Abstract
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 reductase-based 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.
Highlights
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 pathway for production of cellular ATP [1]
We examined both the E. coli FldA/flavodoxin reductase (FNR) pair and a second reducing system composed of an FNR from M. extorquens FNR and an FldA from A. vinelandii
Among the three major classes of enzymatic redox cofactors, there has been a singular lack of understanding regarding the biosynthetic pathway for PQQ production
Summary
Demonstration That the Radical S-Adenosylmethionine (SAM) Enzyme PqqE Catalyzes de Novo Carbon-Carbon Cross-linking within a Peptide Substrate PqqA in the Presence of the Peptide Chaperone PqqD*ࡗ. 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. These findings suggested that PqqD would serve as a chaperone to deliver PqqA to PqqE, functioning as a necessary and heretofore missing component of the subsequent radical-based chemistry This connects PqqD and the PQQ biosynthesis pathway to a growing class of proteins involved in binding and modifying small, ribosomally produced peptides, of which many are radical SAM proteins [13,14,15]. We show that PqqE, in association with the complex, PqqA/D, generates a functional catalyst for the cross-linking of glutamate and tyrosine within PqqA
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