Proline 107 is a major determinant in maintaining the structure of the distal pocket and reactivity of the high-spin heme of MauG

A widely distributed diheme enzyme from Burkholderia that displays an atypically stable bis-Fe(IV) state

Bacterial diheme peroxidases represent a diverse enzyme family with functions that range from hydrogen peroxide (H2O2) reduction to post-translational modifications. By implementing a sequence similarity network (SSN) of the bCCP_MauG superfamily, we present the discovery of a unique diheme peroxidase BthA conserved in all Burkholderia. Using a combination of magnetic resonance, near-IR and Mössbauer spectroscopies and electrochemical methods, we report that BthA is capable of generating a bis-Fe(IV) species previously thought to be a unique feature of the diheme enzyme MauG. However, BthA is not MauG-like in that it catalytically converts H2O2 to water, and a 1.54-Å resolution crystal structure reveals striking differences between BthA and other superfamily members, including the essential residues for both bis-Fe(IV) formation and H2O2 turnover. Taken together, we find that BthA represents a previously undiscovered class of diheme enzymes, one that stabilizes a bis-Fe(IV) state and catalyzes H2O2 turnover in a mechanistically distinct manner.

YhjA - An Escherichia coli trihemic enzyme with quinol peroxidase activity

The trihemic bacterial cytochrome c peroxidase from Escherichia coli, YhjA, is a membrane-anchored protein with a C-terminal domain homologous to the classical bacterial peroxidases and an additional N-terminal (NT) heme binding domain. Recombinant YhjA is a 50 kDa monomer in solution with three c-type hemes covalently bound. Here is reported the first biochemical and spectroscopic characterization of YhjA and of the NT domain demonstrating that NT heme is His63/Met125 coordinated. The reduction potentials of P (active site), NT and E hemes were established to be -170 mV, +133 mV and +210 mV, respectively, at pH 7.5. YhjA has quinol peroxidase activity in vitro with optimum activity at pH 7.0 and millimolar range K_M values using hydroquinone and menadiol (a menaquinol analogue) as electron donors (K_M = 0.6 ± 0.2 and 1.8 ± 0.5 mM H₂O₂, respectively), with similar turnover numbers (k_cat = 19 ± 2 and 13 ± 2 s^-1, respectively). YhjA does not require reductive activation for maximum activity, in opposition to classical bacterial peroxidases, as P heme is always high-spin 6-coordinated with a water-derived molecule as distal axial ligand but shares the need for the presence of calcium ions in the kinetic assays. Formation of a ferryl Fe(IV) = O species was observed upon incubation of fully oxidized YhjA with H₂O₂. The data reported improve our understanding of the biochemical properties and catalytic mechanism of YhjA, a three-heme peroxidase that uses the quinol pool to defend the cells against hydrogen peroxide during transient exposure to oxygenated environments.

Protein-Derived Cofactors Revisited: Empowering Amino Acid Residues with New Functions

A protein-derived cofactor is a catalytic or redox-active site in a protein that is formed by post-translational modification of one or more amino acid residues. These post-translational modifications are irreversible and endow the modified amino acid residues with new functional properties. This Perspective focuses on the following advances in this area that have occurred during recent years. The biosynthesis of the tryptophan tryptophylquinone cofactor is catalyzed by a diheme enzyme, MauG. A bis-Fe^IV redox state of the hemes performs three two-electron oxidations of specific Trp residues via long-range electron transfer. In contrast, a flavoenzyme catalyzes the biosynthesis of the cysteine tryptophylquinone (CTQ) cofactor present in a newly discovered family of CTQ-dependent oxidases. Another carbonyl cofactor, the pyruvoyl cofactor found in classes of decarboxylases and reductases, is formed during an apparently autocatalytic cleavage of a precursor protein at the N-terminus of the cleavage product. It has been shown that in at least some cases, the cleavage is facilitated by binding to an accessory protein. Tyrosylquinonine cofactors, topaquinone and lysine tyrosylquinone, are found in copper-containing amine oxidases and lysyl oxidases, respectively. The physiological roles of different families of these enzymes in humans have been more clearly defined and shown to have significant implications with respect to human health. There has also been continued characterization of the roles of covalently cross-linked amino acid side chains that influence the reactivity of redox-active metal centers in proteins. These include Cys-Tyr species in galactose oxidase and cysteine dioxygenase and the Met-Tyr-Trp species in the catalase-peroxidase KatG.

