7 research outputs found
Dissecting the low catalytic capability of flavin-dependent halogenases
Although flavin-dependent halogenases (FDHs) are attractive biocatalysts, their practical applications are limited because of their low catalytic efficiency. Here, we investigated the reaction mechanisms and structures of tryptophan 6-halogenase (Thal) from Streptomyces albogriseolus using stopped-flow, rapid-quench flow, quantum/mechanics molecular mechanics calculations, crystallography, and detection of intermediate (hypohalous acid [HOX]) liberation. We found that the key flavin intermediate, C4a-hydroperoxyflavin (C4aOOH-FAD), formed by Thal and other FDHs (tryptophan 7-halogenase [PrnA] and tryptophan 5-halogenase [PyrH]), can react with I-, Br-, and Cl- but not F- to form C4a-hydroxyflavin and HOX. Our experiments revealed that I- reacts with C4aOOH-FAD the fastest with the lowest energy barrier and have shown for the first time that a significant amount of the HOX formed leaks out as free HOX. This leakage is probably a major cause of low product coupling ratios in all FDHs. Site-saturation mutagenesis of Lys79 showed that changing Lys79 to any other amino acid resulted in an inactive enzyme. However, the levels of liberated HOX of these variants are all similar, implying that Lys79 probably does not form a chloramine or bromamine intermediate as previously proposed. Computational calculations revealed that Lys79 has an abnormally lower pKa compared with other Lys residues, implying that the catalytic Lys may act as a proton donor in catalysis. Analysis of new X-ray structures of Thal also explains why premixing of FDHs with reduced flavin adenine dinucleotide generally results in abolishment of C4aOOH-FAD formation. These findings reveal the hidden factors restricting FDHs capability which should be useful for future development of FDHs applications.</p
Mechanism of Oxygen Activation in a Flavin-Dependent Monooxygenase: A Nearly Barrierless Formation of C4a-Hydroperoxyflavin via Proton-Coupled Electron Transfer
Understanding how flavin-dependent
enzymes activate oxygen for
their oxidation and oxygenation reactions is one of the most challenging
issues in flavoenzymology. Density functional calculations and transient
kinetics were performed to investigate the mechanism of oxygen activation
in the oxygenase component (C<sub>2</sub>) of <i>p</i>-hydroxyphenylacetate
3-hydroxylase (HPAH). We found that the protonation of dioxygen by
His396 via a proton-coupled electron transfer mechanism is the key
step in the formation of the triplet diradical complex of flavin semiquinone
and <sup>•</sup>OOH. This complex undergoes intersystem crossing
to form the open-shell singlet diradical complex before it forms the
closed-shell singlet C4a-hydroperoxyflavin intermediate (C4aOOH).
Notably, density functional calculations indicated that the formation
of C4aOOH is nearly barrierless, possibly facilitated by the active
site arrangement in which His396 positions the proximal oxygen of
the <sup>•</sup>OOH in an optimum position to directly attack
the C4a atom of the isoalloxazine ring. The nearly barrierless formation
of C4aOOH agrees well with the experimental results; based on transient
kinetics and Eyring plot analyses, the enthalpy of activation for
the formation of C4aOOH is only 1.4 kcal/mol and the formation of
C4aOOH by C<sub>2</sub> is fast (∼10<sup>6</sup> M<sup>–1</sup> s<sup>–1</sup> at 4 °C). The calculations identified
Ser171 as the key residue that stabilizes C4aOOH by accepting a hydrogen
bond from the HÂ(N5) of the isoalloxazine ring. Both Ser171 and Trp112
facilitate H<sub>2</sub>O<sub>2</sub> elimination by donating hydrogen
bonds to the proximal oxygen of the OOH moiety during the proton transfer.
