Two NADPH-dependent 2-ketogluconate reductases involved in 2-ketogluconate assimilation in <i>Gluconobacter</i> sp. strain CHM43

2-Keto-D-gluconate (2KG), a product of incomplete oxidation of glucose by acetic acid bacteria including <i>Gluconobacter</i> spp., is used for various purposes, including in the food industry. <i>Gluconobacter</i> also consumes 2KG via an unclear metabolic pathway. It is reported that <i>Pseudomonas</i> spp. and <i>Cupriavidus necator</i> phosphorylate 2KG in the first step of 2KG metabolism, but some enteric bacteria including <i>Escherichia coli</i> reduce 2KG. This study evaluated the 2KG consumption ability of a mutant derivative of a strain of <i>Gluconobacter</i> that lacks two putative 2KGR-encoding genes. The mutant strain did not consume 2KG at all; the two 2KGRs were each found to catalyze 2KG reduction. It is concluded that reduction of 2KG is the committed step in 2KG metabolism in <i>Gluconobacter</i>. The results presented here give insights that might facilitate improvement of 2KG production systems that use <i>Gluconobacter</i>.