“…In both models, the reaction is initiated by the coordination of HMBPP C 4 -OH to the [4Fe-4S] 2+ unique iron site ( 33 , Figure 6). This step is supported by extensive biochemical (93, 94, 97), Mössbauer (84–86), and crystallographical evidence (82, 95). In the Birch reduction model (66, 82, 84, 93–95), reduction of HMBPP ( 17 ) by the reduced [4Fe-4S] + cluster generates a radical anion intermediate ( 34 , Figure 6B), the formation of which triggers C 4 -dehydration to an allylic radical-[4Fe-4S] 2+ intermediate ( 35 ).…”