Kinetic mechanism and intrinsic isotope effects for the peptidylglycine alpha-amidating enzyme reaction

Capturing the Binuclear Copper State of Peptidylglycine Monooxygenase Using a Peptidyl-Homocysteine Lure

Peptidylglycine monooxygenase is a copper-dependent enzyme that catalyzes C-alpha hydroxylation of glycine extended pro-peptides, a critical post-translational step in peptide hormone processing. The canonical mechanism posits that dioxygen binds at the mononuclear M-center to generate a Cu(II)-superoxo species capable of H atom abstraction from the peptidyl substrate, followed by long-range electron tunneling from the CuH center. Recent crystallographic and biochemical data have challenged this mechanism, suggesting instead that an "open-to-closed" transition brings the copper centers closer, allowing reactivity within a binuclear intermediate. Here we present the first direct observation of an enzyme-bound binuclear copper species, captured by the use of an Ala-Ala-Phe-hCys inhibitor complex. This molecule reacts with the fully reduced enzyme to form a thiolate-bridged binuclear species characterized by EXAFS of the WT and its M314H variant and with the oxidized enzyme to form a novel mixed valence entity characterized by UV/vis and EPR. Mechanistic implications are discussed.

New structures reveal flexible dynamics between the subdomains of peptidylglycine monooxygenase. Implications for an open to closed mechanism

Peptidylglycine monooxygenase (PHM) is essential for the biosynthesis of many neuroendocrine peptides via a copper-dependent hydroxylation of a glycine-extended pro-peptide. The "canonical" mechanism requires the transfer of two electrons from one mononuclear copper (CuH, H-site) to a second mononuclear copper (CuM, M-site) which is the site of oxygen binding and catalysis. In most crystal structures the copper centers are separated by 11 Å of disordered solvent, but recent work has established that a PHM variant H108A forms a closed conformer in the presence of citrate with a reduced Cu-Cu site separation of ~4 Å. Here we report three new PHM structures where the H and M sites are separated by a longer distance of ~14 Å. Variation in Cu-Cu distance is the result of a rotation of the M subdomain about a hinge point centered on the pro199 -leu200 -ile201 triad which forms the linker between subdomains. The energetic cost of domain dynamics is likely small enough to allow free rotation of the subdomains relative to each other, adding credence to recent suggestions that an open-to-closed transition to form a binuclear oxygen binding intermediate is an essential element of catalysis. This inference would explain many experimental observations that are inconsistent with the current canonical mechanism including substrate-induced oxygen activation and isotope scrambling during the peroxide shunt.

Peptidylglycine α-amidating monooxygenase as a therapeutic target or biomarker for human diseases

Peptides play a key role in controlling many physiological and neurobiological pathways. Many bioactive peptides require a C-terminal α-amide for full activity. The bifunctional enzyme catalysing α-amidation, peptidylglycine α-amidating monooxygenase (PAM), is the sole enzyme responsible for amidated peptide biosynthesis, from Chlamydomonas reinhardtii to Homo sapiens. Many neuronal and endocrine functions are dependent upon amidated peptides; additional amidated peptides are growth promoters in tumours. The amidation reaction occurs in two steps, glycine α-hydroxylation followed by dealkylation to generate the α-amide product. Currently, most potentially useful inhibitors target the first reaction, which is rate-limiting. PAM is a membrane-bound enzyme that visits the cell surface during peptide secretion. PAM is then used again in the biosynthetic pathway, meaning that cell-impermeable inhibitors or inactivators could have therapeutic value for the treatment of cancer or psychiatric abnormalities. To date, inhibitor design has not fully exploited the structures and mechanistic details of PAM.

