A gene encoding a putative UDP-glucose 4-epimerase (pGALE) in Pyrococcus horikoshii was cloned and expressed in Escherichia coli. The purified enzyme could reversibly catalyze both the synthesis of UDP-Gal and UDP-Glc but preferred the binding of UDP-Gal by approximately 10-fold. The optimum pH and temperature were 6.5 and 65°C. The enzyme acted effectively without the addition of nicotinamide adenine dinucleotide (NAD(+)), possibly due to the presence of tightly bound NAD(+). In particular, pGALE could be coupled with trehalose synthase (TreT) from P. horikoshii to regenerate UDP-Gal from UDP. The possible byproduct of glycosyltransferase, UDP, was capable of being converted to UDP-Glc with trehalose by TreT, and UDP-Glc was simultaneously converted to UDP-Gal by pGALE. Conclusively, the results suggest that pGALE and TreT with trehalose is an effective one-pot two-enzyme system for the regeneration of UDP-Gal, a high-cost substrate of galactosyltransferase, to complete a sugar nucleotide cycle.
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