According to our combined theoretical and experimental studies, the
existence of a positively charged general acid at the position optimized
for facilitating the proton-coupled electron transfer has emerged
as an important catalytic feature for the oxygen activation process
in flavin-dependent enzymes
Control of C4a-Hydroperoxyflavin Protonation in the Oxygenase Component of <i>p</i>‑Hydroxyphenylacetate-3-hydroxylase
The
protonation status of the peroxide moiety in C4a-(hydro)Âperoxyflavin
of <i>p</i>-hydroxyphenylacetate-3-hydroxylase can be directly
monitored using transient kinetics. The p<i>K</i><sub>a</sub> for the wild-type (WT) enzyme is 9.8 ± 0.2, while the values
for the H396N, H396V, and H396A variants are 9.3 ± 0.1, 7.3 ±
0.2, and 7.1 ± 0.2, respectively. The hydroxylation efficiency
of these mutants is lower than that of the WT enzyme. Solvent kinetic
isotope effect studies indicate that proton transfer is not the rate-limiting
step in the formation of C4a-OOH. All data suggest that His396 may
act as an instantaneous proton provider for the proton-coupled electron
transfer that occurs before the transition state of C4a-OOH formation
Tuning of pK<sub>a</sub> values activates substrates in flavin-dependent aromatic hydroxylases
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.</p
Protonation status and control mechanism of flavin–oxygen intermediates in the reaction of bacterial luciferase
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167808/1/febs15653.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167808/2/febs15653_am.pd
<i>p</i>‑Hydroxyphenylacetate 3‑Hydroxylase as a Biocatalyst for the Synthesis of Trihydroxyphenolic Acids
Trihydroxyphenolic acids such as
3,4,5-trihydroxycinnamic acid (3,4,5-THCA) <b>4c</b> and 2-(3,4,5-trihydroxyphenyl)Âacetic
acid (3,4,5-THPA) <b>2c</b> are strong antioxidants that are
potentially useful as medicinal agents. Our results show that <i>p</i>-hydroxyphenylacetate (HPA) 3-hydroxylase (HPAH) from <i>Acinetobacter baumannii</i> can catalyze the syntheses of 3,4,5-THPA <b>2c</b> and 3,4,5-THCA <b>4c</b> from 4-HPA <b>2a</b> and <i>p</i>-coumaric acid <b>4a</b>, respectively.
The wild-type HPAH can convert 4-HPA <b>2a</b> completely into
3,4,5-THPA <b>2c</b> within 100 min (total turnover number (TTN)
of 100). However, the wild-type enzyme cannot efficiently synthesize
3,4,5-THCA <b>4c</b>. To improve the efficiency, the oxygenase
component of HPAH (C<sub>2</sub>) was rationally engineered in order
to maximize the conversion of <i>p</i>-coumaric acid <b>4a</b> to 3,4,5-THCA <b>4c</b>. Results from site-directed
mutagenesis studies showed that Y398S is significantly more effective
than the wild-type enzyme for the synthesis of 3,4,5-THCA <b>4c</b>; it can catalyze the complete bioconversion of <i>p</i>-coumaric acid <b>4a</b> to 3,4,5-THCA <b>4c</b> within
180 min (TTN ∼ 23 at 180 min). The yield and stability of 3,4,5-THPA <b>2c</b> and 3,4,5-THCA <b>4c</b> were significantly improved
in the presence of ascorbic acid. Thermostability studies showed that
the wild-type C<sub>2</sub> was very stable and remained active after
incubation at 30, 35, and 40 °C for 24 h. Y398S was moderately
stable because its activity was retained for 24 h at 30 °C and
for 15 h at 35 °C. Transient kinetic studies using stopped-flow
spectrophotometry indicated that the key improvement in the reaction
of Y398S with <i>p</i>-coumaric acid <b>4a</b> lies
within the protein–ligand interaction. Y398S binds to <i>p</i>-coumaric acid <b>4a</b> with higher affinity than
the wild-type enzyme, resulting in a shift in equilibrium toward favoring
the productive coupling path instead of the path leading to wasteful
flavin oxidation