21 research outputs found
Electronic Structure of the Ferryl Intermediate in the Ī±āKetoglutarate Dependent Non-Heme Iron Halogenase SyrB2: Contributions to H Atom Abstraction Reactivity
Low
temperature magnetic circular dichroism (LT MCD) spectroscopy
in combination with quantum-chemical calculations are used to define
the electronic structure associated with the geometric structure of
the Fe<sup>IV</sup>ī»O intermediate in SyrB2 that was previously
determined by nuclear resonance vibrational spectroscopy. These studies
elucidate key frontier molecular orbitals (FMOs) and their contribution
to H atom abstraction reactivity. The VT MCD spectra of the enzymatic <i>S</i> = 2 Fe<sup>IV</sup>ī»O intermediate with Br<sup>ā</sup> ligation contain information-rich features that largely
parallel the corresponding spectra of the <i>S</i> = 2 model
complex (TMG<sub>3</sub>tren)ĀFe<sup>IV</sup>ī»O (Srnec, M.;
Wong, S. D.; England, J.; Que, L. Jr.; Solomon, E. I. <i>Proc. Natl.
Acad. Sci. USA</i> <b>2012</b>, <i>109</i>, 14326ā14331).
However, quantitative differences are observed that correlate with
Ļ-anisotropy and oxo donor strength that perturb FMOs and affect
reactivity. Due to Ļ-anisotropy, the Fe<sup>IV</sup>ī»O
active site exhibits enhanced reactivity in the direction of the substrate
cavity that proceeds through a Ļ-channel that is controlled
by perpendicular orientation of the substrate CāH bond relative
to the halideāFe<sup>IV</sup>ī»O plane. Also, the increased
intrinsic reactivity of the SyrB2 intermediate relative to the ferryl
model complex is correlated to a higher oxyl character of the Fe<sup>IV</sup>ī»O at the transition states resulting from the weaker
ligand field of the halogenase
Evidence That the Ī² Subunit of <i>Chlamydia trachomatis</i> Ribonucleotide Reductase Is Active with the Manganese Ion of Its Manganese(IV)/Iron(III) Cofactor in Site 1
The reaction of a class I ribonucleotide reductase (RNR)
begins
when a cofactor in the Ī² subunit oxidizes a cysteine residue
ā¼35 Ć
away in the Ī± subunit, generating a thiyl
radical. In the class Ic enzyme from <i>Chlamydia trachomatis</i> (<i>Ct</i>), the cysteine oxidant is the Mn<sup>IV</sup> ion of a Mn<sup>IV</sup>/Fe<sup>III</sup> cluster, which assembles
in a reaction between O<sub>2</sub> and the Mn<sup>II</sup>/Fe<sup>II</sup> complex of Ī². The heterodinuclear nature of the cofactor
raises the question of which site, 1 or 2, contains the Mn<sup>IV</sup> ion. Because site 1 is closer to the conserved location of the cysteine-oxidizing
tyrosyl radical of class Ia and Ib RNRs, we suggested that the Mn<sup>IV</sup> ion most likely resides in this site (i.e., <sup>1</sup>Mn<sup>IV</sup>/<sup>2</sup>Fe<sup>III</sup>), but a subsequent computational
study favored its occupation of site 2 (<sup>1</sup>Fe<sup>III</sup>/<sup>2</sup>Mn<sup>IV</sup>). In this work, we have sought to resolve
the location of the Mn<sup>IV</sup> ion in <i>Ct</i> RNR-Ī²
by correlating X-ray crystallographic anomalous scattering intensities
with catalytic activity for samples of the protein reconstituted <i>in vitro</i> by two different procedures. In samples containing
primarily Mn<sup>IV</sup>/Fe<sup>III</sup> clusters, Mn preferentially
occupies site 1, but some anomalous scattering from site 2 is observed,
implying that both <sup>1</sup>Mn<sup>II</sup>/<sup>2</sup>Fe<sup>II</sup> and <sup>1</sup>Fe<sup>II</sup>/<sup>2</sup>Mn<sup>II</sup> complexes are competent to react with O<sub>2</sub> to produce the
corresponding oxidized states. However, with diminished Mn<sup>II</sup> loading in the reconstitution, there is no evidence for Mn occupancy
of site 2, and the greater activity of these ālow-Mnā
samples on a per-Mn basis implies that the <sup>1</sup>Mn<sup>IV</sup>/<sup>2</sup>Fe<sup>III</sup>-Ī² is at least the more active
