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3-Hydroxyanthranilate 3,4-dioxygenase (HAO) is an iron-dependent protein that activates O2 and inserts both O atoms into 3- hydroxyanthranilate (3-HAA). An intriguing question is how HAO can rapidly bind O2, even though local O2 concentrations and diffusion rates are relatively low. Here, a close inspection of the HAO structures revealed that substrate- and inhibitor-bound structures exhibit a closed conformation with three hydrophobic loop regions moving toward the catalytic iron center, whereas the ligand-free structure is open. We hypothesized that these loop movements enhance O2 binding to the binary complex of HAO and to 3-HAA. We found that the carboxyl end of 3-HAA triggers the changes in two loop regions and that the third loop movement appears to be driven by an H-bond interaction between Asn-27 and Ile-142. Mutational analyses revealed that N27A, I142A, and I142P variants cannot form a closed conformation, and steady-state kinetic assays indicated that these variants have a substantially higher Km for O2 than wild-type HAO. This observation suggested enhanced hydrophobicity at the iron center resulting from the concerted loop movements after the binding of the primary substrate, which is hydrophilic. Given that O2 is nonpolar, the increased hydrophobicity at the Fe center of the complex appears to be essential for rapid O2 binding and activation, explaining the reason for the 3-HAA–induced loop movements. As substrate binding–induced open-to-closed conformational changes are common, the results reported here may help further our understanding of how oxygen is enriched in the nonheme Fe-dependent dioxygenases.
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Charge Maintenance during Catalysis in Nonheme Iron Oxygenases
Here, the choice of the first coordination shell of the metal center is analyzed from the perspective of charge maintenance in a binary enzyme–substrate complex and an O2-bound ternary complex in the nonheme iron oxygenases. Comparing homogentisate 1,2-dioxygenase and gentisate dioxygenase highlights the significance of charge maintenance after substrate binding as an important factor that drives the reaction coordinate. We then extend the charge analysis to several common types of nonheme iron oxygenases containing either a 2-His-1-carboxylate facial triad or a 3-His or 4-His ligand motif, including extradiol and intradiol ring-cleavage dioxygenases, thiol dioxygenases, α-ketoglutarate-dependent oxygenases, and carotenoid cleavage oxygenases. After forming the productive enzyme–substrate complex, the overall charge of the iron complex at the 0, +1, or +2 state is maintained in the remaining catalytic steps. Hence, maintaining a constant charge is crucial to promote the reaction of the iron center beginning from the formation of the Michaelis or ternary complex. The charge compensation to the iron ion is tuned not only by protein-derived carboxylate ligands but also by substrates. Overall, these analyses indicate that charge maintenance at the iron center is significant when all the necessary components form a productive complex. This charge maintenance concept may apply to most oxygen-activating metalloenzymes systems that do not draw electrons and protons step-by-step from a separate reactant, such as NADH, via a reductase. The charge maintenance perception may also be useful in proposing catalytic pathways or designing prototypical reactions using artificial or engineered enzymes for biotechnological applications.
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- Award ID(s):
- 2204225
- PAR ID:
- 10387657
- Editor(s):
- Pimchai Chaiyen
- Date Published:
- Journal Name:
- ACS Catalysis
- Volume:
- 12
- Issue:
- 10
- ISSN:
- 2155-5435
- Page Range / eLocation ID:
- 6191 to 6208
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acscatal.1c04770
