EPR and Mössbauer studies of protocatechuate 4,5-dioxygenase. Characterization of a new Fe2+ environment

Reaction of NO with the reduced R2 protein of ribonucleotide reductase from Escherichia coli

The active R2 protein of ribonucleotide reductase from Escherichia coli contains a catalytically essential tyrosine radical at position 122 (Tyr122.) that is formed during the reaction of dioxygen with the nearby diiron(II) center. To gain insight into the mode of dioxygen binding, the reaction of the O2 analog NO with the diiron(II) centers of R2red has been investigated by spectroscopic methods. R2red reacts with NO to form an adduct with visible absorption features at 450 and 620 nm and Mössbauer parameters (delta = 0.75 mm/s, delta EQ = -2.13 and -1.73 mm/s) typical of those observed for S = 3/2 [FeNO]7 complexes of other non-heme iron proteins. However, unlike other non-heme [FeNO]7 complexes, this adduct is EPR silent. Our Mössbauer studies show that each iron site of R2red binds one NO to form local S = 3/2 [FeNO]7 centers which then couple antiferromagnetically (J approximately 5 cm-1, H = JS1.S2) to afford an [FeNO]2 center (77% of total iron). This [FeNO]2 center decomposes with a first-order rate constant of 0.013 min-1 to form R2met, accompanied by the release of N2O. These observations suggest that both iron(II) ions of the two diiron(II) centers of R2red have available sites for NO binding, in agreement with the crystallographic results on R2red, and that the bound NO molecules are sufficiently close to each other to permit N-N bond formation to produce N2O. These observations support the proposal that dioxygen binding may also involve both metal ions of the diiron(II) center to form a (mu-1,1-, or mu-1,2-peroxo)-diiron(III) center. This observed reactivity of R2red with NO may contribute to the in vivo inhibition of ribonucleotide reductase by NO.

Substrate, substrate analogue, and inhibitor interactions with the ferrous active site of catechol 2,3-dioxygenase monitored through XAS studies

The interactions of catechol (substrate), 2-hydroxy-pyridine-N-oxide (substrate analogue), and 2-bromophenol (inhibitor) with the extradiol cleaving catechol-2,3-dioxygenase from Pseudomonas putida mt-2 have been monitored through X-ray absorption spectroscopy (XAS). The analysis of the data provides details about the mode of coordination of the substrate and of the inhibitors to the active site of the enzyme.

Aliphatic and aromatic inhibitors binding to the active site of catechol 2,3-dioxygenase from Pseudomonas putida mt-2

The interaction of different classes of inhibitors with the extradiol cleaving catechol 2,3-dioxygenase from Pseudomonas putida mt-2 was monitored by longitudinal and transverse proton relaxation measurements as well as by kinetic activity studies in order to characterize the type of interaction of such inhibitors with the active site of the enzyme. The average distances of the inhibitors from the catalytic iron(II) ion have been estimated from the 1H longitudinal relaxation rates. Phenols and aliphatic ketones appear to be coordinated to the iron(II) ion in the active site.

Purification and characterization of protocatechuate 2,3-dioxygenase from Bacillus macerans: a new extradiol catecholic dioxygenase

Protocatechuate 2,3-dioxygenase (2,3-PCD) from Bacillus macerans JJ1b has been purified to homogeneity for the first time. The enzyme catalyzes proximal extradiol ring cleavage of protocatechuate (PCA) with the attendant incorporation of both atoms of oxygen from O2. The holoenzyme has a mass of 143 +/- 7 kDa as determined by ultracentrifugation and other techniques. It is composed of four apparently identical subunits with M(r)s of 35,500, each containing one iron atom. Mössbauer spectroscopy of 57Fe-enriched enzyme showed that the irons are indistinguishable and are high spin (S = 2) Fe2+ in both the uncomplexed and substrate-bound enzyme. However, the quadrupole splitting, delta EQ, and isomer shift, delta, of the Mössbauer spectrum changed from delta EQ = 2.57 mm/s and delta = 1.29 mm/s to delta EQ = 2.73 mm/s and delta = 1.19 mm/s upon PCA binding to the enzyme, showing that the iron environment is altered when substrate is present. The enzyme was also found to bind variable and substoichiometric amounts of Mn2+, but this metal could be removed without loss of activity or stability. The inherently electron paramagnetic resonance (EPR)-silent Fe2+ of the enzyme reversibly bound nitric oxide to produce an EPR-active species (g = 4.11, 3.95; S = 3/2). The specific activity of the enzyme was found to be correlated with the amount of the S = 3/2 species formed, showing that activity is dependent on Fe2+. Anaerobic addition of substrates to the enzyme-nitric oxide complex significantly altered the EPR spectrum, suggesting that substrates bind to or near the iron. The enzyme was inactivated by reagents that oxidize the Fe2+, such as H2O2 and K3FE(CN)6; full activity was restored after reduction of the iron by ascorbate. Steady-state kinetic data were found to be consistent with an ordered bi-uni mechanism in which the organic substrate must add to 2,3-PCD before O2. The enzyme has the broadest substrate range of any of the well-studied catecholic dioxygenases. All substrates have vicinal hydroxyl groups on the aromatic ring except 4-NH2-3-hydroxybenzoate. This is the first substrate lacking vicinal hydroxyl groups reported for catecholic extradiol dioxygenases. 2,3-PCD is the final member of the PCA dioxygenase family to be purified. It is compared with other members of this family as well as other catecholic dioxygenases.

