Redox properties of vanillyl-alcohol oxidase

Substrate Electronics Dominate the Rate of Reductive Dehalogenation Promoted by the Flavin-Dependent Iodotyrosine Deiodinase

Iodotyrosine deiodinase (IYD) is unusual in its reliance on flavin to promote reductive dehalogenation of halotyrosines under aerobic conditions. Applications of this activity can be envisioned for bioremediation, but expansion of its specificity requires an understanding of the mechanistic steps that limit the rate of turnover. Key processes capable of controlling steady-state turnover have now been evaluated and described in this study. While proton transfer is necessary for converting the electron-rich substrate into an electrophilic intermediate suitable for reduction, kinetic solvent deuterium isotope effects suggest that this process does not contribute to the overall efficiency of catalysis under neutral conditions. Similarly, reconstituting IYD with flavin analogues demonstrates that a change in reduction potential by as much as 132 mV affects kcat by less than 3-fold. Furthermore, kcat/Km does not correlate with reduction potential and indicates that electron transfer is also not rate determining. Catalytic efficiency is most sensitive to the electronic nature of its substrates. Electron-donating substituents on the ortho position of iodotyrosine stimulate catalysis and conversely electron-withdrawing substituents suppress catalysis. Effects on kcat and kcat/Km range from 22- to 100-fold and fit a linear free-energy correlation with a ρ ranging from -2.1 to -2.8 for human and bacterial IYD. These values are consistent with a rate-determining process of stabilizing the electrophilic and nonaromatic intermediate poised for reduction. Future engineering can now focus on efforts to stabilize this electrophilic intermediate over a broad series of phenolic substrates that are targeted for removal from our environment.

1H-Benzo[g]pteridine-2,4-dione

The structure of the title compound, C₁₀H₆N₄O₂, reported by Smalley et al. [(2021). Cryst. Growth Des. 22, 524-534] from powder diffraction data and ¹⁵N NMR spectroscopy, is confirmed using low-temperature data from a twinned crystal. The tautomer in the solid state is alloxazine (1H-benzo[g]pteridine-2,4-dione) rather than isoalloxazine (10H-benzo[g]pteridine-2,4-dione). In the extended structure, the mol-ecules form hydrogen-bonded chains propagating in the [01] direction through alternating centrosymmetric R ² ₂(8) rings with pairwise N-H⋯O inter-actions and centrosymmetric R ² ₂(8) rings with pairwise N-H⋯N inter-actions. The crystal chosen for data collection was found to be a non-merohedral twin (180° rotation about [001]) in a 0.446 (4):0.554 (6) domain ratio.

Sequence Conservation Does Not Always Signify a Functional Imperative as Observed in the Nitroreductase Superfamily

Consensus sequences have the potential to help classify the structure and function of proteins and highlight key regions that may contribute to their biological properties. Often, the level of significance will track with the extent of sequence conservation, but this should not be considered universal. Arg and Lys dominate a position adjacent to the N1 and C2 carbonyl of flavin mononucleotide (FMN) bound in the proteins of the nitroreductase superfamily. Although this placement satisfies expectations for stabilizing the reduced form of FMN, the substitution of these residues in three subfamilies promoting distinct reactions demonstrates their importance to catalysis as only modest. Replacing Arg34 with Lys, Gln, or Glu enhances FMN binding to a flavin destructase (BluB) by twofold and diminishes FMN turnover by no more than 25%. Similarly, replacing Lys14 with Arg, Gln, or Glu in a nitroreductase (NfsB) does not perturb the binding of the substrate nitrofurazone. The catalytic efficiency does decrease by 21-fold for the K14Q variant, but no change in the midpoint potential of FMN was observed with any of the variants. Equivalent substitution at Arg38 in iodotyrosine deiodinase (IYD) affects catalysis even more modestly (<10-fold). While the Arg/Lys to Glu substitution inactivates NfsB and IYD, this change also stabilizes one-electron transfer in IYD contrary to predictions based on other classes of flavoproteins. Accordingly, functional correlations developed in certain structural superfamilies may not necessarily translate well to other superfamilies.

