Hydroxylation of substituted phenols by flavin-dependent monooxygenases is the first step of their biotransformation in various microorganisms. The reaction is thought to proceed via electrophilic aromatic substitution, catalyzed by enzymatic deprotonation of substrate, in single-component hydroxylases that use flavin as a cofactor (group A). However, two-component hydroxylases (group D), which use reduced flavin as a co-substrate, are less amenable to spectroscopic investigation. Herein, we employed 19F NMR in conjunction with fluorinated substrate analogs to directly measure pKa values and to monitor protein events in hydroxylase active sites. We found that the single-component monooxygenase 3-hydroxybenzoate 6-hy-droxylase (3HB6H) depresses the pKa of the bound substrate analog 4-fluoro-3-hydroxybenzoate (4F3HB) by 1.6 pH units, consistent with previously proposed mechanisms. 19F NMR was applied anaerobically to the two-component monooxygenase 4-hydroxyphenylacetate 3-hydroxylase (HPAH), revealing depression of the pKa of 3-fluoro-4-hydroxyphenylacetate by 2.5 pH units upon binding to the C2 component of HPAH. 19F NMR also revealed a pKa of 8.7 ± 0.05 that we attributed to an active-site residue involved in deprotonating bound substrate, and assigned to His-120 based on studies of protein variants. Thus, in both types of hydroxylases, we confirmed that binding favors the phenolate form of substrate. The 9 and 14 kJ/mol magnitudes of the effects for 3HB6H and HPAH-C2, respectively, are consistent with pKa tuning by one or more H-bonding interactions. Our implementation of 19F NMR in anaerobic samples is applicable to other two-component flavin-dependent hydroxylases and promises to expand our understanding of their catalytic mechanisms.
|Number of pages||17|
|Journal||Journal of Biological Chemistry|
|State||Published - Mar 20 2020|
Bibliographical noteFunding Information:
This work was supported by the Vice President for Research of the University of Kentucky, the Center for Pharmaceutical Development, National Sci-ence Foundation Grant I/UCRC IIP-1540011 (to A.-F. M.), a Research Chal-lenge Trust Fund Fellowship (to W. P.), the National Science Foundation, Chemistry of Life Processes Grant CHE-1808433 (to A.-F. M.), the Thailand Research Fund grant RTA5980001, Thailand Science Research and Innova-tion Global Partnership Program and Vidyasirimedhi Institute of Science and Technology (VISTEC) (to P. Chaiyen), and Thailand Research Fund Grants MRG6180156 (to P. Chenprakhon) and RSA5980062 (to J. S.). The authors declare that they have no conflicts of interest with the contents of this article.
© 2020 Pitsawong et al. Published under exclusive license by The American Society for Biochemistry and Molecular Biology, Inc.
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology