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Protein Engineering, Vol. 15, No. 2, 131-140, February 2002
© 2002 Oxford University Press

Alteration of the specificity of the cofactor-binding pocket of Corynebacterium 2,5-diketo-D-gluconic acid reductase A

Scott Banta1,2, Barbara A. Swanson3,4, Shan Wu3, Alisha Jarnagin3 and Stephen Anderson2,,5,6

1 Departments of Chemical and Biochemical Engineering 5 Molecular Biology and Biochemistry, Rutgers, The State University of New Jersey, 2 Center for Advanced Biotechnology and Medicine, 679 Hoes Lane, Piscataway, NJ 08854 3 Genencor International, 925 Page Mill Road, Palo Alto, CA 94304, USA

The NADPH-dependent 2,5-diketo-D-gluconic acid (2,5-DKG) reductase enzyme is a required component in some novel biosynthetic vitamin C production processes. This enzyme catalyzes the conversion of 2,5-DKG to 2-keto-L-gulonic acid, which is an immediate precursor to L-ascorbic acid. Forty unique site-directed mutations were made at five residues in the cofactor-binding pocket of 2,5-DKG reductase A in an attempt to improve its ability to use NADH as a cofactor. NADH is more stable, less expensive and more prevalent in the cell than is NADPH. To the best of our knowledge, this is the first focused attempt to alter the cofactor specificity of a member of the aldo–keto reductase superfamily by engineering improved activity with NADH into the enzyme. Activity of the mutants with NADH or NADPH was assayed using activity-stained native polyacrylamide gels. Eight of the mutants at three different sites were identified as having improved activity with NADH. These mutants were purified and subjected to a kinetic characterization with NADH as a cofactor. The best mutant obtained, R238H, produced an almost 7-fold improvement in catalysis with NADH compared with the wild-type enzyme. Surprisingly, most of this catalytic improvement appeared to be due to an improvement in the apparent kcat for the reaction rather than a large improvement in the affinity of the enzyme for NADH.


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