Figure 2. The two proposed chemical mechanisms for UGMs.

In one mechanism the reduced FAD (1) is depicted to act as a nucleophile forming a flavin-galactose adduct (either via S_N1 or S_N2) (2) and a subsequent iminium ion (3). These steps are followed by ring contraction forming the galactofuranose (4). An alternative mechanism predicts an electron transfer step, in which one electron is transferred from FADH⁻ to an oxocarbenium ion intermediate (6) creating a flavin and a sugar radical, which react and form the galactose-FAD adduct (2), followed by ring contraction.