Plant genomes contain genetically encoded isoforms of most nucleotide sugar interconversion enzymes. Here we show that A. thaliana has five genes encoding functional UDP-D-glucose/UDP-D-galactose 4-epimerase (named UGE1 to –5). All A. thaliana UDP-D-glucose 4-epimerase isoforms are dimeric in solution, maximally active in vitro at 30°C to 40°C and show good activity between pH 7 and pH 9. In vitro, UGE1, -3 and -5 act independently of externally added NAD+, while cofactor addition stimulates the activity of UGE2 and is particularly important for UGE4 activity. UGE1 and UGE3 are most efficiently inhibited by UDP. The five isoforms, display kcatUDP-Gal values between 23 and 128 sec-1 and KmUDP-Gal values between 0.1 and 0.3 mM. This results in enzymatic efficiencies ranging between 97 and 890 mM-1sec-1 for UGE4=UGE1

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