Ascorbate protects the diheme enzyme, MauG, against self-inflicted oxidative damage by an unusual antioxidant mechanism

Ascorbate protects MauG from self-inactivation that occurs during the autoreduction of the reactive bis-FeIV state of its diheme cofactor. The mechanism of protection does not involve direct reaction with reactive oxygen species in solution. Instead, it binds to MauG and mitigates oxidative damage that occurs via internal transfer of electrons from amino acid residues within the protein to the high-valent hemes. The presence of ascorbate does not inhibit the natural catalytic reaction of MauG, which catalyzes oxidative post-translational modifications of a substrate protein that binds to the surface of MauG and is oxidized by the high-valent hemes via long-range electron transfer. Ascorbate was also shown to prolong the activity of a P107V MauG variant that is more prone to inactivation. A previously unknown ascorbate peroxidase activity of MauG was characterized with a kcat of 0.24 s−1 and a Km of 2.2 µM for ascorbate. A putative binding site for ascorbate was inferred from inspection of the crystal structure of MauG and comparison with the structure of soybean ascorbate peroxidase with bound ascorbate. The ascorbate bound to MauG was shown to accelerate the rates of both electron transfers to the hemes and proton transfers to hemes which occur during the multistep autoreduction to the diferric state which is accompanied by oxidative damage. A structural basis for these effects is inferred from the putative ascorbate-binding site. This could be a previously unrecognized mechanism by which ascorbate mitigates oxidative damage to heme-dependent enzymes and redox proteins in nature.

Properties of the high-spin heme of MauG are altered by binding of preMADH at the protein surface 40 Å away

The diheme enzyme MauG catalyzes oxidative post-translational modifications of a protein substrate, precursor protein of methylamine dehydrogenase (preMADH), that binds to the surface of MauG. The high-spin heme iron of MauG is located 40 Å from preMADH. The ferric heme is an equilibrium of five- and six-coordinate states. PreMADH binding increases the proportion of five-coordinate heme three-fold. On reaction of MauG with H₂ O₂ both hemes become Fe^IV . In the absence of preMADH the hemes autoreduce to ferric in a multistep process involving multiple electron and proton transfers. Binding of preMADH in the absence of catalysis alters the mechanism of autoreduction of the ferryl heme. Thus, substrate binding alters the environment in the distal heme pocket of the high-spin heme over very long distance.

Heterolytic OO bond cleavage: Functional role of Glu113 during bis-Fe(IV) formation in MauG

The diheme enzyme MauG utilizes H₂O₂ to perform oxidative posttranslational modification on a protein substrate. A bis-Fe(IV) species of MauG was previously identified as a key intermediate in this reaction. Heterolytic cleavage of the OO bond of H₂O₂ drives the formation of the bis-Fe(IV) intermediate. In this work, we tested a hypothesis that a glutamate residue, Glu113 in the distal pocket of the pentacoordinate heme of MauG, facilitates heterolytic OO bond cleavage, thereby leading to bis-Fe(IV) formation. This hypothesis was proposed based on sequence alignment and structural comparison with other H₂O₂-utilizing hemoenzymes, especially those from the diheme enzyme superfamily that MauG belongs to. Electron paramagnetic resonance (EPR) characterization of the reaction between MauG and H₂O₂ revealed that mutation of Glu113 inhibited heterolytic OO bond cleavage, in agreement with our hypothesis. This result was further confirmed by the HPLC study in which an analog of H₂O₂, cumene hydroperoxide, was used to probe the pattern of OO bond cleavage. Together, our data suggest that Glu113 functions as an acid-base catalyst to assist heterolytic OO bond cleavage during the early stage of the catalytic reaction. This work advances our mechanistic understanding of the H₂O₂-activation process during bis-Fe(IV) formation in MauG.

