Abstract
The flavin transferase ApbE plays essential roles in bacterial physiology, covalently incorporating FMN cofactors into numerous respiratory enzymes that use the integrated cofactors as electron carriers. In this work we performed a detailed kinetic and structural characterization of Vibrio cholerae WT ApbE and mutants of the conserved residue His-257, to understand its role in substrate binding and in the catalytic mechanism of this family. Bi-substrate kinetic experiments revealed that ApbE follows a random Bi Bi sequential kinetic mechanism, in which a ternary complex is formed, indicating that both substrates must be bound to the enzyme for the reaction to proceed. Steady-state kinetic analyses show that the turnover rates of His-257 mutants are significantly smaller than those of WT ApbE, and have increased Km values for both substrates, indicating that the His-257 residue plays important roles in catalysis and in enzyme-substrate complex formation. Analyses of the pH dependence of ApbE activity indicate that the pKa of the catalytic residue (pKES1) increases by 2 pH units in the His-257 mutants, suggesting that this residue plays a role in substrate deprotonation. The crystal structures of WT ApbE and an H257G mutant were determined at 1.61 and 1.92 Å resolutions, revealing that His-257 is located in the catalytic site and that the substitution does not produce major conformational changes. We propose a reaction mechanism in which His-257 acts as a general base that deprotonates the acceptor residue, which subsequently performs a nucleophilic attack on FAD for flavin transfer.
Highlights
The flavin transferase ApbE plays essential roles in bacterial physiology, covalently incorporating FMN cofactors into numerous respiratory enzymes that use the integrated cofactors as electron carriers
ApbE activity was investigated in the presence of potassium, which was recently shown to activate the enzyme [23], to mimic the physiological conditions that V. cholerae encounters during infection
ApbE flavin-transfer activity can be measured by following the fluorescence of covalently-bound FMN to NqrC in SDSPAGE gels exposed to UV light [23]
Summary
The flavin transferase ApbE plays essential roles in bacterial physiology, covalently incorporating FMN cofactors into numerous respiratory enzymes that use the integrated cofactors as electron carriers. It has been recently reported that several families of respiratory enzymes contain covalently-bound FMN attached through a phosphoester bond between FMNЈs phosphate moiety and a threonine residue located within the semiconserved motif SGAT [17,18,19,20,21,22] These covalent bonds have been observed in the ion-pumping NADH:ubiquinone oxidoreductase (NQR)2 [17,18,19,20, 23], which is the first respiratory enzyme and the main sodium pump for Vibrio cholerae (24 –29). This type of covalent attachment of the FMN cofactor has been observed in the membrane-bound ferredoxin:NADϩ oxidoreductase (RNF) [21], urocanate reductase [30], and nitrate reductase regulatory protein NosR [22, 31] These respiratory enzymes, in particular NQR, are essential for pathogenic bacteria including V. cholerae, Pseudomonas aeruginosa, Chlamydia trachomatis, and Klebsiella pneumoniae, sustaining the metabolism, and in some cases supporting antibiotic-resistance development (24 –26, 32–35). This covalent bond was considered exclusive to bacterial enzymes, it was recently reported that eukaryotic fumarate reductases carry this type of covalently-bound FMN [36]
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