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Published on April 15, 2009; 10.1104/pp.109.136929

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Received February 10, 2009
Accepted April 9, 2009

GALACTONOLACTONE DEHYDROGENASE REQUIRES A REDOX-SENSITIVE THIOL FOR OPTIMAL PRODUCTION OF VITAMIN C

Nicole G. H. Leferink , Esther van Duijn , Arjan Barendregt , Albert J. R. Heck , and Willem J. H. van Berkel *

Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands; Biomolecular Mass Spectrometry and Proteomics Group, Bijvoet Center for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands; Netherlands Proteomics Centre

* Corresponding author; email: willem.vanberkel{at}wur.nl.

The mitochondrial flavoenzyme L-galactono-{gamma}-lactone dehydrogenase (GALDH) catalyzes the ultimate step of vitamin C biosynthesis in plants. We found that recombinant GALDH from Arabidopsis thaliana is inactivated by hydrogen peroxide due to selective oxidation of Cys340, located in the cap domain. Electrospray ionization mass spectrometry (ESI-MS) revealed that the partial reversible oxidative modification of Cys340 involves the sequential formation of sulfenic, sulfinic and sulfonic acid states. S-Glutathionylation of the sulfenic acid switches off GALDH activity and protects the enzyme against oxidative damage in vitro. C340A and C340S GALDH variants are insensitive towards thiol oxidation, but exhibit a poor affinity for L-galactono-1,4-lactone. Cys340 is buried beneath the protein surface and its estimated pKBaB of 6.5 suggests the involvement of the thiolate anion in substrate recognition. The indispensability of a redox-sensitive thiol provides a rationale why GALDH was designed as a dehydrogenase and not, like related aldonolactone oxidoreductases, as an oxidase.






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