A Suicide Mutation Affecting Proton Transfers to High-Valent Hemes Causes Inactivation of MauG during Catalysis

In the absence of its substrate, the autoreduction of the high-valent bis-Fe^IV state of the hemes of MauG to the diferric state proceeds via a Compound I-like and then a Compound II-like intermediate. This process is coupled to oxidative damage to specific methionine residues and inactivation of MauG. The autoreduction of a P107V MauG variant, which is more prone to oxidative damage, proceeds directly from the bis-Fe^IV to the Compound II-like state with no detectable Compound I intermediate. Comparison of the crystal structures of native and P107V MauG reveals that this mutation alters the positions of amino acid residues in the heme site as well as the water network that delivers protons from the solvent to the hemes during their reduction. Kinetic, spectroscopic, and solvent kinetic isotope effect studies demonstrate that these changes in the heme site affect the protonation state of the ferryl heme and the relative efficiencies of two alternative pathways for the transfer of protons from solvent to the hemes. These changes enhance the rate of autoreduction of P107V MauG such that it competes with the catalytic reaction with substrate and causes the enzyme to inactivate itself during the steady-state reaction with H₂O₂ and its substrate. Thus, while this mutation has negligible effects on the initial steady-state kinetic parameters of MauG, it is a fatal mutation as it causes inactivation during catalysis.

Mechanism of protein oxidative damage that is coupled to long-range electron transfer to high-valent haems

In the absence of its substrate, the auto-reduction of the high-valent bis-Fe(IV) state of the dihaem enzyme MauG is coupled to oxidative damage of a methionine residue. Transient kinetic and solvent isotope effect studies reveal that this process occurs via two sequential long-range electron transfer (ET) reactions from methionine to the haems. The first ET is coupled to proton transfer (PT) to the haems from solvent via an ordered water network. The second ET is coupled to PT at the methionine site and occurs during the oxidation of the methionine to a sulfoxide. This process proceeds via Compound I- and Compound II-like haem intermediates. It is proposed that the methionine radical is stabilized by a two-centre three-electron (2c3e) bond. This provides insight into how oxidative damage to proteins may occur without direct contact with a reactive oxygen species, and how that damage can be propagated through the protein.

Roles of multiple-proton transfer pathways and proton-coupled electron transfer in the reactivity of the bis-FeIV state of MauG

The high-valent state of the diheme enzyme MauG exhibits charge-resonance (CR) stabilization in which the major species is a bis-Fe(IV) state with one heme present as Fe(IV)=O and the other as Fe(IV) with axial heme ligands provided by His and Tyr side chains. In the absence of its substrate, the high-valent state is relatively stable and returns to the diferric state over several minutes. It is shown that this process occurs in two phases. The first phase is redistribution of the resonance species that support the CR. The second phase is the loss of CR and reduction to the diferric state. Thermodynamic analysis revealed that the rates of the two phases exhibited different temperature dependencies and activation energies of 8.9 and 19.6 kcal/mol. The two phases exhibited kinetic solvent isotope effects of 2.5 and 2.3. Proton inventory plots of each reaction phase exhibited extreme curvature that could not be fit to models for one- or multiple-proton transfers in the transition state. Each did fit well to a model for two alternative pathways for proton transfer, each involving multiple protons. In each case the experimentally determined fractionation factors were consistent with one of the pathways involving tunneling. The percent of the reaction that involved the tunneling pathway differed for the two reaction phases. Using the crystal structure of MauG it was possible to propose proton-transfer pathways consistent with the experimental data using water molecules and amino acid side chains in the distal pocket of the high-spin heme.

A T67A mutation in the proximal pocket of the high-spin heme of MauG stabilizes formation of a mixed-valent FeII/FeIII state and enhances charge resonance stabilization of the bis-FeIV state

The diheme enzyme MauG catalyzes a six-electron oxidation required for posttranslational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. One heme is low-spin with ligands provided by His205 and Tyr294, and the other is high-spin with a ligand provided by His35. The side chain methyl groups of Thr67 and Leu70 are positioned at a distance of 3.4Å on either side of His35, maintaining a hydrophobic environment in the proximal pocket of the high-spin heme and restricting the movement of this ligand. Mutation of Thr67 to Ala in the proximal pocket of the high-spin heme prevented reduction of the low-spin heme by dithionite, yielding a mixed-valent state. The mutation also enhanced the stabilization of the charge-resonance-transition of the high-valent bis-FeIV state that is generated by addition of H2O2. The rates of electron transfer from TTQ biosynthetic intermediates to the high-valent form of T67A MauG were similar to that of wild-type MauG. These results are compared to those previously reported for mutation of residues in the distal pocket of the high-spin heme that also affected the redox properties and charge resonance transition stabilization of the high-valent state of the hemes. However, given the position of residue 67, the structure of the variant protein and the physical nature of the T67A mutation, the basis for the effects of the T67A mutation must be different from those of the mutations of the residues in the distal heme pocket.

