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Journal of Bacteriology, March 1999, p. 1739-1747, Vol. 181, No. 6
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Tyrosine Aminotransferase Catalyzes the Final Step of Methionine Recycling in Klebsiella pneumoniae

Jacqueline Heilbronn,1,2 Judith Wilson,1 and Bradley J. Berger1,*

Department of Biochemistry, University of Dundee,1 and Scottish Crop Research Institute, Invergowrie,2 Dundee, United Kingdom

Received 8 September 1998/Accepted 12 January 1999

An aminotransferase which catalyzes the final step in methionine recycling from methylthioadenosine, the conversion of alpha -ketomethiobutyrate to methionine, has been purified from Klebsiella pneumoniae and characterized. The enzyme was found to be a homodimer of 45-kDa subunits, and it catalyzed methionine formation primarily using aromatic amino acids and glutamate as the amino donors. Histidine, leucine, asparagine, and arginine were also functional amino donors but to a lesser extent. The N-terminal amino acid sequence of the enzyme was determined and found to be almost identical to the N-terminal sequence of both the Escherichia coli and Salmonella typhimurium tyrosine aminotransferases (tyrB gene products). The structural gene for the tyrosine aminotransferase was cloned from K. pneumoniae and expressed in E. coli. The deduced amino acid sequence displayed 83, 80, 38, and 34% identity to the tyrosine aminotransferases from E. coli, S. typhimurium, Paracoccus denitrificans, and Rhizobium meliloti, respectively, but it showed less than 13% identity to any characterized eukaryotic tyrosine aminotransferase. Structural motifs around key invariant residues placed the K. pneumoniae enzyme within the Ia subfamily of aminotransferases. Kinetic analysis of the aminotransferase showed that reactions of an aromatic amino acid with alpha -ketomethiobutyrate and of glutamate with alpha -ketomethiobutyrate proceed as favorably as the well-known reactions of tyrosine with alpha -ketoglutarate and tyrosine with oxaloacetate normally associated with tyrosine aminotransferases. The aminotransferase was inhibited by the aminooxy compounds canaline and carboxymethoxylamine but not by substrate analogues, such as nitrotyrosine or nitrophenylalanine.

* Corresponding author. Mailing address: Div. of Molecular Parasitology & Biological Chemistry, Wellcome Trust Bldg., Dept. of Biochemistry, University of Dundee, Dundee, United Kingdom DD1 5EH. Phone: 44-(0)1382-345761. Fax: 44-(0)1382-345893. E-mail: bjberger{at}bad.dundee.ac.uk.

Journal of Bacteriology, March 1999, p. 1739-1747, Vol. 181, No. 6
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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