Combined optical and resonance Raman studies have revealed the formation of an O2-adduct upon exposure of 4e- chemically reduced stearoyl-acyl carrier protein Delta9 desaturase to stearoyl-ACP and 1 atm O2. The observed intermediate has a broad absorption band at 700 nm and is remarkably stable at room temperature (t1/2 approximately 26 min). Resonance Raman studies using 16O2 gas reveal vibrational features of a bound peroxide [Vs(Fe-O2), 442 cm-1; Vas(Fe-O2), 490 cm-1; V(O-O), 898 cm-1] that undergo the expected mass-dependent shifts when prepared in (16)O(18)O or 18(O2). The appearance of two Fe-O2 vibrations, each having a single peak of intermediate frequency with 16(O)18(O), provs that the peroxide is bound symmetrically between the two iron atoms in a mu-1,2 configuration. The same results have been obtained in the accompanying resonance Raman study of ribonucleotide reductase isoform W48F/D84E [P. Moënne-Loccoz, J. Baldwin, B. A. Ley, T. M. Loehr, and J. M. Bollinger, Jr. (1998) Biochemistry 37, 14659-14663], thus making it likely that other members of the class II diiron enzymes form related peroxodiferric intermediates. Study of the reactivity of peroxodiferric Delta9D revealed that this intermediate underwent 2e- reduction leading to an oxidase reaction and recovery of the resting ferric homodimer. In contrast, biological reduction of the same enzyme preparations using ferredoxin reductase and [2Fe-2S] ferredoxin gave catalytic desaturation with a turnover number of 20-30 min-1. The profound difference in catalytic outcome for chemically and enzymatically reduced Delta9D suggests that redox-state dependent conformational changes cause partition of reactivity between desaturase and oxidase chemistries. The Delta9D oxidase reaction represents a new type of reactivity for the acyl-ACP desaturases and provides a two-step catalytic precedent for the "alternative oxidase" activity recently proposed for a membrane diiron enzyme in plants and trypanosomes.

Stearoyl-ACP Delta(9)-desaturase (Delta 9D) is a diiron enzyme that catalyzes 18:0-ACP desaturation. Each subunit of homodimeric resting Delta 9D contains a diferric cluster, while chemical reduction by 4e(-) produces a diferrous cluster in each subunit. Reaction of 4e(-)-reduced Delta 9D with 18:0-ACP and O(2) yields a blue chromophore (lambda(max) approximately 700 nm) that exhibits a vibrational spectrum indicative of a micro-1,2-peroxo complex; this species has been designated peroxo Delta9D. In contrast to other enzymic peroxodiiron intermediates, peroxo Delta 9D is long-lived (t(1/2) approximately 30 min at 25 degrees C) and decays via an oxidase reaction without formation of either H(2)O(2) or product (18:1-ACP). In this work, optical, transient kinetic, and Mössbauer techniques have been used to further investigate the origin and nature of this unusual peroxodiiron complex. Rapid mixing of 4e(-) Delta 9D with O(2)-equilibrated 18:0-ACP produced peroxo Delta 9D as revealed by a temperature-dependent, pseudo-first-order absorption increase at 700 nm (k = 46 s(-)(1) at 6 degrees C). The Mössbauer spectrum of peroxo Delta 9D, accounting for 96% of the total iron, consists of two quadrupole doublets present in equal proportions: delta(1) = 0.68(1) mm/s, and Delta E(Q)(1) = 1.90(2) mm/s; delta(2) = 0.64(1) mm/s, and Delta E(Q)(2) = 1.06(2) mm/s. Decay of the 700 nm optical band (k = 0.004 min(-)(1) at 6 degrees C) correlates with the complete conversion of peroxo Delta 9D into a complex called peroxo-cycled Delta 9D, which exhibits two new doublets present in equal proportions: delta(1) = 0.57(2) mm/s, and Delta E(Q)(1) = 1. 91(3) mm/s; delta(2) = 0.52(2) mm/s, and Delta E(Q)(2) = 1.41(3) mm/s. Thus, peroxo Delta 9D contains two asymmetric diferric clusters and reacts to yield peroxo-cycled Delta 9D, also containing two asymmetric diferric clusters that most probably represent a substrate complex state. The clusters of both peroxo Delta 9D and peroxo-cycled Delta 9D have a diamagnetic ground state. Because peroxo Delta 9D and peroxo-cycled Delta 9D are observed only in the presence of 18:0-ACP, substrate binding appears to have introduced asymmetry into the Delta 9D diiron clusters. In situ photolysis of peroxo Delta 9D at 4.2 K in the Mössbauer cryostat caused the release of O(2) and the reappearance of a diferrous Delta 9D.18:0-ACP complex with slightly changed parameters, suggesting a constrained cluster configuration was produced by the photolysis event. Annealing the photolyzed sample for 30 min at 77 K quantitatively restored the Mössbauer spectrum of peroxo Delta 9D, showing that the released O(2) was effectively sequestered within the active site.

