The radical S-adenosylmethionine (SAM) protein PqqE is predicted to function in the pyrroloquinoline quinone (PQQ) biosynthetic pathway via catalysis of carbon-carbon bond formation between a glutamate and tyrosine side chain within the small peptide substrate PqqA. We report here that PqqE activity is dependent on the accessory protein PqqD, which was recently shown to bind PqqA tightly. In addition, PqqE activity in vitro requires the presence of a flavodoxin-and flavodoxin reductasebased reduction system, with other reductants leading to an uncoupled cleavage of the co-substrate SAM. These results indicate that PqqE, in conjunction with PqqD, carries out the first step in PQQ biosynthesis: a radical-mediated formation of a new carbon-carbon bond between two amino acid side chains on PqqA.Pyrroloquinoline quinone (PQQ) 2 is employed by a wide variety of bacteria, where it functions as a redox cofactor in substrate oxidation via an alternate (non-glycolytic) pathway for production of cellular ATP (1). Hundreds of bacterial species are thought to produce PQQ, based on the presence in their genomes of the strongly conserved pqq operon (2) (Fig. 1A). Although the existence of this important redox cofactor has been known for decades, knowledge of the biosynthetic pathway for PQQ has remained scant (3, 4). The PQQ molecule derives from the evolutionarily conserved glutamate and tyrosine side chains within a ribosomally produced peptide substrate PqqA (Fig. 1, B and C). To produce PQQ, a large number of chemical modifications are required: (i) the formation of a new carbon-carbon bond between the ␥-carbon of glutamate and the 3-position of tyrosine (labeled in green in Fig. 1C); (ii) the addition of two additional hydroxyl groups to the tyrosine ring; (iii) a condensation between the 4-OH of tyrosine and the backbone amine of glutamate to produce a heterocyclic ring; and (iv) an 8-electron oxidation catalyzed by PqqC, a cofactorless enzyme that converts 3a-(2-amino-2-carboxyethyl)-4,5-dioxo-4,5,6,7,8,9-hexahydroquinoline-7,9-dicarboxylic acid (AHQQ) to PQQ in the final step of the pathway (5-7).The radical SAM enzyme PqqE has been the primary candidate as the catalyst for the formation of the new carbon-carbon bond between glutamate and tyrosine. Radical SAM proteins use the reductive cleavage of S-adenosyl methionine to initiate free radical chemistry, and can accomplish a wide variety of reactions, including carbon-carbon bond formation (8, 9). PqqE is a founding member of the SPASM domain-containing radical SAM proteins, which contain one or more auxiliary clusters in their C-terminal regions, and of which several are known to modify small peptides or proteins (10, 11). Recently, a small (ϳ10 kDa) protein, PqqD, has been shown to form a strong, sub-micromolar K D complex with the peptide substrate PqqA. This complex was further demonstrated to associate with PqqE (12). These findings suggested that PqqD would serve as a chaperone to deliver PqqA to PqqE, functioning as a necessary and heretofore missing componen...
CONSPECTUS: Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world's energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production−consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earthabundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal−polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH 3 CN) 2 ] 2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me 2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me 2 )MoO] 2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H 2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H 2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me 2 )MoS 2 ] 2+ , which bears a molecular MoS 2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me 2 )MoS 2 ] 2+ onsets at 400 mV of overpotential, whereas [(PY5Me 2 )MoO] 2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me 2 ligand with an appropriate Co(II) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me 2 )Co(H 2 O)] 2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF 3 SO 3 )] 1+ system, we showed that introducing a redox-active moiety can facilitate the electro-and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we...
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