Pre-Steady-State Reactivity of Peptidylglycine Monooxygenase Implicates Ascorbate in Substrate Triggering of the Active Conformer

Peptidylglycine monooxygenase (PHM) is essential for the posttranslational amidation of neuroendocrine peptides. An important aspect of the PHM mechanism is the complete coupling of oxygen reduction to substrate hydroxylation, which implies no oxygen reactivity of the fully reduced enzyme in the absence of peptidyl substrates. As part of studies aimed at investigating this feature of the PHM mechanism, we explored pre-steady-state kinetics using chemical quench (CQ) and rapid freeze-quench (RFQ) studies of the fully reduced ascorbate-free PHM enzyme. First, we confirmed the absence of Cu(I)-enzyme oxidation by O₂ at catalytic rates in the absence of peptidyl substrate. Next, we investigated reactivity in the presence of the substrate dansyl-YVG. Surprisingly, when ascorbate-free di-Cu(I) PHM was shot against oxygenated buffer containing the dansyl-YVG substrate, <15% of the expected product was formed. Substoichiometric reactivity was confirmed by stopped-flow and RFQ EPR spectroscopy. Product generation reached a maximum of 70% by the addition of increasing amounts of the ascorbate cosubstrate in a process that was not the result of multiple turnovers. FTIR spectroscopy of the Cu(I)-CO reaction chemistry was then used to show that increasing ascorbate concentrations correlated with a substrate-induced Cu(I)M-CO species characteristic of an altered conformation. We conclude that ascorbate and peptidyl substrate work together to induce a transition from an inactive to an active conformation and suggest that the latter may represent the "closed" conformation (Cu-Cu of ∼4 Å) recently observed for both PHM and its sister enzyme DBM by crystallography.

Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O-H, N-H, and C-H Substrates

Instability of end-on superoxocopper(II) complexes, with respect to conversion to peroxo-bridged dicopper(II) complexes, has largely constrained their study to very low temperatures. This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of bulky ligands, Ar₃-TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O₂ to yield [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ species, which are stable against dimerization at all temperatures. Binding of O₂ saturates at subambient temperatures and can be reversed by warming. The onset of oxygenation for the Ar = tpb and dpb systems is observed at 25 °C, and all three [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ complexes are stable against self-decay at temperatures of ≤-20 °C. This provides a wide temperature window for study of these complexes, which was exploited by performing extensive reaction kinetics measurements for [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ using a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants (k₂) versus oxidation potentials (E_ox) for a range of phenols, construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicate that reaction with all substrates proceeds via H atom transfer (HAT), reaction with the phenols proceeds with significant charge transfer, and full tunneling of both H and D atoms occurs in the case of 1,2-diphenylhydrazine and 4-methoxy-2,6-di-tert-butylphenol. Oxidation of C-H bonds proved to be kinetically challenging, and whereas [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ can oxidize moderately strong O-H and N-H bonds, it is only able to oxidize very weak C-H bonds.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

Multiple roles for peptidylglycine α-amidating monooxygenase in the response to hypoxia

The biosynthesis of many of the peptides involved in homeostatic control requires peptidylglycine α-amidating monooxygenase (PAM), an ancient, highly conserved copper- and ascorbate-dependent enzyme. Using the production of amidated chromogranin A to monitor PAM function in tumor cells, physiologically relevant levels of hypoxia were shown to inhibit this monooxygenase. The ability of primary pituitary cells exposed to hypoxic conditions for 4 h to produce amidated chromogranin A was similarly inhibited. The affinity of the purified monooxygenase for oxygen (K_m  = 99 ± 19 μM) was consistent with this result. The ability of PAM to alter secretory pathway behavior under normoxic conditions required its monooxygenase activity. Under normoxic conditions, hypoxia-inducible factor 1a levels in dense cultures of corticotrope tumor cells expressing high levels of PAM exceeded those in control cells; expression of inactive monooxygenase did not have this effect. The effects of hypoxia on levels of two PAM-regulated genes (activating transcription factor 3 [Atf3] and FK506 binding protein 2 [Fkbp2]) differed in cells expressing high versus low levels of PAM. Putative hypoxia response elements occur in both human and mouse PAM, and hPAM has consistently been identified as one of the genes upregulated in response to hypoxia. Expression of PAM is also known to alter gene expression. A quarter of the genes consistently upregulated in response to hypoxia were downregulated following increased expression of PAM. Taken together, our data suggest roles for PAM and amidated peptide secretion in the coordination of tissue-specific responses to hypoxia.

