Glucose-4-epimerase

Phase 2. Recruitment and conversion of an NDP-activated and neutral hex-ose from primary metabolism this is most likely UDP-D-glucose, which by the action of an UDP-glucose 4-epimerase, either the HygJ or HygK protein, respectively, becomes converted into the suggested precursor UDP-D-galactose. 

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

Ribulose 5-phosphate 3-epimerase UDP-glucose 4-epimerase D-Glyceraldehyde 3-phosphate ketol isomerase... 

In contrast, the UDP-D-glucose 4-epimerases from various sources... 

A model reaction in which UDP-D-glucose 4-epimerase was shown20,1698 to form an inactive NADH-enzyme complex on incubation with either D-glucose or D-galactose and UMP has been shown1690 to be unrelated to the epimerization reaction. The label from D-galactose-J-t was transferred to NAD, and D-galactonic acid was isolated. Oxidation therefore occurred at C-l, which is normally involved in combination with UDP. 

The use of dTDP-6-deoxy-D-glucose-4- as a substrate for dTDP-D-glucose oxidoreductase (see p. 169) has permitted the stereochemistry of this normally closed system to be determined.169 The B side of NAD accepts the hydrogen atom. UDP-D-glucose 4-epimerase has also been shown168 to use this side of the NAD molecule. 

Then UDP-glucose is formed by the isomerization of galactose catalyzed by UDP-glucose-4-epimerase ... 

Various carbohydrate epimerases such as UDP-D-glucose-4 -epimerase (E.C. class 5.1.3) ... 

Wilson and Hogness showed that extensively purified UDP-D-glucose 4-epimerase from Escherichia coli contains 1 mole of tightly bound NAD per mole of enzyme, and, as an absorption peak almost identical in shape to that of NADH could be observed on addition of the substrate to the enzyme, this evidence is in accord with an oxidation-reduction mechanism involving C-4 of the hexosyl group of the UDP-hexoses. 

The enzyme catalyzing this reaction requires NAD , and it thus resembles UDP-D-glucose 4-epimerase in this respect. 

It has now been found that short treatment of a UDP-D-glucose-4-epimerase-less mutant of E. coli with dilute (ethylenedinitrilo)tetra-acetic acid specifically releases a ternary complex of LPS, phos-phatidyl-2-aminoethanol, and UDP-D-galactose LPS D-galactosyl transferase, without causing significant lysis or killing of the cells. ... 

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