Probing bis-Fe(IV) MauG: experimental evidence for the long-range charge-resonance model

The biosynthesis of tryptophan tryptophylquinone, a protein-derived cofactor, involves a long-range reaction mediated by a bis-Fe(IV) intermediate of a diheme enzyme, MauG. Recently, a unique charge-resonance (CR) phenomenon was discovered in this intermediate, and a biological, long-distance CR model was proposed. This model suggests that the chemical nature of the bis-Fe(IV) species is not as simple as it appears; rather, it is composed of a collection of resonance structures in a dynamic equilibrium. Here, we experimentally evaluated the proposed CR model by introducing small molecules to, and measuring the temperature dependence of, bis-Fe(IV) MauG. Spectroscopic evidence was presented to demonstrate that the selected compounds increase the decay rate of the bis-Fe(IV) species by disrupting the equilibrium of the resonance structures that constitutes the proposed CR model. The results support this new CR model and bring a fresh concept to the classical CR theory.

Bis-Fe(IV): nature's sniper for long-range oxidation

Iron-dependent enzymes are prevalent in nature and participate in a wide range of biological redox activities. Frequently, high-valence iron intermediates are involved in the catalytic events of iron-dependent enzymes, especially when the activation of peroxide or molecular oxygen is involved. Building on the fundamental framework of iron-oxygen chemistry, these reactive intermediates constantly attract significant attention from the enzymology community. During the past few decades, tremendous efforts from a number of laboratories have been dedicated to the capture and characterization of these intermediates to improve mechanistic understandings. In 2008, an unprecedented bis-Fe(IV) intermediate was reported in a c-type diheme enzyme, MauG, which is involved in the maturation of a tryptophan tryptophylquinone cofactor of methylamine dehydrogenase. This intermediate, although chemically equivalent to well-characterized high-valence iron intermediates, such as compound I, compound ES, and intermediate Q in methane monooxygenase, as well as the hypothetical Fe(V) species in Rieske non-heme oxygenases, is orders of magnitude more stable than these other high-valence species in the absence of its primary substrate. It has recently been discovered that the bis-Fe(IV) intermediate exhibits a unique near-IR absorption feature which has been attributed to a novel charge-resonance phenomenon. This review compares the properties of MauG with structurally related enzymes, summarizes the current knowledge of this new high-valence iron intermediate, including its chemical origin and structural basis, explores the formation and consequences of charge resonance, and recounts the long-range catalytic mechanism in which bis-Fe(IV) participates. Biological strategies for storing oxidizing equivalents with iron ions are also discussed.

Site-directed mutagenesis of Gln103 reveals the influence of this residue on the redox properties and stability of MauG

The diheme enzyme MauG catalyzes a six-electron oxidation that is required for the posttranslational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived cofactor, tryptophan tryptophylquinone (TTQ). Crystallographic and computational studies have implicated Gln103 in stabilizing the Fe^IV=O moiety of the bis-Fe^IV state by hydrogen bonding. The role of Gln103 was probed by site-directed mutagenesis. Q103L and Q103E mutations resulted in no expression and very little expression of the protein, respectively. Q103A MauG exhibited oxidative damage when isolated. Q103N MauG was isolated at levels comparable to that of wild-type MauG and exhibited normal activity in catalyzing the biosynthesis of TTQ from preMADH. The crystal structure of the Q103N MauG–preMADH complex suggests that a water may mediate hydrogen bonding between the shorter Asn103 side chain and the Fe^IV=O moiety. The Q103N mutation caused the two redox potentials associated with the diferric/diferrous redox couple to become less negative, although the redox cooperativity of the hemes of MauG was retained. Upon addition of H₂O₂, Q103N MauG exhibits changes in the absorbance spectrum in the Soret and near-IR regions consistent with formation of the bis-Fe^IV redox state. However, the rate of spontaneous return of the spectrum in the Soret region was 4.5-fold greater for Q103N MauG than for wild-type MauG. In contrast, the rate of spontaneous decay of the absorbance at 950 nm, which is associated with charge-resonance stabilization of the high-valence state, was similar for wild-type MauG and Q103N MauG. This suggests that as a consequence of the mutation a different distribution of resonance structures stabilizes the bis-Fe^IV state. These results demonstrate that subtle changes in the structure of the side chain of residue 103 can significantly affect the overall protein stability of MauG and alter the redox properties of the hemes.

