The mechanism of a unique fumarate reductase is explored using the hybrid density functional B3LYP method. The calculations show a two-step mechanism, initiated with a hydride transfer from FAD ( flavin adenine dinucleotide) to fumarate, followed by a proton shift from Arg402. The rate-limiting process is assigned to the hydride transfer, and the energetics are consistent with experimental data. It is shown that the enzyme is essential to correctly position the substrate in the active site, stabilizing its extremely anionic character.