sentence: If DMSP binds in a bidentate fashion when NO is present , it would indicate that the Fe ( II ) center is bound to three protein residues ; however , it is possible for the iron cofactor to be bound to four residues and only bind DMSP in a monodentate fashion.Based on these studies , a mechanism of DddW-catalyzed reaction is proposed , wherein the metal center would possibly be coordinated by 3His-1-Glu ligands ( H81 , H83 , E87 , and H121 ) and the substrate can bind either in monodentate or bidentate mode , with the remaining coordination sphere around Fe ( II ) occupied by water molecules ( Fig 10 ) .