Theory Demonstrated a "Coupled" Mechanism for O2 Activation and Substrate Hydroxylation by Binuclear Copper Monooxygenases

Multiscale simulations have been performed to address the longstanding issue of "dioxygen activation" by the binuclear copper monooxygenases (PHM and DβM), which have been traditionally classified as "noncoupled" binuclear copper enzymes. Our QM/MM calculations rule out that Cu_M(II)-O₂^• is an active species for H-abstraction from the substrate. In contrast, Cu_M(II)-O₂^• would abstract an H atom from the cosubstrate ascorbate to form a Cu_M(II)-OOH intermediate in PHM and DβM. Consistent with the recently reported structural features of DβM, the umbrella sampling shows that the "open" conformation of the Cu_M(II)-OOH intermediate could readily transform into the "closed" conformation in PHM, in which we located a mixed-valent μ-hydroperoxodicopper(I,II) intermediate, (μ-OOH)Cu(I)Cu(II). The subsequent O-O cleavage and OH moiety migration to Cu_H generate the unexpected species (μ-O^•)(μ-OH)Cu(II)Cu(II), which is revealed to be the reactive intermediate responsible for substrate hydroxylation. We also demonstrate that the flexible Met ligand is favorable for O-O cleavage reactions, while the replacement of Met with the strongly bound His ligand would inhibit the O-O cleavage reactivity. As such, the study not only demonstrates a "coupled" mechanism for O₂ activation by binuclear copper monooxygenases but also deciphers the full catalytic cycle of PHM and DβM in accord with the available experimental data. These findings of O₂ activation and substrate hydroxylation by binuclear copper monooxygenases could expand our understanding of the reactivities of the synthetic monocopper complexes.

Rational Design of a Histidine-Methionine Site Modeling the M-Center of Copper Monooxygenases in a Small Metallochaperone Scaffold

Mononuclear copper monooxygenases peptidylglycine monooxygenase (PHM) and dopamine β-monooxygenase (DBM) catalyze the hydroxylation of high energy C-H bonds utilizing a pair of chemically distinct copper sites (CuH and CuM) separated by 11 Å. In earlier work, we constructed single-site PHM variants that were designed to allow the study of the M- and H-centers independently in order to place their reactivity sequentially along the catalytic pathway. More recent crystallographic studies suggest that these single-site variants may not be truly representative of the individual active sites. In this work, we describe an alternative approach that uses a rational design to construct an artificial PHM model in a small metallochaperone scaffold. Using site-directed mutagenesis, we constructed variants that provide a His₂Met copper-binding ligand set that mimics the M-center of PHM. The results show that the model accurately reproduces the chemical and spectroscopic properties of the M-center, including details of the methionine coordination, and the properties of Cu(I) and Cu(II) states in the presence of endogenous ligands such as CO and azide. The rate of reduction of the Cu(II) form of the model by the chromophoric reductant N,N'-dimethyl phenylenediamine (DMPD) has been compared with that of the PHM M-center, and the reaction chemistry of the Cu(I) forms with molecular oxygen has also been explored, revealing an unusually low reactivity toward molecular oxygen. This latter finding emphasizes the importance of substrate triggering of oxygen reactivity and implies that the His₂Met ligand set, while necessary, is insufficient on its own to activate oxygen in these enzyme systems.

Effects of copper occupancy on the conformational landscape of peptidylglycine α-hydroxylating monooxygenase

The structures of metalloproteins that use redox-active metals for catalysis are usually exquisitely folded in a way that they are prearranged to accept their metal cofactors. Peptidylglycine α-hydroxylating monooxygenase (PHM) is a dicopper enzyme that catalyzes hydroxylation of the α-carbon of glycine-extended peptides for the formation of des-glycine amidated peptides. Here, we present the structures of apo-PHM and of mutants of one of the copper sites (H107A, H108A, and H172A) determined in the presence and absence of citrate. Together, these structures show that the absence of one copper changes the conformational landscape of PHM. In one of these structures, a large interdomain rearrangement brings residues from both copper sites to coordinate a single copper (closed conformation) indicating that full copper occupancy is necessary for locking the catalytically competent conformation (open). These data suggest that in addition to their required participation in catalysis, the redox-active metals play an important structural role.