of the two oxidized forms and may be the only active form
Vanadyl as a Stable Structural Mimic of Reactive Ferryl Intermediates in Mononuclear Nonheme-Iron Enzymes
The ironĀ(II)- and
2-(oxo)Āglutarate-dependent (Fe/2OG) oxygenases catalyze an array of
challenging transformations via a common ironĀ(IV)-oxo (ferryl) intermediate,
which in most cases abstracts hydrogen (Hā¢) from an aliphatic
carbon of the substrate. Although it has been shown that the relative
disposition of the FeāO and CāH bonds can control the
rate of Hā¢ abstraction and fate of the resultant substrate
radical, there remains a paucity of structural information on the
actual ferryl states, owing to their high reactivity. We demonstrate
here that the stable vanadyl ion [(V<sup>IV</sup>-oxo)<sup>2+</sup>] binds along with 2OG or its decarboxylation product, succinate,
in the active site of two different Fe/2OG enzymes to faithfully mimic
their transient ferryl states. Both ferryl and vanadyl complexes of
the Fe/2OG halogenase, SyrB2, remain stably bound to its carrier protein
substrate (l-aminoacyl-<i>S</i>-SyrB1), whereas
the corresponding complexes harboring transition metals (Fe, Mn) in
lower oxidation states dissociate. In the well-studied taurine:2OG
dioxygenase (TauD), the disposition of the substrate CāH bond
relative to the vanadyl ion defined by pulse electron paramagnetic
resonance methods is consistent with the crystal structure of the
reactant complex and computational models of the ferryl state. Vanadyl
substitution may thus afford access to structural details of the key
ferryl intermediates in this important enzyme class
Evidence for Only Oxygenative Cleavage of Aldehydes to Alk(a/e)nes and Formate by Cyanobacterial Aldehyde Decarbonylases
Cyanobacterial aldehyde decarbonylases (ADs) catalyze
the conversion
of C<sub><i>n</i></sub> fatty aldehydes to formate (HCO<sub>2</sub><sup>ā</sup>) and the corresponding C<sub><i>n</i>ā1</sub> alkĀ(a/e)Ānes. Previous studies of the <i>Nostoc
punctiforme</i> (<i>Np</i>) AD produced in <i>Escherichia coli</i> (<i>Ec</i>) showed that this
apparently hydrolytic reaction is actually a cryptically redox oxygenation
process, in which one O-atom is incorporated from O<sub>2</sub> into
formate and a protein-based reducing system (NADPH, ferredoxin, and
ferredoxin reductase; N/F/FR) provides all four electrons needed for
the complete reduction of O<sub>2</sub>. Two subsequent publications
by Marsh and co-workers [Das, et al. (2011) Angew. Chem.
Int. Ed. 50, 7148ā7152; Eser, et al. (2011) Biochemistry 50, 10743ā10750] reported that their <i>Ec</i>-expressed <i>Np</i> and <i>Prochlorococcus marinus</i> (<i>Pm</i>) AD preparations transform aldehydes to the same products
more rapidly by an O<sub>2</sub>-independent, truly hydrolytic process,
which they suggested proceeded by transient substrate reduction with
obligatory participation by the reducing system (they used a chemical
system, NADH and phenazine methosulfate; N/PMS). To resolve this discrepancy,
we re-examined our preparations of both AD orthologues by a combination
of (i) activity assays in the presence and absence of O<sub>2</sub> and (ii) <sup>18</sup>O<sub>2</sub> and H<sub>2</sub><sup>18</sup>O isotope-tracer experiments with <i>direct</i> mass-spectrometric
detection of the HCO<sub>2</sub><sup>ā</sup> product. For multiple
combinations of the AD orthologue (<i>Np</i> and <i>Pm</i>), reducing system (protein-based and chemical), and substrate
(<i>n</i>-heptanal and <i>n</i>-octadecanal),
our preparations strictly require O<sub>2</sub> for activity and do
not support detectable hydrolytic formate production, despite having
catalytic activities similar to or greater than those reported by
Marsh and co-workers. Our results, especially of the <sup>18</sup>O-tracer experiments, suggest that the activity observed by Marsh
