Toxoflavin Lyase Requires a Novel 1-His-2-Carboxylate Facial Triad,

摘要

High-resolution crystal structures are reported for apo, holo, and substrate-bound forms of a toxoflavin-ndegrading metalloenzyme (TflA). In addition, the degradation reaction is shown to be dependent on oxygen,Mn(II),nand dithiothreitol in vitro. Despite its low sequence identity with proteins of known structure, TflA is structurallynhomologous to proteins of the vicinal oxygen chelate superfamily. Like other metalloenzymes in this superfamily, thenTflA fold contains four modules that associate to form a metal binding site; however, the fold displays a rarenrearrangement of the structuralmodules indicative of domain permutation.Moreover, unlike the 2-His-1-carboxylatenfacial triadcommonlyutilizedbyvicinaloxygenchelate dioxygenases and other dioxygen-activating non-hemenFe(II) enzymes, the metal center in TflA consists of a 1-His-2-carboxylate facial triad. The substrate-nbound complex shows square-pyramidal geometry in which one position is occupied by O5 of toxoflavin.nThe open coordination site is predicted to be the dioxygen binding site. TflA appears to stabilize the reducednform of toxoflavin through second-sphere interactions. This anionic species is predicted to be the electronnsource responsible for reductive activation of oxygen to produce a peroxytoxoflavin intermediate.