Mechanism of the reaction catalyzed by dehydroascorbate reductase from spinach chloroplasts

Eur J Biochem. 2003 Mar;270(5):921-8. doi: 10.1046/j.1432-1033.2003.03452.x.

Abstract

Dehydroascorbate reductase (DHAR) reduces dehydroascorbate (DHA) to ascorbate with glutathione (GSH) as the electron donor. We analyzed the reaction mechanism of spinach chloroplast DHAR, which had a much higher reaction specificity for DHA than animal enzymes, using a recombinant enzyme expressed in Escherichia coli. Kinetic analysis suggested that the reaction proceeded by a bi-uni-uni-uni-ping-pong mechanism, in which binding of DHA to the free, reduced form of the enzyme was followed by binding of GSH. The Km value for DHA and the summed Km value for GSH were determined to be 53 +/- 12 micro m and 2.2 +/- 1.0 mm, respectively, with a turnover rate of 490 +/- 40 s-1. Incubation of 10 microm DHAR with 1 mm DHA and 10 microm GSH resulted in stable binding of GSH to the enzyme. Bound GSH was released upon reduction of the GSH-enzyme adduct by 2-mercaptoethanol, suggesting that the adduct is a reaction intermediate. Site-directed mutagenesis indicated that C23 in DHAR is indispensable for the reduction of DHA. The mechanism of catalysis of spinach chloroplast DHAR is proposed.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Blotting, Western
  • Catalysis
  • Chloroplasts / enzymology*
  • Electrophoresis, Polyacrylamide Gel
  • Escherichia coli / genetics
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Oxidative Stress
  • Oxidoreductases / chemistry
  • Oxidoreductases / genetics
  • Oxidoreductases / metabolism*
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Spinacia oleracea / enzymology*

Substances

  • Recombinant Proteins
  • Oxidoreductases
  • glutathione dehydrogenase (ascorbate)