and co-workers could have arisen from contaminating O<sub>2</sub> in
their assays. The definitive reaffirmation of the oxygenative nature
of the reaction implies that the enzyme, initially designated as aldehyde
decarbonylase when the C1-derived coproduct was thought to be carbon
monoxide rather than formate, should be redesignated as aldehyde-deformylating
oxygenase (ADO)
Direct Measurement of the Radical Translocation Distance in the Class I Ribonucleotide Reductase from <i>Chlamydia trachomatis</i>
Ribonucleotide reductases (RNRs)
catalyze conversion of ribonucleotides
to deoxyribonucleotides in all organisms via a free-radical mechanism
that is essentially conserved. In class I RNRs, the reaction is initiated
and terminated by radical translocation (RT) between the Ī± and
Ī² subunits. In the class Ic RNR from <i>Chlamydia trachomatis</i> (<i>Ct</i> RNR), the initiating event converts the active <i>S</i> = 1 MnĀ(IV)/FeĀ(III) cofactor to the <i>S</i> = 1/2 Mn(III)/Fe(III) āRT-productā
form
in the Ī² subunit and generates a cysteinyl radical in the Ī±
active site. The radical can be trapped via the well-described decomposition
reaction of the mechanism-based inactivator, 2ā²-azido-2ā²-deoxyuridine-5ā²-diphosphate,
resulting in the generation of a long-lived, nitrogen-centered radical
(N<sup>ā¢</sup>) in Ī±. In this work, we have determined
the distance between the MnĀ(III)/FeĀ(III) cofactor in Ī² and N<sup>ā¢</sup> in Ī± to be 43 Ā± 1 Ć
by using double
electronāelectron resonance experiments. This study provides
the first structural data on the <i>Ct</i> RNR holoenzyme
complex and the first direct experimental measurement of the inter-subunit
RT distance in any class I RNR
Circular Dichroism, Magnetic Circular Dichroism, and Variable Temperature Variable Field Magnetic Circular Dichroism Studies of Biferrous and Mixed-Valent <i>myo</i>-Inositol Oxygenase: Insights into Substrate Activation of O<sub>2</sub> Reactivity
<i>myo</i>-Inositol oxygenase (MIOX) catalyzes the 4e<sup>ā</sup> oxidation of <i>myo</i>-inositol (MI) to d-glucuronate
using a substrate activated FeĀ(II)ĀFeĀ(III) site.
The biferrous and FeĀ(II)ĀFeĀ(III) forms of MIOX were studied with circular
dichroism (CD), magnetic circular dichroism (MCD), and variable temperature
variable field (VTVH) MCD spectroscopies. The MCD spectrum of biferrous
MIOX shows two ligand field (LF) transitions near 10000 cm<sup>ā1</sup>, split by ā¼2000 cm<sup>ā1</sup>, characteristic of
six coordinate (6C) FeĀ(II) sites, indicating that the modest reactivity
of the biferrous form toward O<sub>2</sub> can be attributed to the
saturated coordination of both irons. Upon oxidation to the FeĀ(II)ĀFeĀ(III)
state, MIOX shows two LF transitions in the ā¼10000 cm<sup>ā1</sup> region, again implying a coordinatively saturated FeĀ(II) site. Upon
MI binding, these split in energy to 5200 and 11200 cm<sup>ā1</sup>, showing that MI binding causes the FeĀ(II) to become coordinatively
unsaturated. VTVH MCD magnetization curves of unbound and MI-bound
FeĀ(II)ĀFeĀ(III) forms show that upon substrate binding, the isotherms
become more nested, requiring that the exchange coupling and ferrous
zero-field splitting (ZFS) both decrease in magnitude. These results
imply that MI binds to the ferric site, weakening the FeĀ(III)āĪ¼-OH
bond and strengthening the FeĀ(II)āĪ¼-OH bond. This perturbation
results in the release of a coordinated water from the FeĀ(II) that
enables its O<sub>2</sub> activation
Experimental Correlation of Substrate Position with Reaction Outcome in the Aliphatic Halogenase, SyrB2
The ironĀ(II)- and
2-(oxo)Āglutarate-dependent (Fe/2OG) oxygenases
catalyze an array of challenging transformations, but how individual
members of the enzyme family direct different outcomes is poorly understood.