Sweta Maheshwari et al. present X-ray crystal structures of the two-copper enzyme peptidylglycine α-hydroxylating monooxygenase and three inactive mutant forms. They show that full copper occupancy is needed to maintain the catalytically competent (open) conformation of the enzyme.

Substrate-Induced Carbon Monoxide Reactivity Suggests Multiple Enzyme Conformations at the Catalytic Copper M-Center of Peptidylglycine Monooxygenase

The present study uses CO as a surrogate for oxygen to probe how substrate binding triggers oxygen activation in peptidylglycine monooygenase (PHM). Infrared stretching frequencies (ν(C ≡ O)) of the carbonyl (CO) adducts of copper proteins are sensitive markers of Cu(I) coordination and are useful in probing oxygen reactivity because the electronic properties of O₂ and CO are similar. The carbonyl chemistry has been explored using PHM WT and a number of active site variants in the absence and presence of peptidyl substrates. We have determined that upon carbonylation (i) a major CO band at 2092 cm^-1 and a second minor CO band at 2063 cm^-1 are observed in the absence of peptide substrate Ac-YVG; (ii) the presence of peptide substrate amplifies the minor CO band and causes it to partially interconvert with the CO band at 2092 cm^-1; (iii) the substrate-induced CO band is associated with a second conformer at CuM; and (iv) the CuH-site mutants, which are inactive, fail to generate any substrate-induced CO bands. The total intensity of both bands is constant, suggesting that the Cu(I)M-site partitions between the two carbonylated enzyme states. Together, these data provide evidence for two conformers at CuM, one of which is induced by binding of the peptide substrate with the implication that this represents the conformation that also allows binding and activation of O₂.

Mechanism of O2 activation and substrate hydroxylation in noncoupled binuclear copper monooxygenases

Peptidylglycine α-hydroxylating monooxygenase (PHM) and dopamine β-monooxygenase (DβM) are copper-dependent enzymes that are vital for neurotransmitter regulation and hormone biosynthesis. These enzymes feature a unique active site consisting of two spatially separated (by 11 Å in PHM) and magnetically noncoupled copper centers that enables 1e^- activation of O₂ for hydrogen atom abstraction (HAA) of substrate C-H bonds and subsequent hydroxylation. Although the structures of the resting enzymes are known, details of the hydroxylation mechanism and timing of long-range electron transfer (ET) are not clear. This study presents density-functional calculations of the full reaction coordinate, which demonstrate: (i) the importance of the end-on coordination of superoxide to Cu for HAA along the triplet spin surface; (ii) substrate radical rebound to a Cu^II hydroperoxide favors the proximal, nonprotonated oxygen; and (iii) long-range ET can only occur at a late step with a large driving force, which serves to inhibit deleterious Fenton chemistry. The large inner-sphere reorganization energy at the ET site is used as a control mechanism to arrest premature ET and dictate the correct timing of ET.

Copper(I)-Dioxygen Adducts and Copper Enzyme Mechanisms

Primary copper(I)-dioxygen (O₂) adducts, cupric-superoxide complexes, have been proposed intermediates in copper-containing dioxygen-activating monooxygenase and oxidase enzymes. Here, mechanisms of C-H activation by reactive copper-(di)oxygen intermediates are discussed, with an emphasis on cupric-superoxide species. Over the past 25 years, many synthetically derived cupric-superoxide model complexes have been reported. Due to the thermal instability of these intermediates, early studies focused on increasing their stability and obtaining physical characterization. More recently, in an effort to gain insight into the possible substrate oxidation step in some copper monooxygenases, several cupric-superoxide complexes have been used as surrogates to probe substrate scope and reaction mechanisms. These cupric superoxides are capable of oxidizing substrates containing weak O-H and C-H bonds. Mechanistic studies for some enzymes and model systems have supported an initial hydrogen-atom abstraction via the cupric-superoxide complex as the first step of substrate oxidation.

