Catalyzes the reduction of 2,5-diketo-D-gluconic acid (25DKG) to 2-keto-L-gulonic acid (2KLG). 5-keto-D-fructose and dihydroxyacetone can also serve as substrates. 25DKGR-A exhibits a greater selectivity for the substrate and higher thermal stability than 25DKGR-B. 2-dehydro-L-idonate + NADP(+) = 2,5-didehydro-D-gluconate + H(+) + NADPH Inhibited by Zn(2+), Fe(3+), Cu(2+) and Ni(2+). Optimum pH is 6.4. Active over a broad pH range. Monomer. Introduced by genetic manipulation and expressed in Erwinia herbicola by Anderson et al. The resultant organism is able to convert in a single fermentative step D-glucose into 2-keto-L-gulonic acid, a key precursor in the industrial production of L-ascorbic acid (vitamin C). However, the technology still needs some working on and is not used in commercial production at present. Modeling study indicates that the active site may not be optimized for 2,5-diketo-D-gluconic acid. Belongs to the aldo/keto reductase family.