Fig. 4: Parallels and differences between biocatalysis of BurG and canonical KARIs.

a, Structures of BurG with the reaction products 6 and DMS and the trapped enolate intermediate 14 after co-crystallization with gonydiol 5 (PDB 7PCN). Two magnesium atoms (A and B) are shown as green spheres. b, Interaction of active site residues of BurG with 6, 14 and DMS. c, Structural overlay of BurG bound to 12, 13 and 14 indicates a strong interaction of the positively charged sulfonium moiety of 14 with E232 as compared to neutral 12 and 13. d, Tight lock of the oxygen-functionalized compounds 6 and 7 through interaction with Mg²⁺ ions in the active centre of BurG. W_apo indicates the binding sites of water molecules as found in the apo structure that are replaced by ligand atoms in the depicted structures. e, Structure of unliganded holo-BurG illustrates a hydroxide ion bridged between two Mg²⁺ centres (PDB 7PCC). f, Comparison of BurG and KARIs displays a similar binding mode of substrates in the Mg²⁺ cluster. g, Proposed reaction mechanism¹⁹ for KARIs with cyclopropane transition state. h, Reaction mechanism proposed for BurG involving a concealed (transient) redox reaction.