Characterization of Arabidopsis serine:glyoxylate aminotransferase, AGT1, as an asparagine aminotransferase

Asparagine (Asn) is a major form of nitrogen transported to sink tissues. Results from a previous study have shown that an Arabidopsis mutant lacking asparaginase activity develops relatively normally, high-lighting a possible compensation by other types of asparagine metabolic enzymes. Prior studies with barley and tobacco mutants have associated Asn aminotransferase activity with the photorespiratory enzyme, serine (Ser):glyoxylate aminottansferase. This enzyme is encoded by AGT1 in Arabidopsis thaliana. Recombinant N-terminal His-tagged AGT1 purified from Escherichia coli was characterized with Ser, alanine (Ala) and Asn as amino acid donors and glyoxylate, pyruvate and hydroxypyruvate as organic acid acceptors. The V-max of AGT1 with Asn was higher than with Ser or Ala by ca. 5- to 20-fold. As a result, the catalytic efficiency (V-max/K-m) was slightly higher with Asn than with the two other amino acids. In the roots of 10-day-old seedlings treated for 2 h with 20 mM Asn, the AGT1 transcript levels were raised by 2-fold. During this treatment, the concentration of Asn in root was raised by ca. 5-fold. These results suggest that AGT1 is involved in Asn metabolism in Arabidopsis. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.