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J. Biol. Chem., Vol. 281, Issue 25, 17276-17285, June 23, 2006

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Distinct Properties of the Five UDP-D-glucose/UDP-D-galactose 4-Epimerase Isoforms of Arabidopsis thaliana^*

From the Department of Cell and Developmental Biology, John Innes Centre, Colney, Norwich NR4 7UH, United Kingdom

Plant genomes contain genetically encoded isoforms of most nucleotide sugar interconversion enzymes. Here we show that Arabidopsis thaliana has five genes encoding functional UDP-D-glucose/UDP-D-galactose 4-epimerase (named UGE1 to UGE5). All A. thaliana UDP-D-glucose 4-epimerase isoforms are dimeric in solution, maximally active in vitro at 30-40 °C, and show good activity between pH 7 and pH 9. In vitro, UGE1, -3, and -5 act independently of externally added NAD⁺, whereas 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 values between 23 and 128 s^- and values between 0.1 and 0.3 mM. This results in enzymatic efficiencies ranging between 97 and 890 mM^-1 s^-1 for UGE4 = UGE1 < UGE3 < UGE5 < UGE2. The values, derived from the Haldane relationship, were 0.76 mM for UGE1, 0.56 mM for UGE4, and between 0.13 and 0.23 mM for UGE2, -3, and -5. The expression of UGE isoforms is ubiquitous and displays developmental and cell type-dependent variations. UGE1 and -3 expression patterns globally resemble enzymes involved in carbohydrate catabolism, and UGE2, -4, and -5 expression is more related to carbohydrate biosynthesis. UGE1, -2, and -4 are present in the cytoplasm, whereasUGE4 is additionally enriched close to Golgi stacks. All UGE genes tested complement the UGE4^rhd1 phenotype, confer increased galactose tolerance in planta, and complement the galactose metabolization deficiency in the Saccharomyces cerevisiae gal10 mutant. We suggest that plant UGE isoforms function in different metabolic situations and that enzymatic properties, gene expression pattern, and subcellular localization contribute to the differentiation of isoform function.

Received for publication, November 29, 2005 , and in revised form, April 27, 2006.

^* C. B. performed enzyme kinetic experiments, J. R. performed biophysical studies and yeast experiments, A. R. contributed to data mining and K. F. performed immunogold labeling and electron microscopy. G. J. S. conceived of and supervised the project in the K. R. laboratory, and wrote the manuscript. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Supported by the John Innes Foundation and recipient of additional funding from the United Kingdom Scholarship for International Research Students and the Janggen-Pöhn Foundation.

² Recipient of a John Innes Centre MSc scholarship.

³ To whom correspondence should be addressed: Dept. of Cell and Developmental Biology, John Innes Centre, Colney, Norwich NR4 7UH, United Kingdom. Tel.: 44-1603-450460; E-mail: georg.seifert{at}bbsrc.ac.uk.

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