Dioxygen Binding, Activation, and Reduction to H2O by Cu Enzymes

Oxygen intermediates in copper enzymes exhibit unique spectroscopic features that reflect novel geometric and electronic structures that are key to reactivity. This perspective will describe: (1) the bonding origin of the unique spectroscopic features of the coupled binuclear copper enzymes and how this overcomes the spin forbiddenness of O2 binding and activates monooxygenase activity, (2) how the difference in exchange coupling in the non-coupled binuclear Cu enzymes controls the reaction mechanism, and (3) how the trinuclear Cu cluster present in the multicopper oxidases leads to a major structure/function difference in enabling the irreversible reductive cleavage of the O-O bond with little overpotential and generating a fully oxidized intermediate, different from the resting enzyme studied by crystallography, that is key in enabling fast PCET in the reductive half of the catalytic cycle.

Stopped-Flow Studies of the Reduction of the Copper Centers Suggest a Bifurcated Electron Transfer Pathway in Peptidylglycine Monooxygenase

Peptidylglycine monooxygenase (PHM) is a dicopper enzyme that plays a vital role in the amidation of glycine-extended pro-peptides. One of the crucial aspects of its chemistry is the transfer of two electrons from an electron-storing and -transferring site (CuH) to the oxygen binding site and catalytic center (CuM) over a distance of 11 Å during one catalytic turnover event. Here we present our studies of the first electron transfer (ET) step (reductive phase) in wild-type (WT) PHM as well as its variants. Stopped flow was used to record the reduction kinetic traces using the chromophoric agent N,N-dimethyl-p-phenylenediamine dihydrochloride (DMPD) as the reductant. The reduction was found to be biphasic in the WT PHM with an initial fast phase (17.2 s(-1)) followed by a much slower phase (0.46 s(-1)). We were able to ascribe the fast and slow phase to the CuH and CuM sites, respectively, by making use of the H242A and H107AH108A mutants that contain only the CuH site and CuM site, respectively. In the absence of substrate, the redox potentials determined by cyclic voltammetry were 270 mV (CuH site) and -15 mV (CuM site), but binding of substrate (Ac-YVG) was found to alter both potentials so that they converged to a common value of 83 mV. Substrate binding also accelerated the slow reductive phase by ~10-fold, an effect that could be explained at least partially by the equalization of the reduction potential of the copper centers. Studies of H108A showed that the ET to the CuM site is blocked, highlighting the role of the H108 ligand as a component of the reductive ET pathway. Strikingly, the rate of reduction of the H172A variant was unaffected despite the rate of catalysis being 3 orders of magnitude slower than that of the WT PHM. These studies strongly indicate that the reductive phase and catalytic phase ET pathways are different and suggest a bifurcated ET pathway in PHM. We propose that H172 and Y79 form part of an alternate pathway for the catalytic phase ET while the H108 ligand along with the water molecules and substrate form the reductive phase ET pathway.

Oxygen-18 Kinetic Isotope Effects of Nonheme Iron Enzymes HEPD and MPnS Support Iron(III) Superoxide as the Hydrogen Abstraction Species

Nonheme iron oxygenases that carry out four-electron oxidations of substrate have been proposed to employ iron(III) superoxide species to initiate this reaction [Paria, S.; Que, L.; Paine, T. K. Angew. Chem. Int. Ed.2011, 50, 11129]. Here we report experimental evidence in support of this proposal. ¹⁸O KIEs were measured for two recently discovered mononuclear nonheme iron oxygenases: hydroxyethylphosphonate dioxygenase (HEPD) and methylphosphonate synthase (MPnS). Competitive ¹⁸O KIEs measured with deuterated substrates are larger than those measured with unlabeled substrates, which indicates that C–H cleavage must occur before an irreversible reductive step at molecular oxygen. A similar observation was previously used to implicate copper(II) superoxide in the H-abstraction reactions catalyzed by dopamine β-monooxygenase [Tian, G. C.; Klinman, J. P. J. Am. Chem. Soc.1993, 115, 8891] and peptidylglycine α-hydroxylating monooxygenase [Francisco, W. A.; Blackburn, N. J.; Klinman, J. P. Biochemistry2003, 42, 1813].

