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First published online August 13, 2004; 10.1104/pp.104.041806 Plant Physiology 135:1946-1955 (2004) © 2004 American Society of Plant Biologists Biochemical and Structural Characterization of Benzenoid Carboxyl Methyltransferases Involved in Floral Scent Production in Stephanotis floribunda and Nicotiana suaveolens1Department of Biological Sciences, University of Rostock, 18059 Rostock, Germany (M.B.P., F.H., S.S., U.E., B.P.); Department of Molecular, Cellular, and Developmental Biology, University of Michigan, Ann Arbor, Michigan 48109–1048 (F.C., E.P.); Department of Plant Physiology Bio III, Rheinisch-Westfälische Technische Hochschule Aachen University, 52074 Aachen, Germany (I.K., A.S.); and Structural Biology Laboratory, The Salk Institute for Biological Studies and Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, 92307 (J.R., J.P.N.)
Flower-specific benzenoid carboxyl methyltransferases from Stephanotis floribunda and Nicotiana suaveolens were biochemically and structurally characterized. The floral scents of both these species contain higher levels of methyl benzoate and lower levels of methyl salicylate. The S. floribunda enzyme has a 12-fold lower Km value for salicylic acid (SA) than for benzoic acid (BA), and results of in silico modeling of the active site of the S. floribunda enzyme, based on the crystal structure of Clarkia breweri salicylic acid methyltransferase (SAMT), are consistent with this functional observation. The enzyme was therefore designated SAMT. The internal concentration of BA in S. floribunda flowers is three orders of magnitude higher than the SA concentration, providing a rationale for the observation that these flowers synthesize and emit more methyl benzoate than methyl salicylate. The N. suaveolens enzyme has similar Km values for BA and SA, and the in silico modeling results are again consistent with this in vitro observation. This enzyme was therefore designated BSMT. However, the internal concentration of BA in N. suaveolens petals was also three orders of magnitude higher than the concentration of SA. Both S. floribunda SAMT and N. suaveolens BSMT are able to methylate a range of other benzenoid-related compounds and, in the case of S. floribunda SAMT, also several cinnamic acid derivatives, an observation that is consistent with the larger active site cavity of each of these two enzymes compared to the SAMT from C. breweri, as shown by the models. Broad substrate specificity may indicate recent evolution or an adaptation to changing substrate availability.
1 This work was supported by the Deutsche Forschungsgemeinschaft Pi 153/17–1 and 17–2 to B.P.), by Cusanuswerk (to M.B.P.), and by the National Science Foundation (grant nos. IBN–0211697 and MCB–0312449 to E.P. and MCB–0312466 to J.P.N.). 2 Present address: Carnegie Institution of Washington, Department of Plant Biology, 260 Panama Street, Stanford, CA 94305. 3 Present address: Max-Planck-Institute for Biochemistry, Am Klopferspitz 18a, 82152 Martinsried, Germany. Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.041806. * Corresponding author; e-mail birgit.piechulla{at}biologie.uni-rostock.de; fax 49–(0)381–4986132. Received February 27, 2004; returned for revision May 26, 2004; accepted May 26, 2004. This article has been cited by other articles:
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