MauG, a diheme enzyme that catalyzes tryptophan tryptophylquinone biosynthesis by remote catalysis

MauG contains two c-type hemes with atypical physical and catalytic properties. While most c-type cytochromes function simply as electron transfer mediators, MauG catalyzes the completion of tryptophan tryptophylquinone (TTQ)(1) biosynthesis within a precursor protein of methylamine dehydrogenase. This posttranslational modification is a six-electron oxidation that requires crosslinking of two Trp residues, oxygenation of a Trp residue and oxidation of the resulting quinol to TTQ. These reactions proceed via a bis-Fe(IV) state in which one heme is present as Fe(IV)O and the other is Fe(IV) with axial heme ligands provided by His and Tyr side chains. Catalysis does not involve direct contact between the protein substrate and either heme of MauG. Instead it is accomplished by remote catalysis using a hole hopping mechanism of electron transfer in which Trp residues of MauG are reversibly oxidized. In this process, long range electron transfer is coupled to the radical mediated chemical reactions that are required for TTQ biosynthesis.

Mutation of Trp(93) of MauG to tyrosine causes loss of bound Ca(2+) and alters the kinetic mechanism of tryptophan tryptophylquinone cofactor biosynthesis

The dihaem enzyme MauG catalyses a six-electron oxidation required for post-translational modification of preMADH (precursor of methylamine dehydrogenase) to complete the biosynthesis of its TTQ (tryptophan tryptophylquinone) cofactor. Trp93 of MauG is positioned midway between its two haems, and in close proximity to a Ca2+ that is critical for MauG function. Mutation of Trp93 to tyrosine caused loss of bound Ca2+ and changes in spectral features similar to those observed after removal of Ca2+ from WT (wild-type) MauG. However, whereas Ca2+-depleted WT MauG is inactive, W93Y MauG exhibited TTQ biosynthesis activity. The rate of TTQ biosynthesis from preMADH was much lower than that of WT MauG and exhibited highly unusual kinetic behaviour. The steady-state reaction exhibited a long lag phase, the duration of which was dependent on the concentration of preMADH. The accumulation of reaction intermediates, including a diradical species of preMADH and quinol MADH (methylamine dehydrogenase), was detected during this pre-steady-state phase. In contrast, steady-state oxidation of quinol MADH to TTQ, the final step of TTQ biosynthesis, exhibited no lag phase. A kinetic model is presented to explain the long pre-steady-state phase of the reaction of W93Y MauG, and the role of this conserved tryptophan residue in MauG and related dihaem enzymes is discussed.

Oxidative damage in MauG: implications for the control of high-valent iron species and radical propagation pathways

The di-heme enzyme MauG catalyzes the oxidative biosynthesis of a tryptophan tryptophylquinone cofactor on a precursor of the enzyme methylamine dehydrogenase (preMADH). Reaction of H2O2 with the diferric form of MauG, or reaction of O2 with diferrous MauG, forms the catalytic intermediate known as bis-Fe(IV), which acts as the key oxidant during turnover. The site of substrate oxidation is more than 40 Å from the high-spin heme iron where H2O2 initially reacts, and catalysis relies on radical hopping through an interfacial residue, Trp199 of MauG. In the absence of preMADH, the bis-Fe(IV) intermediate is remarkably stable, but repeated exposure to H2O2 results in suicide inactivation. Using mass spectrometry, we show that this process involves the oxidation of three Met residues (108, 114, and 116) near the high-spin heme through ancillary electron transfer pathways engaged in the absence of substrate. The mutation of a conserved Pro107 in the distal pocket of the high-spin heme results in a dramatic increase in the level of oxidation of these Met residues. These results illustrate structural mechanisms by which MauG controls reaction with its high-valent heme cofactor and limits uncontrolled oxidation of protein residues and loss of catalytic activity. The conservation of Met residues near the high-spin heme among MauG homologues from different organisms suggests that eventual deactivation of MauG may function in a biological context. That is, methionine oxidation may represent a protective mechanism that prevents the generation of reactive oxygen species by MauG in the absence of preMADH.

Posttranslational biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone

Methylamine dehydrogenase (MADH) catalyzes the oxidative deamination of methylamine to formaldehyde and ammonia. Tryptophan tryptophylquinone (TTQ) is the protein-derived cofactor of MADH required for this catalytic activity. TTQ is biosynthesized through the posttranslational modification of two tryptophan residues within MADH, during which the indole rings of two tryptophan side chains are cross-linked and two oxygen atoms are inserted into one of the indole rings. MauG is a c-type diheme enzyme that catalyzes the final three reactions in TTQ formation. In total, this is a six-electron oxidation process requiring three cycles of MauG-dependent two-electron oxidation events using either H2O2 or O2. The MauG redox form responsible for the catalytic activity is an unprecedented bis-Fe(IV) species. The amino acids of MADH that are modified are ≈ 40 Å from the site where MauG binds oxygen, and the reaction proceeds by a hole hopping electron transfer mechanism. This review addresses these highly unusual aspects of the long-range catalytic reaction mediated by MauG.