Solvent and Temperature Probes of the Long-Range Electron-Transfer Step in Tyramine β-Monooxygenase: Demonstration of a Long-Range Proton-Coupled Electron-Transfer Mechanism

Tyramine β-monooxygenase (TβM) belongs to a family of physiologically important dinuclear copper monooxygenases that function with a solvent-exposed active site. To accomplish each enzymatic turnover, an electron transfer (ET) must occur between two solvent-separated copper centers. In wild-type TβM, this event is too fast to be rate limiting. However, we have recently shown [Osborne, R. L.; et al. Biochemistry2013, 52, 1179] that the Tyr216Ala variant of TβM leads to rate-limiting ET. In this study, we present a pH–rate profile study of Tyr216Ala, together with deuterium oxide solvent kinetic isotope effects (KIEs). A solvent KIE of 2 on k_cat is found in a region where k_cat is pH/pD independent. As a control, the variant Tyr216Trp, for which ET is not rate determining, displays a solvent KIE of unity. We conclude, therefore, that the observed solvent KIE arises from the rate-limiting ET step in the Tyr216Ala variant, and show how small solvent KIEs (ca. 2) can be fully accommodated from equilibrium effects within the Marcus equation. To gain insight into the role of the enzyme in the long-range ET step, a temperature dependence study was also pursued. The small enthalpic barrier of ET (E_a = 3.6 kcal/mol) implicates a significant entropic barrier, which is attributed to the requirement for extensive rearrangement of the inter-copper environment during PCET catalyzed by the Tyr216Ala variant. The data lead to the proposal of a distinct inter-domain pathway for PCET in the dinuclear copper monooxygenases.

Mechanistic insights into the oxidation of substituted phenols via hydrogen atom abstraction by a cupric-superoxo complex

To obtain mechanistic insights into the inherent reactivity patterns for copper(I)-O2 adducts, a new cupric-superoxo complex [(DMM-tmpa)Cu(II)(O2(•-))](+) (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation-oxygenation reactions. Compound 2 is characterized by UV-vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (k(B)T/e) ln k2 versus E(ox) of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, (18)O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a Cu(II)-OO-(ArO') intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O-O cleavage) mainly occurs for p-R-DTBP which gives (18)O-labeled DTBQ and elimination of the R group.

Reaction mechanism of the bicopper enzyme peptidylglycine α-hydroxylating monooxygenase

Peptidylglycine α-hydroxylating monooxygenase is a noninteracting bicopper enzyme that stereospecifically hydroxylates the terminal glycine of small peptides for its later amidation. Neuroendocrine messengers, such as oxytocin, rely on the biological activity of this enzyme. Each catalytic turnover requires one oxygen molecule, two protons from the solvent, and two electrons. Despite this enzyme having been widely studied, a consensus on the reaction mechanism has not yet been found. Experiments and theoretical studies favor a pro-S abstraction of a hydrogen atom followed by the rebinding of an OH group. However, several hydrogen-abstracting species have been postulated; because two protons are consumed during the reaction, several protonation states are available. An electron transfer between the copper atoms could play a crucial role for the catalysis as well. This leads to six possible abstracting species. In this study, we compare them on equal footing. We perform quantum mechanics/molecular mechanics calculations, considering the glycine hydrogen abstraction. Our results suggest that the most likely mechanism is a protonation of the abstracting species before the hydrogen abstraction and another protonation as well as a reduction before OH rebinding.