The efficient Δ1-dehydrogenation of a wide spectrum of 3-ketosteroids in a broad pH range by 3-ketosteroid dehydrogenase from Sterolibacterium denitrificans

Cholest-4-en-3-one Δ¹-dehydrogenase (AcmB) from Sterolibacterium denitrificans, a key enzyme of the central degradation pathway of cholesterol, is a protein catalyzing Δ¹-dehydrogenation of a wide range of 3-ketosteroids. In this study, we demonstrate the application of AcmB in the synthesis of 1-dehydro-3-ketosteroids and investigate the influence of reaction conditions on the catalytic performance of the enzyme. The recombinant AcmB expressed in E. coli BL21(DE3)Magic exhibits a broad pH optimum and pH stability in the range of 6.5 to 9.0. The activity-based pH optimum of AcmB reaction depends on the type of electron acceptor (2,6-dichloroindophenol - DCPIP, phenazine methosulfate - PMS or potassium hexacyanoferrate - K₃[Fe(CN)₆]) used in the biocatalytic process yielding the best kinetic properties for the reaction with a DCPIP/PMS mixture (k_cat/K_m = 1.4·10⁵ s^-1·M^-1 at pH 9.0) followed by DCPIP (k_cat/K_m = 1.0·10⁵ s^-1·M^-1 at pH = 6.5) and K₃[Fe(CN)₆] (k_cat/K_m = 0.5·10² s^-1·M^-1 at pH = 8.0). The unique feature of AcmB is its capability to convert both testosterone derivatives (C20-C22) as well as steroids substituted at C17 (C27-C30) such as cholest-4-en-3-one or (25R)-spirost-4-en-3-one (diosgenone). Apparent steady-state kinetic parameters were determined for both groups of AcmB substrates. In a batch reactor synthesis, the solubility of water-insoluble steroids was facilitated by the addition of a solubilizer, 2-hydroxypropyl-β-cyclodextrin, and organic co-solvent, 2-methoxyethanol. Catalytic properties characterization of AcmB was tested in fed-batch reactor set-ups, using 0.81 μM of isolated enzyme, PMS and aerobic atmosphere resulting in >99% conversion of the C17-C20 3-ketosteroids within 2 h. Finally, the whole cell E. coli system with recombinant enzyme was demonstrated as an efficient biocatalyst in the synthesis of 1-dehydro-3-ketosteroids.

Redox control of iodotyrosine deiodinase

The redox chemistry of flavoproteins is often gated by substrate and iodotyrosine deiodinase (IYD) has the additional ability to switch between reaction modes based on the substrate. Association of fluorotyrosine (F-Tyr), an inert substrate analog, stabilizes single electron transfer reactions of IYD that are not observed in the absence of this ligand. The co-crystal of F-Tyr and a T239A variant of human IYD have now been characterized to provide a structural basis for control of its flavin reactivity. Coordination of F-Tyr in the active site of this IYD closely mimics that of iodotyrosine and only minor perturbations are observed after replacement of an active site Thr with Ala. However, loss of the side chain hydroxyl group removes a key hydrogen bond from flavin and suppresses the formation of its semiquinone intermediate. Even substitution of Thr with Ser decreases the midpoint potential of human IYD between its oxidized and semiquinone forms of flavin by almost 80 mV. This decrease does not adversely affect the kinetics of reductive dehalogenation although an analogous Ala variant exhibits a 6.7-fold decrease in its kcat /Km . Active site ligands lacking the zwitterion of halotyrosine are not able to induce closure of the active site lid that is necessary for promoting single electron transfer and dehalogenation. Under these conditions, a basal two-electron process dominates catalysis as indicated by preferential reduction of nitrophenol rather than deiodination of iodophenol.

