0428同步年報-2021-全

Life Science 051 Fig. 2 : Electron density of PQQ soaked into crystals of apo - MDH crystals: top view (upper left) and side view (upper right). More complete electron density in the region of C4 and C5 of PQQ is observed in the PQQ-soaked MDH. Compared to the holo -MDH in state (bottom left and right), there is evidently greater electron density between the Ca 2+ atom and the PQQ in the PQQ- soaked MDH structure based on this high-resolution structure, indicating reduction of the PQQ at the C5 position. [Reproduced from Ref. 1] Fig. 3 : Electron density of PQQ in the MDH after soaking with CH 3 OH: Top view (left) and side view (right) at resolution 1.80 Å. The electron density of CH 3 OH shows that the −OH group is interacting with C5 of PQQ. [Reproduced from Ref. 1] radicals are only ~5 Å apart; the two electron spins are therefore coupled with a strong magnetic dipolar interaction. In light of these findings, they revised the catalytic cycle of MDH to encompass biradical formation instead of a two-step reoxidation of the reduced PQQ ethenediol following the hydride transfer during CH 3 OH oxidation. These observations were corroborated with electron-density changes between the two cysteine sulfurs of the disulfide bridge as well as between the bound Ca 2+ ion and the O5=C5 bond of PQQ in the high-resolution (up to 1.8 Å) X-ray structures ( Figs. 2 and 3 ). On the basis of these findings, they proposed a novel mechanism for the controlled redistribution of the two electrons during hydride transfer from the CH 3 OH in the alcohol oxidation without formation of reduced PQQ ethenediol, a biradical mechanism that allows for possible recovery of the hydride for transfer to an external NAD + oxidant in the regeneration of the PQQ cofactor for multiple catalytic turnovers ( Fig. 4 , see next page). In support of this mechanism, a steady level of the disulfide radical anion was observed during turnover of the MDH in the presence of CH 3 OH and NAD + . The direct involvement of a protein redox- active residue to facilitate PQQ- dependent hydride transfer is a strategy that should find wide applicability in enzymes that use the PQQ prosthetic group or related quinone cofactors in their catalytic chemistry. In summary, the high-resolution X-ray structures of methanol dehydrogenase from M. capsulatus (Bath) under various conditions have been determined, revealing that the vicinal disulfide bridge near the PQQ cofactor can be broken in particular states. In conjunction with spectral and biochemical studies on these structures, the mechanism of the PQQ-dependent hydride transfer chemistry in the oxidation of methanol to formaldehyde has finally been clarified after decades of confusion. This breakthrough paves the way toward the development of a biomimetic catalyst for the controlled conversion of methanol into formaldehyde under ambient conditions, an important next step in C1 chemistry. (Reported by Chun-Jung Chen) This report features the work of Chun-Jung Chen and his collaborators published in J. Am. Chem. Soc. 143 , 3359 (2021).