We propose that arginine side chains often play a previously unappreciated general structural role in the maintenance of tertiary structure in proteins, wherein the positively charged guanidinium group forms multiple hydrogen bonds to backbone carbonyl oxygens. Using as a criterion for a "structural" arginine one that forms 4 or more hydrogen bonds to 3 or more backbone carbonyl oxygens, we have used molecular graphics to locate arginines of interest in 4 proteins: Arg 180 in Thermus thermophilus manganese superoxide dismutase, Arg 254 in human carbonic anhydrase 11, Arg 31 in Streptomyces rubiginosusxylose isomerase, and Arg 313 in Rhodospirillum rubrum ribulose-l,S-bisphosphate carboxylase/oxygenase. Arg 180 helps to mold the active site channel of superoxide dismutase, whereas in each of the other enzymes the structural arginine is buried in the "mantle" (i.e., inside, but near the surface) of the protein interior well removed from the active site, where it makes 5 hydrogen bonds to 4 backbone carbonyl oxygens. Using a more relaxed criterion of 3 or more hydrogen bonds to 2 or more backbone carbonyl oxygens, arginines that play a potentially important structural role were found in yeast enolase, Bacillus stearothermophilus glyceraldehyde-3-phosphate dehydrogenase, bacteriophage T4 and human lysozymes, Enteromorpha prolifera plastocyanin, HIV-1 protease, Trypanosoma brucei brucei and yeast triosephosphate isomerases, and Escherichia coli trp aporepressor (but not trp repressor or the trp repressor/operator complex). In addition to helping form the active site funnel in superoxide dismutase, the structural arginines found in this study play such diverse roles as stapling together 3 strands of backbone from different regions of the primary sequence, and tying a-helix to a-helix, &turn to 6-turn, and subunit to subunit.Keywords: backbone carbonyl oxygens; buried arginine; carbonic anhydrase; enolase; glyceraldehyde-3-phosphate dehydrogenase; HIV-1 protease; lysozyme; manganese superoxide dismutase; multiple hydrogen bonds; plastocyanin; ribulose-l,5-bisphosphate carboxylase/oxygenase; structural arginine; triosephosphate isomerase; trp aporepressor; xylose isomeraseThe guanidinium group of arginine is the most polar of all the common amino acid side chains found in proteins (Wolfenden, 1983) and is thus the residue most likely to be found on the surface in an aqueous environment. The side chain almost always has a pK, 112 and the positively charged planar guanidinium group can be a hydrogen donor in the formation of up to 5 hydrogen bonds (Fig. 1). Arginine plays a major role in the binding of negatively charged substrates, cofactors, and effectors to

Stearoyl-acyl carrier protein (stearoyl-ACP) Δ9-desaturase (Δ9D) catalyzes the insertion of a cis double bond between the 9 and 10 positions of the stearoyl-ACP to convert it to oleoyl-ACP. The binuclear non-heme iron active site of the fully reduced enzyme (reduced Δ9D) and its substrate-bound form (stearoyl-ACP Δ9D) have been studied using a combination of circular dichroism (CD) and magnetic circular dichroism (MCD) to probe their geometric and electronic structures. CD and MCD in the near-IR region probe the ligand-field d−d transitions of the ferrous sites. Variable-temperature variable-field (VTVH) MCD combined with a spin-Hamiltonian analysis including the zero-field splitting (ZFS) of both irons and the exchange coupling (J) between the irons due to bridging ligation is used to probe their ground-state properties. These ground- and excited-state results indicate that the active site of reduced Δ9D has two equivalent 5-coordinate irons in a distorted square pyramidal geometry. They are weakly antiferromagnetically coupled with large negative and equivalent ZFSs (D 1 = D 2 < 0), which gives a diamagnetic ground state interacting with low-lying paramagnetic excited states. Addition of substrate causes a significant change in both the excited states and the nature of the ground state. These spectral changes indicate that one of the irons becomes 4-coordinate, while the other distorts toward a trigonal bipyramidal geometry. The two irons remain weakly antiferromagnetically coupled. However, this geometry change modifies their ZFSs (D 1 < 0 and D 2 > 0), which results in a new ground state, M s = ±1, with a low-lying M s = ±2 first excited state. These results are the first direct evidence that the stearoyl-ACP binding strongly perturbs the active site, creating an additional open coordination position that correlates with enhanced dioxygen reactivity. These results are correlated with the X-ray crystal structure and compared to the related enzyme, ribonucleotide reductase, to gain insight into geometric and electronic structure contributions to dioxygen reactivity.