On the pathways of biologically relevant diatomic gases through proteins. Dioxygen and heme oxygenase from the perspective of molecular dynamics

This work deals with dioxygen (O2 ) binding sites and pathways through inducible human heme oxygenase (HO-1). The experimentally known distal binding site 1, and sites 2-3 above it, could be reproduced by means of non-deterministic random-acceleration molecular-dynamics (RAMD) simulations. In addition, RAMD revealed the proximal binding site 5, a deeply-seated binding site 4, which lies behind heme, as well as a few gates communicating with the external medium. In getting from site 1 to the main gate, which lies on the protein front opposed to site 4, O2 follows chiefly the shortest direct pathway. Less frequently, O2 visits intermediate sites 2, 4, or 5 along longer pathways. A similarity between HO-1, myoglobin, and cytoglobin in using, for diatomic gas delivery, the direct shortest pathway from the heme center to the surrounding medium, is emphasized. Otherwise, comparing other proteins and diatomic gases, each system reveals its peculiarities as to sites, gates, and pathways. Thus, relating these properties to the physiological functions of the proteins remains in general a challenge for future studies.

Carboxyl group of Glu113 is required for stabilization of the diferrous and bis-Fe(IV) states of MauG

The diheme enzyme MauG catalyzes a six-electron oxidation required for post-translational modification of a precursor of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Crystallographic studies have implicated Glu113 in the formation of the bis-Fe(IV) state of MauG, in which one heme is Fe(IV)═O and the other is Fe(IV) with His-Tyr axial ligation. An E113Q mutation had no effect on the structure of MauG but significantly altered its redox properties. E113Q MauG could not be converted to the diferrous state by reduction with dithionite but was only reduced to a mixed valence Fe(II)/Fe(III) state, which is never observed in wild-type (WT) MauG. Addition of H2O2 to E113Q MauG generated a high valence state that formed more slowly and was less stable than the bis-Fe(IV) state of WT MauG. E113Q MauG exhibited no detectable TTQ biosynthesis activity in a steady-state assay with preMADH as the substrate. It did catalyze the steady-state oxidation of quinol MADH to the quinone, but 1000-fold less efficiently than WT MauG. Addition of H2O2 to a crystal of the E113Q MauG-preMADH complex resulted in partial synthesis of TTQ. Extended exposure of these crystals to H2O2 resulted in hydroxylation of Pro107 in the distal pocket of the high-spin heme. It is concluded that the loss of the carboxylic group of Glu113 disrupts the redox cooperativity between hemes that allows rapid formation of the diferrous state and alters the distribution of high-valence species that participate in charge-resonance stabilization of the bis-Fe(IV) redox state.

Tryptophan-mediated charge-resonance stabilization in the bis-Fe(IV) redox state of MauG

The diheme enzyme MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor protein to generate a tryptophan tryptophylquinone cofactor. The MauG-catalyzed reaction proceeds via a bis-Fe(IV) intermediate in which one heme is present as Fe(IV)=O and the other as Fe(IV) with axial histidine and tyrosine ligation. Herein, a unique near-infrared absorption feature exhibited specifically in bis-Fe(IV) MauG is described, and evidence is presented that it results from a charge-resonance-transition phenomenon. As the two hemes are physically separated by 14.5 Å, a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes is reversibly oxidized and reduced to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(IV) MauG. Analysis of the MauG structure reveals that electron transfer via this mechanism is rapid enough to enable a charge-resonance stabilization of the bis-Fe(IV) state without direct contact between the hemes. The finding of the charge-resonance-transition phenomenon explains why the bis-Fe(IV) intermediate is stabilized in MauG and does not permanently oxidize its own aromatic residues.

