Mechanism of Aromatic Hydroxylation by an Activated Fe<sup>IV</sup>O Core in Tetrahydrobiopterin‐Dependent Hydroxylases

Bassan, Arianna; Blomberg, Margareta R. A.; Siegbahn, Per E. M.

doi:10.1002/chem.200304768

Cited by 72 publications

(102 citation statements)

References 51 publications

(80 reference statements)

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“…Cleavage of the dioxygen molecule could then give rise to simultaneous hydroxylated L-Phe and BH 4 , in agreement with the finding that both substrates are being released at the same time ( 1 ). There are some very important differences observed in our DFT results as compared to the study by Bassan et al (21). Although only one Fe-O distance is reported in the study by these authors, their septet dioxygen complex appears to be significantly more asymmetric (thus endon) as judged from the figures given in the paper.…”

Section: Implications For the Reactioncontrasting

confidence: 90%

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

Teigen

Jensen²,

Martı́nez³

2005

Pteridines

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Phenylalanine hydroxylase (PAH) is a non-heme iron and tetrahydrobiopterin-dependent enzyme that catalyzes the hydroxylation of L-phenylalanine to L-tyrosine using dioxygen as additional substrate. The cofactor tetrahydrobiopterin accepts one of the oxygen atoms of dioxygen during catalysis and also seems to be involved in prereduction of the active site iron from the ferric to the activated ferrous form. Structures of the truncated form of PAH in complex with substrate and cofactor are available, but the oxygen binding site and the actual mechanism of electron transfer are uncertain. It is believed that dioxygen binds directly to the metal, where it is activated, and several reaction mechanisms involving end-on binding of 0 2 have been proposed based on both experimental studies and quantum mechanical calculations. However, in this work we aimed to investigate the possibility of side-on binding of dioxygen to the iron. Furthermore, NMR and recent high-resolution crystallographic studies also place the cofactor in closer proximity to the iron, challenging the mechanistic conclusions from earlier crystallographic and computational studies. In this paper we report preliminary results from a density functional theory (DFT) study of the coordination of dioxygen to a structural model of PAH based on a recent crystallographic structure. These results are compared with existing computational and experimental data and their implications for the mechanism of the PAH-reaction are discussed. Particular attention is paid to the binding-mode of dioxygen and the iron-cofactor distance.

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Section: Implications For the Reactioncontrasting

confidence: 90%

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

Teigen

Jensen²,

Martı́nez³

2005

Pteridines

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“…The conversion that VioE facilitates involves oxidative chemistry, but the most unusual aspect of the reaction, a 1,2 shift of an indole ring, is a reaction that is not known to be catalyzed by other characterized enzymes. Other shift reactions have been characterized (51)(52)(53)(54), and each of the responsible enzymes utilize iron as a cofactor (as a heme or non-heme iron), whereas VioE requires no cofactors or metals. The lack of cofactors of VioE might mean that this indole shift reaction is mechanistically more facile than other observed, enzyme-mediated shift reactions.…”

Section: Discussionmentioning

confidence: 99%

The Violacein Biosynthetic Enzyme VioE Shares a Fold with Lipoprotein Transporter Proteins

Ryan

Balibar

Turo

et al. 2008

Journal of Biological Chemistry

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VioE, an unusual enzyme with no characterized homologues, plays a key role in the biosynthesis of violacein, a purple pigment with antibacterial and cytotoxic properties. Without bound cofactors or metals, VioE, from the bacterium Chromobacterium violaceum, mediates a 1,2 shift of an indole ring and oxidative chemistry to generate prodeoxyviolacein, a precursor to violacein. Our 1.21 Å resolution structure of VioE shows that the enzyme shares a core fold previously described for lipoprotein transporter proteins LolA and LolB. For both LolB and VioE, a bound polyethylene glycol molecule suggests the location of the binding and/or active site of the protein. Mutations of residues near the bound polyethylene glycol molecule in VioE have identified the active site and five residues important for binding or catalysis. This structural and mutagenesis study suggests that VioE acts as a catalytic chaperone, using a fold previously associated with lipoprotein transporters to catalyze the production of its prodeoxyviolacein product.Violacein 5 (Fig. 1) is a purple pigment that gives tropical bacterium Chromobacterium violaceum its characteristic coloring (1-5). Violacein 5 has potential medical applications as an antibacterial (6 -8), anti-tryptanocidal (8 -10), anti-ulcerogenic (11), and anti-cancer drug (8,(12)(13)(14)(15)(16)(17). The molecule, made of L-tryptophan and molecular oxygen (18 -21), was originally thought to be synthesized by the action of just four enzymes VioABCD (22, 23), but an additional open reading frame (24), also in the operon, was shown to encode the enzyme VioE (25, 26). A 22-kDa protein with few homologues based on sequence analysis, VioE is required for formation of prodeoxyviolacein 3, a precursor to violacein 5 (25-27). Without the presence of VioE, a shunt product, chromopyrrolic acid 4, is produced, rather than 3 (25-27). VioE is therefore required for the 1,2 shift of the indole ring and subsequent chemistry that gives violacein 5 its distinctive architecture. Surprisingly, despite carrying out this chemistry, VioE lacks any bound cofactors or metal ions (26).Using genetic and biochemical analysis, an enzymatic pathway for violacein 5 production has been determined ( Fig. 1), and intermediates have been identified (26), with the major exception that the substrate of VioE (product of VioB) is still unknown. It has, however, been established that the product of VioB is the substrate for VioE; when VioA and VioB are incubated in a reaction mixture with the VioA substrate L-tryptophan for 10 min, passed through a 10-kDa filter (removing VioA and VioB), and then incubated with VioE, prodeoxyviolacein 3 is produced (26). By contrast, in the absence of VioE, no prodeoxyviolacein 3 is made (26). This result reveals three things: first, the VioB product is a small molecule that can pass through a 10-kDa filter; second, no interaction between VioB and VioE is necessary for product formation; and third, VioE is a catalyst. The small molecule passed through the filter (the product of VioB and substra...