The initial characterization of the iron environment in lipoxygenase by Mössbauer spectroscopy

The incorporation of 57Fe into two lipoxygenase isoenzymes from soybeans has been achieved making possible the first observations of the iron environment in these proteins using Mössbauer spectroscopy. Immature soybean seeds were grown in tissue culture medium supplied with 57Fe. The iron in the active lipoxygenases that were isolated from the cultured seeds was readily detected in Mössbauer measurements. It was unequivocally demonstrated that the native enzyme contains high-spin Fe(II). Based on the sign of the electric field gradient, the most likely ligand sphere for the iron in native lipoxygenase consists of oxygen and nitrogen ligands in a roughly octahedral field of symmetry. It was possible to detect Mössbauer signals in highly concentrated samples of native lipoxygenases containing 57Fe at natural abundance. The spectra obtained for enriched and natural abundance native enzyme had the same high-spin Fe(II) Mössbauer parameters. This confirmed that the environment of the iron in enzymes isolated from cultured seeds and dry soybeans were the same. The Mössbauer spectra (4.2-250 K) for samples of both isoenzymes after oxidation of the iron in native enzyme by the product of lipoxygenase catalysis were extremely broad (20 mm/s) with no obvious narrow resonance lines. This was the result of the existence of paramagnetically broadened spectra for such samples even at relatively high temperature as evidenced by the appropriate EPR signal. A small molecule containing an iron site sharing many of these Mössbauer and electron paramagnetic resonance properties with lipoxygenase was identified: Fe(II)/(III).diethylenetriaminepentaacetic acid.

Molecular cloning of the protocatechuate 4,5-dioxygenase genes of Pseudomonas paucimobilis

We determined the nucleotide sequence of a 1.9-kilobase fragment of Pseudomonas paucimobilis SYK6 chromosomal DNA that included genes encoding protocatechuate 4,5-dioxygenase, the enzyme responsible for the aromatic ring fission of protocatechuate. Two open reading frames of 417 and 906 base pairs were found that had no homology with previously reported sequences, including those encoding protocatechuate 3,4-dioxygenase. Since both open reading frames were indispensable for the enzyme activity, they should encode the subunits of protocatechuate 4,5-dioxygenase. We named these genes ligA and ligB. Protocatechuate 4,5-dioxygenase was efficiently expressed in Escherichia coli with the aid of the lac promoter, and the polypeptides of the ligA and ligB gene products were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino acid sequencing.

Microbial enzymes for oxidation of organic molecules

Enzymatic systems employed by microorganisms for oxidative transformation of various organic molecules include laccases, ligninases, tyrosinases, monooxygenases, and dioxygenases. Reactions performed by these enzymes play a significant role in maintaining the global carbon cycle through either transformation or complete mineralization of organic molecules. Additionally, oxidative enzymes are instrumental in modification or degradation of the ever-increasing man-made chemicals constantly released into our environment. Due to their inherent stereo- and regioselectivity and high efficiency, oxidative enzymes have attracted attention as potential biocatalysts for various biotechnological processes. Successful commercial application of these enzymes will be possible through employing new methodologies, such as use of organic solvents in the reaction mixtures, immobilization of either the intact microorganisms or isolated enzyme preparations on various supports, and genetic engineering technology.

Purification of a high specific activity methane monooxygenase hydroxylase component from a type II methanotroph

The purification of the hydroxylase component of a 3 component methane monooxygenase from the type II methanotroph Methylosinus trichosporium OB3b is reported. The enzyme (240 kDa) has an (alpha beta gamma)2 subunit structure as observed for hydroxylases isolated from other Type I and Type II methanotrophs, but it exhibits a 5 to 10 fold higher specific activity and is isolated in 2 to 10 fold higher yield. EPR and Mössbauer spectra of the hydroxylase show that it contains a coupled iron center containing an even number of iron atoms. The spectra are similar to those of proteins known to contain oxo-bridged binuclear iron centers. The presence of such a center is unprecedented in a monooxygenase and suggests that a novel mechanism is utilized.

Mössbauer investigation of the cofactor iron of putidamonooxin

Mononuclear non-heme cofactor iron of putidamonooxin has been investigated in the binary oxidized 'enzyme X substrate' complex and in the ternary 'enzyme X substrate X NO' complex via Mössbauer spectroscopy. The experimental spectra were analyzed on the basis of the spin-Hamiltonian formalism. The resulting fine and hyperfine structure parameters are compared with literature values of similar compounds. From this comparison we conclude that in the binary complex (reduced and oxidized) the mononuclear non-heme cofactor iron has a coordination number higher than four. Additionally, the cofactor iron shows remarkable spectral similarities with iron in protocatechuate 3,4-dioxygenase, though the catalytic properties of the iron sites in the two proteins are different. The data obtained form the ternary 'enzyme X substrate X NO' complex indicate that the cofactor iron (a) is in the ferric intermediate spin state (S = 3/2) and (b) is pentacoordinated, which means that upon NO binding to the reduced cofactor iron at least one ligand has to be released. Comparing our data with literature values suggests that the cofactor iron in the binary as well as in the ternary NO complex is not directly bound to a sulfur atom, though biochemical arguments seem to indicate the opposite.