The power of integrating kinetic isotope effects into the formalism of the Michaelis-Menten equation

The final arbiter of enzyme mechanism is the ability to establish and test a kinetic mechanism. Isotope effects play a major role in expanding the scope and insight derived from the Michaelis-Menten equation. The integration of isotope effects into the formalism of the Michaelis-Menten equation began in the 1970s and has continued until the present. This review discusses a family of eukaryotic copper proteins, including dopamine β-monooxygenase, tyramine β-monooxygenase and peptidylglycine α-amidating enzyme, which are responsible for the synthesis of neuroactive compounds, norepinephrine, octopamine and C-terminally carboxamidated peptides, respectively. The review highlights the results of studies showing how combining kinetic isotope effects with initial rate parameters permits the evaluation of: (a) the order of substrate binding to multisubstrate enzymes; (b) the magnitude of individual rate constants in complex, multistep reactions; (c) the identification of chemical intermediates; and (d) the role of nonclassical (tunnelling) behaviour in C-H activation.

Interdomain long-range electron transfer becomes rate-limiting in the Y216A variant of tyramine β-monooxygenase

The enzyme tyramine β-monooxygenase (TβM) belongs to a small eukaryotic family of physiologically important mononuclear dicopper monooxygenases. The properties of this family include noncoupled mononuclear copper centers ~11 Å apart, with Cu(M) performing C-H and O(2) activation and Cu(H) functioning as an electron storage site [Klinman, J. P. (2006) J. Biol. Chem. 281, 3013-3016]. A conserved tyrosine (Y216 in TβM) is positioned between the copper domains and is associated with Cu(H) (through an interaction with a Cu(H)-coordinating histidine). Mutations at Y216 (to W, I, and A) indicate little or no difference in electron paramagnetic resonance spectra, while X-ray absorption spectroscopy studies show only a very small decrease in distance between Cu(M) and its Met471 ligand in reduced enzyme. High-performance liquid chromatography assays demonstrate that turnover of substrate is complete with Y216W and Y216I, whereas Y216A undergoes a secondary inactivation that is linked to oxidation of ligands at Cu(M). Steady-state kinetic and isotope effect measurements were investigated. The significantly elevated K(m,Tyr) for Y216A, together with a very large (D)(k(cat)/K(m,Tyr)) of ~12, indicates a major impact on the binding of substrate at the Cu(M) site. The kinetic and isotopic parameters lead to estimated rate constants for C-H bond cleavage, dissociation of substrate from the Cu(M) site, and, in the case of Y216A, the rate of electron transfer (ET) from Cu(H) to Cu(M). These studies uncover a rate-limiting ET within the solvent-filled interface and lead to a paradigm shift in our understanding of the mononuclear dicopper monooxygenases.

Coordination of peroxide to the Cu(M) center of peptidylglycine α-hydroxylating monooxygenase (PHM): structural and computational study

Many bioactive peptides, such as hormones and neuropeptides, require amidation at the C terminus for their full biological activity. Peptidylglycine α-hydroxylating monooxygenase (PHM) performs the first step of the amidation reaction-the hydroxylation of peptidylglycine substrates at the Cα position of the terminal glycine. The hydroxylation reaction is copper- and O(2)-dependent and requires 2 equiv of exogenous reductant. The proposed mechanism suggests that O(2) is reduced by two electrons, each provided by one of two nonequivalent copper sites in PHM (Cu(H) and Cu(M)). The characteristics of the reduced oxygen species in the PHM reaction and the identity of the reactive intermediate remain uncertain. To further investigate the nature of the key intermediates in the PHM cycle, we determined the structure of the oxidized form of PHM complexed with hydrogen peroxide. In this 1.98-Å-resolution structure (hydro)peroxide binds solely to Cu(M) in a slightly asymmetric side-on mode. The O-O interatomic distance of the copper-bound ligand is 1.5 Å, characteristic of peroxide/hydroperoxide species, and the Cu-O distances are 2.0 and 2.1 Å. Density functional theory calculations using the first coordination sphere of the Cu(M) active site as a model system show that the computed energies of the side-on L(3)Cu(M)(II)-O(2) (2-) species and its isomeric, end-on structure L(3)Cu(M)(I)-O(2) (·-) are similar, suggesting that both these intermediates are significantly populated within the protein environment. This observation has important mechanistic implications. The geometry of the observed side-on coordinated peroxide ligand in L(3)Cu(M)(II)O(2) (2-) is in good agreement with the results of a hybrid quantum mechanical-molecular mechanical optimization of this species.

