The iron(II)- and 2-(oxo)glutarate-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 iron-nitrosyl adduct, a surrogate for reaction intermediates. The Fe-2H distances and N-Fe-2H angles, which vary from 4.2 Å and 85° for threonine to 3.4 Å and 65° 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.

It has been speculated that the presence of OH(H2O)n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.

Recently, we reported the spectroscopic and kinetic characterizations of cytochrome P450 compound I in CYP119A1, effectively closing the catalytic cycle of cytochrome P450-mediated hydroxylations. In this minireview, we focus on the developments that made this breakthrough possible. We examine the importance of enzyme purification in the quest for reactive intermediates and report the preparation of compound I in a second P450 (P450 ST ). In an effort to bring clarity to the field, we also examine the validity of controversial reports claiming the production of P450 compound I through the use of peroxynitrite and laser flash photolysis.A significant amount of research in the field of bioinorganic chemistry is focused on discerning the intimate details of enzyme catalysis. Critical to these efforts is the preparation of reactive intermediates that form the stations of the catalytic cycle of an enzyme. The study of these transient species is driven by the hope that insights gleaned from their electronic, structural, and kinetic characterizations will guide the design of next-generation catalysts or point the way to inhibitors that could serve as drugs for a variety of maladies. Although a thorough characterization of all the intermediates in a catalytic cycle is required for a detailed dissection of the catalytic mechanism, there are certain species that have special significance and, as such, are considered high-value targets for characterization. These species are generally the highly reactive intermediates that are ultimately responsible for the most important, difficult, or chemically interesting transformation in the catalytic mechanism.Recently, we reported the capture and characterization of one of the most highly sought intermediates in biological chemistry, P450 compound I (P450-I) 4 (1). This iron(IV)-oxo (or ferryl) radical species (7 in Fig. 1) had long been thought to be the principal intermediate in cytochrome P450 catalysis, but due to its highly reactive nature, it had eluded definitive characterization for over 40 years. The existence of P450-I was postulated based on the observation of a "shunt pathway" (Fig. 1) allowing the oxidation of substrates through the use of oxygen donors such as hydrogen peroxide and meta-chloroperbenzoic acid. These oxidants were known to generate high-valent iron-oxo species in heme peroxidases (2, 3), suggesting that a similar intermediate might be involved in P450 catalysis. However, 4 decades worth of searching for the elusive P450-I had led to questions about not only its competence as a hydroxylating agent but also its role in P450 catalysis (4 -6).Our investigations confirmed the existence and the reactive nature of the intermediate. P450-I is capable of hydroxylating unactivated C-H bonds with the remarkable rate constant of 1 ϫ 10 7 M Ϫ1 s Ϫ1 (1). Kinetic isotope effects support a mechanism in which P450-I abstracts hydrogen from substrate, forming an iron(IV)-hydroxide complex that rapidly recombines with substrate to yield hydroxylated product (7-9 in Fig. 1...

The class Ia ribonucleotide reductase (RNR) from Escherichia coli (Ec) employs a free-radical mechanism, which involves bidirectional translocation of a radical equivalent or “hole” over a distance of ∼35 Å from the stable diferric/tyrosyl-radical (Y122•) cofactor in the β subunit to cysteine 439 (C439) in the active site of the α subunit. This long-range, inter-subunit electron transfer occurs by a multi-step “hopping” mechanism via formation of transient amino acid radicals along a specific pathway and is thought to be conformationally gated and coupled to local proton transfers. Whereas constituent amino acids of the hopping pathway have been identified, details of the proton-transfer steps and conformational gating within the β sununit have remained obscure; specific proton couples have been proposed, but no direct evidence has been provided. In the key first step, the reduction of Y122• by the first residue in the hopping pathway, a water ligand to Fe1 of the diferric cluster was suggested to donate a proton to yield the neutral Y122. Here we show that forward radical translocation is associated with perturbation of the Mössbauer spectrum of the diferric cluster, especially the quadrupole doublet associated with Fe1. Density functional theory (DFT) calculations verify the consistency of the experimentally observed perturbation with that expected for deprotonation of the Fe1-coordinated water ligand. The results thus provide the first evidence that the diiron cluster of this prototypical class Ia RNR functions not only in its well-known role as generator of the enzyme's essential Y122•, but also directly in catalysis.