Playing Flavin Ping-Pong♦

As a result of widespread use and its ability to resist degradation, the chelating agent EDTA has slowly become a major pollution concern. Several bacteria have developed enzymes to break down this organic agent, though, and the structure of one such enzyme, the flavin oxidoreductase EmoB, is presented in this Paper of the Week. EmoB is part of a two-component enzyme complex; it produces a reduced flavin molecule (FMNH₂) utilized by its partner, the monooxygenase EmoA, to carry out the first step of EDTA degradation. Mark Nissen and colleagues solved the crystal structure of EmoB at three different stages of catalysis (apo-form, NADH:FMN-bound, and FMN: FMN) to reveal a detailed picture of the EmoB reaction cycle. This enzyme undergoes a ping-pong bi-bi mechanism, whereby NADH reduces tightly bound FMN to FMNH₂ and leaves as NAD⁺, at which point a second FMN molecule diffuses in and is reduced to FMNH₂; this molecule is then used by EmoA while the enzyme-bound flavin is restored to FMN to begin the cycle anew.⇓

Structure of the EmoB monomer with bound flavin mononucleotide (FMN).