UDP-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... 

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

Three different enzymes may catalyze the formation of UDP-galactose (1) UDP-glucose 4-epimerase (UDP-galactose 4-epimerase) (2) UDP-galactose pyrophosphorylase and (3) galactose 1-phosphate uridyl... 

Since the aspects of galactose metabolism which have been touched upon briefly above are covered in many reviews (see above), the following discussion will be concaned mainly with some recent work on the mechanism of the UDP-glucose 4-epimerase reaction. 

Scheme 15. 1) sucrose synthase, UTP 2) UDP-glucose-4 -epimerase 3) P(l-4)galactosyl transferase 4) a(l-3)galactosyl transferase. 

Overexpression of selected enzymes involved in galactose biosynthesis, either independently or collectively as an entire metabolic pathway, may be a route to increased levels of galactosylation in CHO cells. These include the UDP-glucose-4-epimerase [36], the phosphoglucomutase [43], and the UDP-glucose pyrophosphor-ylase [44] and the UDP-galactose transferase [45]. All of the genes for these enzymes have been isolated from either mammalian or microbial sources. 

---