and possibly three additional enzymes are necessary for the biosynthesis of the end product. This sequence of transformations is illustrated in Figure 3. The first step is the conversion of the 4-keto-6-deoxy-hexose intermediate described above. The intermediates in brackets are postulated and are assumed to be bound to the enzyme. An enzyme (or enzymes), referred to below as 3,5-isomerase, catalyzes epimerizations at carbons 3 and 5, probably via the enediol form. The epimerizations are followed by a... 

A summary of some of the common features for the first three reactions described in this preparation, TDPG-oxidoreductase, UDP-galac-tose-4-epimerase, and UDP-glucuronic acid carboxylase, are shown in Figure 14. Our initial discussion is restricted to these three examples since we have more information about them. 

Figure 14. Common features of the TDPG-oxidoreductase, UDP-galactose-4-epimerase, and UDP-glucuronic acid carboxylase...

Model for the regulation of enzymes of the GAL system, (a) Three structural genes synthesize distinct mRNAs and enzymes (transferase, epimerase, and kinase). Arrows next to the genes indicate the direction of transcription. Synthesis requires the GAL4 protein. However, it is inactive in the absence of inducer because of complex formation with the GAL80 protein. The GAL80 protein does not... 

S Kovacevic, MB Tobin, JR Miller. The (3-lactam biosynthesis genes for isopenicillin N epimerase and deacetoxycephalosporin C synthetase are expressed from a single transcript in Streptomyces clavuligerus. J Bacteriol 172 3952-3958, 1990. 

Fig. 2. Metabolic pathways for PHA biosyntheis in fad mutant E. coli strains used in this study. Enoyl-CoA hydratase, epimerase, and 3-ketoacyl-CoA or ACP reductase have been suggested to supply PHA precursors from inhibited b-oxidation pathway. The crosses indicate inactivation of corresponding enzymes. The question mark represents uncharacterized enzyme. Enzymes involved in the metabolic pathways shown have been described previously FabG (21,32), YfcX (24,33), MaoC (34), PhaA (36), and PhaB (36).

Although the fatty acid oxidation scheme works neatly for even-numbered chain lengths, it can t work completely for fatty acids that contain an odd number of carbons. P-oxidation of these compounds leads to propionyl-CoA and acetyl-CoA, rather than to two acetyl-CoA at the final step. The propionyl-CoA is not a substrate for the TCA cycle or other simple pathways. Propionyl-CoA undergoes a carboxylation reaction to form methylmalonyl-CoA. This reaction requires biotin as a cofactor, and is similar to an essential step in fatty acid biosynthesis. Methylmalonyl-CoA is then isomerized by an epimerase and then by methylmalonyl-CoA mutase—an enzyme that uses Vitamin Bi2 as a cofactor—to form succinyl-CoA, which is a TCA-cycle intermediate. 

The 5-carbon sugar phosphates are interconverted by the action of epimerase and isomerase to yield ribulose-5-phosphate, which is phosphorylated by the enzyme ribulose phosphate kinase to make RuBP, the acceptor of C02. 

UDPG) dehydrogenase, UDPG epimerase and UDP-glucuronate epimerase. Acta Parasitologica Polonica, 28 187-96. 

It is this transferase that is missing in galactosemia. UDP-galactose is normally transformed to UDP-glucose by UDP-galactose epimerase, and UDP-glucose may then form glycogen. 

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