Active Site Binding Is Not Sufficient for Reductive Deiodination by Iodotyrosine Deiodinase

The minimal requirements for substrate recognition and turnover by iodotyrosine deiodinase were examined to learn the basis for its catalytic specificity. This enzyme is crucial for iodide homeostasis and the generation of thyroid hormone in chordates. 2-Iodophenol binds only very weakly to the human enzyme and is dehalogenated with a k_cat/K_m that is more than 4 orders of magnitude lower than that for iodotyrosine. This discrimination likely protects against a futile cycle of iodinating and deiodinating precursors of thyroid hormone biosynthesis. Surprisingly, a very similar catalytic selectivity was expressed by a bacterial homologue from Haliscomenobacter hydrossis. In this example, discrimination was not based on affinity since 4-cyano-2-iodophenol bound to the bacterial deiodinase with a K_d lower than that of iodotyrosine and yet was not detectably deiodinated. Other phenols including 2-iodophenol were deiodinated but only very inefficiently. Crystal structures of the bacterial enzyme with and without bound iodotyrosine are nearly superimposable and quite similar to the corresponding structures of the human enzyme. Likewise, the bacterial enzyme is activated for single electron transfer after binding to the substrate analogue fluorotyrosine as previously observed with the human enzyme. A cocrystal structure of bacterial deiodinase and 2-iodophenol indicates that this ligand stacks on the active site flavin mononucleotide (FMN) in a orientation analogous to that of bound iodotyrosine. However, 2-iodophenol association is not sufficient to activate the FMN chemistry required for catalysis, and thus the bacterial enzyme appears to share a similar specificity for halotyrosines even though their physiological roles are likely very different from those in humans.

Single Amino Acid Switch between a Flavin-Dependent Dehalogenase and Nitroreductase

A single mutation within a flavoprotein is capable of switching the catalytic activity of a dehalogenase into a nitroreductase. This change in function correlates with a destabilization of the one-electron-reduced flavin semiquinone that is differentially expressed in the nitro-FMN reductase superfamily during redox cycling. The diversity of function within such a superfamily therefore has the potential to arise from rapid evolution, and its members should provide a convenient basis for developing new catalysts with an altered specificity of choice.

A switch between one- and two-electron chemistry of the human flavoprotein iodotyrosine deiodinase is controlled by substrate

Reductive dehalogenation is not typical of aerobic organisms but plays a significant role in iodide homeostasis and thyroid activity. The flavoprotein iodotyrosine deiodinase (IYD) is responsible for iodide salvage by reductive deiodination of the iodotyrosine derivatives formed as byproducts of thyroid hormone biosynthesis. Heterologous expression of the human enzyme lacking its N-terminal membrane anchor has allowed for physical and biochemical studies to identify the role of substrate in controlling the active site geometry and flavin chemistry. Crystal structures of human IYD and its complex with 3-iodo-l-tyrosine illustrate the ability of the substrate to provide multiple interactions with the isoalloxazine system of FMN that are usually provided by protein side chains. Ligand binding acts to template the active site geometry and significantly stabilize the one-electron-reduced semiquinone form of FMN. The neutral form of this semiquinone is observed during reductive titration of IYD in the presence of the substrate analog 3-fluoro-l-tyrosine. In the absence of an active site ligand, only the oxidized and two-electron-reduced forms of FMN are detected. The pH dependence of IYD binding and turnover also supports the importance of direct coordination between substrate and FMN for productive catalysis.

Electrooxidation of Vanillyl Alcohol in Acidic Aqueous Solution Using Rotating Ring-Disk Electrode Voltammetry

The electrooxidation of benzylic alcohol derivative in acidic aqueous solution shows an oxidation pre-peak in the cyclic voltammogram, which means that the reaction is proceeding via an ECE mechanism where the second electron transfer occurs at a less positive potential. From the result of the rotating ring-disk electrode voltammetry, the initial oxidation response of the electrode reaction can be extracted.