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Single-Step Replacement of an Unreactive C–H Bond by a C–S Bond Using Polysulfide as the Direct Sulfur Source in the Anaerobic Ergothioneine Biosynthesis

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journal contribution
posted on 30.07.2020, 14:18 by Ronghai Cheng, Lian Wu, Rui Lai, Chao Peng, Nathchar Naowarojna, Weiyao Hu, Xinhao Li, Stephen A. Whelan, Norman Lee, Juan Lopez, Changming Zhao, Youhua Yong, Jiahui Xue, Xuefeng Jiang, Mark W. Grinstaff, Zixin Deng, Jiesheng Chen, Qiang Cui, Jiahai Zhou, Pinghua Liu
Ergothioneine, a natural longevity vitamin and antioxidant, is a thiol-histidine derivative. Recently, two types of biosynthetic pathways were reported. In the aerobic ergothioneine biosyntheses, non-heme iron enzymes incorporate a sulfoxide into an sp2 C–H bond from trimethyl-histidine (hercynine) through oxidation reactions. In contrast, in the anaerobic ergothioneine biosynthetic pathway in a green-sulfur bacterium, Chlorobium limicola, a rhodanese domain containing protein (EanB), directly replaces this unreactive hercynine C–H bond with a C–S bond. Herein, we demonstrate that polysulfide (HSSnSR) is the direct sulfur source in EanB catalysis. After identifying EanB’s substrates, X-ray crystallography of several intermediate states along with mass spectrometry results provide additional mechanistic details for this reaction. Further, quantum mechanics/molecular mechanics (QM/MM) calculations reveal that the protonation of Nπ of hercynine by Tyr353 with the assistance of Thr414 is a key activation step for the hercynine sp2 C–H bond in this trans-sulfuration reaction.