The Fe/2OG halogenase, SyrB2, chlorinates C4 of its native substrate, l-threonine appended to the carrier protein, SyrB1, but hydroxylates
C5 of l-norvaline and, to a lesser extent, C4 of l-aminobutyric acid when SyrB1 presents these non-native amino acids.
To test the hypothesis that positioning of the targeted carbon dictates
the outcome, we defined the positions of these three substrates by
measuring hyperfine couplings between substrate deuterium atoms and
the stable, EPR-active ironānitrosyl adduct, a surrogate for
reaction intermediates. The Feā<sup>2</sup>H distances and
NāFeā<sup>2</sup>H angles, which vary from 4.2 Ć
and 85Ā° for threonine to 3.4 Ć
and 65Ā° for norvaline,
rationalize the trends in reactivity. This experimental correlation
of position to outcome should aid in judging from structural data
on other Fe/2OG enzymes whether they suppress hydroxylation or form
hydroxylated intermediates on the pathways to other outcomes
Branched Intermediate Formation Is the Slowest Step in the Protein Splicing Reaction of the Ala1 KlbA Intein from <i>Methanococcus jannaschii</i>
We report the first detailed investigation of the kinetics
of protein
splicing by the <i>Methanococcus jannaschii</i> KlbA (<i>Mja</i> KlbA) intein. This intein has an N-terminal Ala in place
of the nucleophilic Cys or Ser residue that normally initiates splicing
but nevertheless splices efficiently in vivo [Southworth, M. W., Benner,
J., and Perler, F. B. (2000) <i>EMBO J.</i> <i>19</i>, 5019ā5026]. To date, the spontaneous nature of the cis splicing
reaction has hindered its examination in vitro. For this reason, we
constructed an <i>Mja</i> KlbA inteināmini-extein
precursor using intein-mediated protein ligation and engineered a
disulfide redox switch that permits initiation of the splicing reaction
by the addition of a reducing agent such as dithiothreitol (DTT).
A fluorescent tag at the C-terminus of the C-extein permits monitoring
of the progress of the reaction. Kinetic analysis of the splicing
reaction of the wild-type precursor (with no substitutions in known
nucleophiles or assisting groups) at various DTT concentrations shows
that formation of the branched intermediate from the precursor is
reversible (forward rate constant of 1.5 Ć 10<sup>ā3</sup> s<sup>ā1</sup> and reverse rate constant of 1.7 Ć 10<sup>ā5</sup> s<sup>ā1</sup> at 42 Ā°C), whereas the
productive decay of this intermediate to form the ligated exteins
is faster and occurs with a rate constant of 2.2 Ć 10<sup>ā3</sup> s<sup>ā1</sup>. This finding conflicts with reports about
standard inteins, for which Asn cyclization has been assigned as the
rate-determining step of the splicing reaction. Despite being the
slowest step of the reaction, branched intermediate formation in the <i>Mja</i> KlbA intein is efficient in comparison with those of
other intein systems. Interestingly, it also appears that this intermediate
is protected against thiolysis by DTT, in contrast to other inteins.