A Trp199Glu MauG variant reveals a role for Trp199 interactions with pre-methylamine dehydrogenase during tryptophan tryptophylquinone biosynthesis

MauG catalyzes posttranslational modifications of a methylamine dehydrogenase precursor (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. Trp199 is present at the site of interaction between MauG and preMADH and is critical to this process as it mediates hole hopping during the inter-protein electron transfer that is required for catalysis. Trp199 was converted to Glu and the structure and reactivity of the W199E/preMADH complex were characterized. The results reveal that the nature of residue 199 is also important for productive complex formation between preMADH and MauG.

MauG: a di-heme enzyme required for methylamine dehydrogenase maturation

Methylamine dehydrogenase (MADH) requires the cofactor tryptophan tryptophylquinone (TTQ) for activity. TTQ is a posttranslational modification that results from an 8-electron oxidation of two specific tryptophans in the MADH β-subunit. The final 6-electron oxidation is catalyzed by an unusual c-type di-heme enzyme, MauG. The di-ferric enzyme can react with H(2)O(2), but atypically for c-type hemes the di-ferrous enzyme can react with O(2) as well. In both cases, an unprecedented bis-Fe(IV) redox state is formed, composed of a ferryl heme (Fe(IV)=O) with the second heme as Fe(IV) stabilized by His-Tyr axial ligation. Bis-Fe(IV) MauG acts as a potent 2-electron oxidant. Catalysis is long-range and requires a hole hopping electron transfer mechanism. This review highlights the current knowledge and focus of research into this fascinating system.

On CO egress from, and re-uptake by, the enzyme MauG, as a mimic of the acquisition of oxidizing agents by the pre-MADH_MauG system. A molecular mechanics approach

In this work, by applying a non-deterministic, randomly-oriented minimal force to the dissociated CO ligand of the MauG-CO system, the molecular-dynamics (MD) behavior of this system could be quickly unraveled. It turned out that CO has no marked directional egress from the high-spin c-heme iron distal pocket. Rather, CO is able to exploit all interstices created during the protein fluctuations. Nonetheless, no steady route toward the surrounding solvent was ever observed: CO jumped first into other binding pockets before being able to escape the protein. In a few cases, on hitting the surrounding H(2)O molecules, CO was observed to reverse direction, re-entering the protein. A contention that conformational inversion of the P107 ring provides a gate to the iron ion is not supported by the present simulations.

Effects of the loss of the axial tyrosine ligand of the low-spin heme of MauG on its physical properties and reactivity

MauG catalyzes posttranslational modifications of methylamine dehydrogenase to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. MauG possesses a five-coordinate high-spin and a six-coordinate low-spin ferric heme, the latter with His-Tyr ligation. Replacement of this tyrosine with lysine generates a MauG variant with only high-spin ferric heme and altered spectroscopic and redox properties. Y294K MauG cannot stabilize the bis-Fe(IV) redox state required for TTQ biosynthesis but instead forms a compound I-like species on reaction with peroxide. The results clarify the role of Tyr ligation of the five-coordinate heme in determining the physical and redox properties and reactivity of MauG.

Characterization of electron tunneling and hole hopping reactions between different forms of MauG and methylamine dehydrogenase within a natural protein complex

Respiration, photosynthesis, and metabolism require the transfer of electrons through and between proteins over relatively long distances. It is critical that this electron transfer (ET) occur with specificity to avoid cellular damage, and at a rate that is sufficient to support the biological activity. A multistep hole hopping mechanism could, in principle, enhance the efficiency of long-range ET through proteins as it does in organic semiconductors. To explore this possibility, two different ET reactions that occur over the same distance within the protein complex of the diheme enzyme MauG and different forms of methylamine dehydrogenase (MADH) were subjected to kinetic and thermodynamic analysis. An ET mechanism of single-step direct electron tunneling from diferrous MauG to the quinone form of MADH is consistent with the data. In contrast, the biosynthetic ET from preMADH, which contains incompletely synthesized tryptophan tryptophylquinone, to the bis-Fe(IV) form of MauG is best described by a two-step hole hopping mechanism. Experimentally determined ET distances matched the distances determined from the crystal structure that would be expected for single-step tunneling and multistep hopping. Experimentally determined relative values of electronic coupling (H(AB)) for the two reactions correlated well with the relative H(AB) values predicted from computational analysis of the structure. The rate of the hopping-mediated ET reaction is also 10-fold greater than that of the single-step tunneling reaction despite a smaller overall driving force for the hopping-mediated ET reaction. These data provide insight into how the intervening protein matrix and redox potentials of the electron donor and acceptor determine whether the ET reaction proceeds via single-step tunneling or multistep hopping.