Purification of three aminotransferases from Hydrogenobacter thermophilus TK-6 – novel types of alanine or glycine aminotransferase

Enzymes and catalysis

Authors


M. Ishii, Department of Biotechnology, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
Fax: +81 3 5841 5272
Tel: +81 3 5841 5143
E-mail: amishii@mail.ecc.u-tokyo.ac.jp

Abstract

Aminotransferases catalyse synthetic and degradative reactions of amino acids, and serve as a key linkage between central carbon and nitrogen metabolism in most organisms. In this study, three aminotransferases (AT1, AT2 and AT3) were purified and characterized from Hydrogenobacter thermophilus, a hydrogen-oxidizing chemolithoautotrophic bacterium, which has been reported to possess unique features in its carbon and nitrogen anabolism. AT1, AT2 and AT3 exhibited glutamate:oxaloacetate aminotransferase, glutamate:pyruvate aminotransferase and alanine:glyoxylate aminotransferase activities, respectively. In addition, both AT1 and AT2 catalysed a glutamate:glyoxylate aminotransferase reaction. Interestingly, phylogenetic analysis showed that AT2 belongs to aminotransferase family IV, whereas known glutamate:pyruvate aminotransferases and glutamate:glyoxylate aminotransferases are members of family Iγ. In contrast, AT3 was classified into family I, distant from eukaryotic alanine:glyoxylate aminotransferases which belong to family IV. Although Thermococcus litoralis alanine:glyoxylate aminotransferase is the sole known example of family I alanine:glyoxylate aminotransferases, it is indicated that this alanine:glyoxylate aminotransferase and AT3 are derived from distinct lineages within family I, because neither high sequence similarity nor putative substrate-binding residues are shared by these two enzymes. To our knowledge, this study is the first report of the primary structure of bacterial glutamate:glyoxylate aminotransferase and alanine:glyoxylate aminotransferase, and demonstrates the presence of novel types of aminotransferase phylogenetically distinct from known eukaryotic and archaeal isozymes.

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