Evidence is presented in support of a tight coupling between the N-terminal
and C-terminal cleavage steps, despite the fact that the C-terminal
single-cleavage reaction occurs in variant <i>Mja</i> KlbA
inteins in the absence of N-terminal cleavage. We posit that the splicing
events in the <i>Mja</i> KlbA system are tightly coordinated
by a network of intra- and interdomain noncovalent interactions, rendering
its function particularly sensitive to minor disruptions in the intein
or extein environments
A 2.8 Ć FeāFe Separation in the Fe<sub>2</sub><sup>III/IV</sup> Intermediate, X, from <i>Escherichia coli</i> Ribonucleotide Reductase
A class
Ia ribonucleotide reductase (RNR) employs a Ī¼-oxo-Fe<sub>2</sub><sup>III/III</sup>/tyrosyl radical cofactor in its Ī²
subunit to oxidize a cysteine residue ā¼35 Ć
away in its
Ī± subunit; the resultant cysteine radical initiates substrate
reduction. During self-assembly of the <i>Escherichia coli</i> RNR-Ī² cofactor, reaction of the proteinās Fe<sub>2</sub><sup>II/II</sup> complex with O<sub>2</sub> results in accumulation
of an Fe<sub>2</sub><sup>III/IV</sup> cluster, termed <b>X</b>, which oxidizes the adjacent tyrosine (Y<sub>122</sub>) to the radical
(Y<sub>122</sub><sup>ā¢</sup>) as the cluster is converted to
the Ī¼-oxo-Fe<sub>2</sub><sup>III/III</sup> product. As the first
high-valent non-heme-iron enzyme complex to be identified and the
key activating intermediate of class Ia RNRs, <b>X</b> has been
the focus of intensive efforts to determine its structure. Initial
characterization by extended X-ray absorption fine structure (EXAFS)
spectroscopy yielded a FeāFe separation (<i>d</i><sub>FeāFe</sub>) of 2.5 Ć
, which was interpreted to
imply the presence of three single-atom bridges (O<sup>2ā</sup>, HO<sup>ā</sup>, and/or Ī¼-1,1-carboxylates). This short
distance has been irreconcilable with computational and synthetic
models, which all have <i>d</i><sub>FeāFe</sub> ā„
2.7 Ć
. To resolve this conundrum, we revisited the EXAFS characterization
of <b>X</b>. Assuming that samples containing increased concentrations
of the intermediate would yield EXAFS data of improved quality, we
applied our recently developed method of generating O<sub>2</sub> <i>in situ</i> from chlorite using the enzyme chlorite dismutase
to prepare <b>X</b> at ā¼2.0 mM, more than 2.5 times the
concentration realized in the previous EXAFS study. The measured <i>d</i><sub>FeāFe</sub> = 2.78 Ć
is fully consistent
with computational models containing a (Ī¼-oxo)<sub>2</sub>-Fe<sub>2</sub><sup>III/IV</sup> core. Correction of the <i>d</i><sub>FeāFe</sub> brings the experimental data and computational
models into full conformity and informs analysis of the mechanism
by which <b>X</b> generates Y<sub>122</sub><sup>ā¢</sup>
Installation of the Ether Bridge of Lolines by the Iron- and 2āOxoglutarate-Dependent Oxygenase, LolO: Regio- and Stereochemistry of Sequential Hydroxylation and Oxacyclization Reactions
The core of the loline
family of insecticidal alkaloids is the
bicyclic pyrrolizidine unit with an additional strained ether bridge
between carbons 2 and 7. Previously reported genetic and <i>in
vivo</i> biochemical analyses showed that the presumptive iron-
and 2-oxoglutarate-dependent (Fe/2OG) oxygenase, LolO, is required
for installation of the ether bridge upon the pathway intermediate,
1-<i>exo</i>-acetamidopyrrolizidine (AcAP). Here we show
that LolO is, in fact, solely responsible for this biosynthetic four-electron
oxidation. In sequential 2OG- and O<sub>2</sub>-consuming steps, LolO
removes hydrogens from C2 and C7 of AcAP to form both carbonāoxygen
bonds in <i>N</i>-acetylnorloline (NANL), the precursor
to all other lolines. When supplied with substoichiometric 2OG, LolO
only hydroxylates AcAP. At higher 2OG:AcAP ratios, the enzyme further
processes the alcohol to the tricyclic NANL. Characterization of the
alcohol intermediate by mass spectrometry and nuclear magnetic resonance
spectroscopy shows that it is 2-<i>endo</i>-hydroxy-1-<i>exo</i>-acetamidopyrrolizidine (2-<i>endo</i>-OH-AcAP).
Kinetic and spectroscopic analyses of reactions with site-specifically
deuteriated AcAP substrates confirm that the C2āH bond is cleaved
first and that the responsible intermediate is, as expected, an Fe<sup>IV</sup>āoxo (ferryl) complex. Analyses of the loline products
from cultures fed with stereospecifically deuteriated AcAP precursors,
proline and aspartic acid, establish that LolO removes the endo hydrogens
from C2 and C7 and forms both new CāO bonds with retention
of configuration. These findings delineate the pathway to an important
class of natural insecticides and lay the foundation for mechanistic
dissection of the chemically challenging oxacyclization reaction