Kinetic analysis of Arabidopsis glucosyltransferase UGT74B1 illustrates a general mechanism by which enzymes can escape product inhibition

Author:

Kopycki Jakub1,Wieduwild Elizabeth1,Kohlschmidt Janine2,Brandt Wolfgang3,Stepanova Anna N.4,Alonso Jose M.4,Pedras M. Soledade C.5,Abel Steffen1,Grubb C. Douglas1

Affiliation:

1. Department of Molecular Signal Processing, Leibniz Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany

2. Department of Applied Biosciences and Process Engineering, Anhalt University of Applied Sciences, 06366 Köthen, Germany

3. Department of Bioorganic Chemistry, Leibniz Institute of Plant Biochemistry, Halle (Saale), Germany

4. Department of Genetics, North Carolina State University, Raleigh, NC 27695, U.S.A.

5. Department of Chemistry, University of Saskatchewan, Saskatoon, SK, Canada, S7N 5C9

Abstract

Plant genomes encode numerous small molecule glycosyltransferases which modulate the solubility, activity, immunogenicity and/or reactivity of hormones, xenobiotics and natural products. The products of these enzymes can accumulate to very high concentrations, yet somehow avoid inhibiting their own biosynthesis. Glucosyltransferase UGT74B1 (UDP-glycosyltransferase 74B1) catalyses the penultimate step in the core biosynthetic pathway of glucosinolates, a group of natural products with important functions in plant defence against pests and pathogens. We found that mutation of the highly conserved Ser284 to leucine [wei9-1 (weak ethylene insensitive)] caused only very mild morphological and metabolic phenotypes, in dramatic contrast with knockout mutants, indicating that steady state glucosinolate levels are actively regulated even in unchallenged plants. Analysis of the effects of the mutation via a structural modelling approach indicated that the affected serine interacts directly with UDP-glucose, but also predicted alterations in acceptor substrate affinity and the kcat value, sparking an interest in the kinetic behaviour of the wild-type enzyme. Initial velocity and inhibition studies revealed that UGT74B1 is not inhibited by its glycoside product. Together with the effects of the missense mutation, these findings are most consistent with a partial rapid equilibrium ordered mechanism. This model explains the lack of product inhibition observed both in vitro and in vivo, illustrating a general mechanism whereby enzymes can continue to function even at very high product/precursor ratios.

Publisher

Portland Press Ltd.

Subject

Cell Biology,Molecular Biology,Biochemistry

Reference42 articles.

1. Glycosyltransferases of lipophilic small molecules;Bowles;Annu. Rev. Plant Biol.,2006

2. Glycosyltransferases: structures, functions, and mechanisms;Lairson;Annu. Rev. Biochem.,2008

3. Glucosinolate metabolism and its control;Grubb;Trends Plant Sci.,2006

4. Biology and biochemistry of glucosinolates;Halkier;Annu. Rev. Plant Biol.,2006

5. Biosynthesis of glucosinolates: gene discovery and beyond;Sonderby;Trends Plant Sci.,2010

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