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“…Alternatively, the Fe(IV)=O species may abstract one electron to yield an intermediate ring radical and an Fe(III)-oxo species. Subsequent reaction of the oxo species with the substrate radical would yield the same arenium cation as the first mechanism 76 . The second mechanism is more in line with reactions of high valent iron-oxo in other systems, but the final intermediate in either case would collapse to yield the hydroxylated product and leave the Fe(II) resting state of the enzyme.…”

Section: Tetrahydropterin-containing Oxygenasesmentioning

confidence: 91%

Versatility of biological non-heme Fe(II) centers in oxygen activation reactions

2008

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Oxidase and oxygenase enzymes allow the use of relatively unreactive O 2 in biochemical reactions. Many of the mechanistic strategies employed in nature for this key reaction are represented within the 2-His-1-carboxylate facial triad family of non-heme Fe(II) containing enzymes. The open face of the metal coordination sphere opposite the three endogenous ligands participates directly in the reaction chemistry. Here, data from several studies are presented showing that reductive O 2 activation within this family is initiated by substrate (and in some cases co-substrate or cofactor) binding, which then allows coordination of O 2 to the metal. From this starting point, both the O 2 activation process and the reactions with substrates diverge broadly. The reactive species formed in these reactions have been proposed to encompass four oxidation states of iron and all forms of reduced O 2 as well as several of the reactive oxygen species that derive from O-O bond cleavage.Dioxygen serves at least three quite different roles that profoundly impact aerobic life. The most commonly appreciated role is to serve as the terminal electron acceptor in processes such as oxidative phosphorylation that yield the central energy-rich molecules used throughout metabolism. The second, no less important role is to serve as the source for many of the oxygen atoms found in the essential molecules of biological systems such as steroid hormones, aromatic amino acids, neurotransmitters, signalling molecules, and regulatory factors 1 . Also, processes operating on a global scale, such as the recovery of the enormous quantities of carbon sequestered as lignin in plant life or the oxidation of the billions of tons of methane generated by anaerobes before it can enter the atmosphere, also involve oxygen incorporation from O 2 2 ,3 . On a more local scale, biodegradation of both aliphatic and aromatic toxic compounds often begins with the incorporation of oxygen 4 . The third, and least appreciated role played by dioxygen in aerobic organisms involves neither energy conversion nor oxygen incorporation. Rather, some enzymes can convert dioxygen to alternative forms that are, in effect, highly specialized reagents that are used to catalyze the synthesis of important biomolecules. An excellent example of the latter role for O 2 is the biosynthesis of penicillintype antibiotics 5,6 , which is discussed in more detail later in this review.Dioxygen is an attractive reagent for use in a biological system because its high potential reactivity is held in check by its molecular structure. The triplet ground state of O 2 that results from the presence of two unpaired electrons in degenerate molecular orbitals makes the direct reaction with singlet molecules, the spin-paired state of most potential reaction partners, a forbidden process 7 . The central question that has faced chemists and biochemists for the half Competing Interests StatementThe authors declare no competing financial interests. Of course, the answers to this question are of fundam...

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Mechanism of Aromatic Hydroxylation by an Activated Fe^IVO Core in Tetrahydrobiopterin‐Dependent Hydroxylases

Cited by 72 publications

References 51 publications

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

The Violacein Biosynthetic Enzyme VioE Shares a Fold with Lipoprotein Transporter Proteins

Versatility of biological non-heme Fe(II) centers in oxygen activation reactions

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Mechanism of Aromatic Hydroxylation by an Activated FeIVO Core in Tetrahydrobiopterin‐Dependent Hydroxylases

Cited by 72 publications

References 51 publications

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

The Reaction Mechanism of Phenylalanine Hydroxylase. – A Question of Coordination

The Violacein Biosynthetic Enzyme VioE Shares a Fold with Lipoprotein Transporter Proteins

Versatility of biological non-heme Fe(II) centers in oxygen activation reactions

Contact Info

Product

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Mechanism of Aromatic Hydroxylation by an Activated Fe^IVO Core in Tetrahydrobiopterin‐Dependent Hydroxylases