Journal Article

Auxin Amidohydrolases from <i>Brassica rapa</i> Cleave the Alanine Conjugate of Indolepropionic Acid as a Preferable Substrate: A Biochemical and Modeling Approach

Bojana Savić, Sanja Tomić, Volker Magnus, Kristina Gruden, Katja Barle, Renata Grenković, Jutta Ludwig-Müller and Branka Salopek-Sondi

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 50, issue 9, pages 1587-1599
Published in print September 2009 | ISSN: 0032-0781
Published online July 2009 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcp101
Auxin Amidohydrolases from Brassica rapa Cleave the Alanine Conjugate of Indolepropionic Acid as a Preferable Substrate: A Biochemical and Modeling Approach

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Two auxin amidohydrolases, BrIAR3 and BrILL2, from Chinese cabbage [Brassica rapa L. ssp. pekinensis (Lour.) Hanelt] were produced by heterologous expression in Escherichia coli, purified, and screened for activity towards N-(indol-3-ylacetyl)–l-alanine (IAA-Ala) and the long-chain auxin–amino acid conjugates, N-[3-(indol-3-yl)propionyl]–l-alanine (IPA-Ala) and N-[4-(indol-3-yl)butyryl]–l-alanine (IBA-Ala). IPA-Ala was shown to be the favored substrate of both enzymes, but BrILL2 was approximately 15 times more active than BrIAR3. Both enzymes cleaved IBA-Ala and IAA-Ala to a lesser extent. The enzyme kinetics were measured for BrILL2 and the obtained parameters suggested similar binding affinities for the long-chain auxin–amino acid conjugates (IPA-Ala and IBA-Ala). The velocity of the hydrolyzing reaction decreased in the order IPA-Ala > IBA-Ala > IAA-Ala. In a root growth bioassay, higher growth inhibition was caused by IPA-Ala and IBA-Ala in comparison with IAA-Ala. Neither these conjugates nor the corresponding free auxins affected the expression of the BrILL2 gene. A modeling study revealed several possible modes of IPA-Ala binding to BrILL2. Based on these results, two possible scenarios for substrate hydrolysis are proposed. In one the metal binding water is activated by the carboxyl group of the substrate itself, and in the other by a glutamate residue from the active site of the enzyme.

Keywords: Auxin amidohydrolase; Auxin–amino acid conjugate; Brassica rapa; Chinese cabbage; Enzyme kinetics; Modeling

Journal Article.  7478 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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