Identification and characterization of a strict and a promiscuous N-acetylglucosamine-1-P uridylyltransferase in Arabidopsis

UDP-GlcNAc is an essential precursor for glycoprotein and glycolipid synthesis. In the present study, a functional nucleotidyltransferase gene from Arabidopsis encoding a 58.3 kDa GlcNAc1pUT-1 (N-acetylglucosamine-1-phosphate uridylyltransferase) was identified. In the forward reaction the enzyme catalyses the formation of UDP-N-acetylglucosamine and PP_i from the respective monosaccharide 1-phosphate and UTP. The enzyme can utilize the 4-epimer UDP-GalNAc as a substrate as well. The enzyme requires divalent ions (Mg²⁺ or Mn²⁺) for activity and is highly active between pH 6.5 and 8.0, and at 30–37 °C. The apparent K_m values for the forward reaction were 337 μM (GlcNAc-1-P) and 295 μM (UTP) respectively. Another GlcNAc1pUT-2, which shares 86% amino acid sequence identity with GlcNAc1pUT-1, was found to convert, in addition to GlcNAc-1-P and GalNAc-1-P, Glc-1-P into corresponding UDP-sugars, suggesting that subtle changes in the UT family cause different substrate specificities. A three-dimensional protein structure model using the human AGX1 as template showed a conserved catalytic fold and helped identify key conserved motifs, despite the high sequence divergence. The identification of these strict and promiscuous gene products open a window to indentify new roles of amino sugar metabolism in plants and specifically their role as signalling molecules. The ability of GlcNAc1pUT-2 to utilize three different substrates may provide further understanding as to why biological systems have plasticity.