UDP-glucose dehydrogenase (UGDH), an oxidoreductase, catalyzes the NAD⁺-dependent four-electron oxidation of UDP-glucose to UDP-glucuronic acid. The catalytic mechanism of UGDH remains controversial despite extensive investigation and is classified into two types according to whether an aldehyde intermediate is generated in the first oxidation step. The first type, which involves the presence of this putative aldehyde, is inconsistent with some experimental findings. In contrast, the second type, which indicates that the first oxidation step bypasses the aldehyde via an NAD⁺-dependent bimolecular nucleophilic substitution (S_N2) reaction, is consistent with the experimental phenomena, including those that cannot be explained by the first type. This NAD⁺-dependent S_N2 mechanism is thus more reasonable and likely applicable to other oxidoreductases that catalyze four-electron oxidation reactions.