Inactivation of Met471Cys tyramine β-monooxygenase results from site-specific cysteic acid formation

Tyramine β-monooxygenase (TβM), the insect homologue of dopamine β-monooxygenase, is a neuroregulatory enzyme that catalyzes the β-hydroxylation of tyramine to yield octopamine. Mutation of the methionine (Met) ligand to Cu(M) of TβM, Met471Cys, yielded a form of TβM that is catalytically active but susceptible to inactivation during turnover [Hess, C. R., Wu, Z., Ng, A., Gray, E. E., McGuirl, M. M., and Klinman, J. P. (2008) J. Am. Chem. Soc. 130, 11939-11944]. Further, although the wild-type (WT) enzyme undergoes coordination of Met471 to Cu(M) in its reduced form, the generation of Met471Cys almost completely eliminates this interaction [Hess, C. R., Klinman, J. P., and Blackburn, N. J. (2010) J. Biol. Inorg. Chem. 15, 1195-1207]. The aim of this study is to identify the chemical consequence of the poor ability of Cys to coordinate Cu(M). We show that Met471Cys TβM is ~5-fold more susceptible to inactivation than the WT enzyme in the presence of the cosubstrate/reductant ascorbate and that this process is not facilitated by the substrate tyramine. The resulting 50-fold smaller ratio for turnover to inactivation in the case of Met471Cys prevents full turnover of the substrate under all conditions examined. Liquid chromatography-tandem mass spectrometry analysis of proteolytic digests of inactivated Met471Cys TβM leads to the identification of cysteic acid at position 471. While both Met and Cys side chains are expected to be similarly subject to oxidative damage in proteins, the enhanced reactivity of Met471Cys toward solution oxidants in TβM is attributed to its weaker interaction with Cu(I)(M).

Dioxygen activation by a non-heme iron(II) complex: formation of an iron(IV)-oxo complex via C-H activation by a putative iron(III)-superoxo species

Iron(III)-superoxo intermediates are believed to play key roles in oxygenation reactions by non-heme iron enzymes. We now report that a non-heme iron(II) complex activates O(2) and generates its corresponding iron(IV)-oxo complex in the presence of substrates with weak C-H bonds (e.g., olefins and alkylaromatic compounds). We propose that a putative iron(III)-superoxo intermediate initiates the O(2)-activation chemistry by abstracting a H atom from the substrate, with subsequent generation of a high-valent iron(IV)-oxo intermediate from the resulting iron(III)-hydroperoxo species.

Evidence for substrate preorganization in the peptidylglycine α-amidating monooxygenase reaction describing the contribution of ground state structure to hydrogen tunneling

Peptidylglycine α-amidating monooxygenase (PAM) is a bifunctional enzyme which catalyzes the post-translational modification of inactive C-terminal glycine-extended peptide precursors to the corresponding bioactive α-amidated peptide hormone. This conversion involves two sequential reactions both of which are catalyzed by the separate catalytic domains of PAM. The first step, the copper-, ascorbate-, and O(2)-dependent stereospecific hydroxylation at the α-carbon of the C-terminal glycine, is catalyzed by peptidylglycine α-hydroxylating monooxygenase (PHM). The second step, the zinc-dependent dealkylation of the carbinolamide intermediate, is catalyzed by peptidylglycine amidoglycolate lyase. Quantum mechanical tunneling dominates PHM-dependent C(α)-H bond activation. This study probes the substrate structure dependence of this chemistry using a set of N-acylglycine substrates of varying hydrophobicity. Primary deuterium kinetic isotope effects (KIEs), molecular mechanical docking, alchemical free energy perturbation, and equilibrium molecular dynamics were used to study the role played by ground-state substrate structure on PHM catalysis. Our data show that all Ν-acylglycines bind sequentially to PHM in an equilibrium-ordered fashion. The primary deuterium KIE displays a linear decrease with respect to acyl chain length for straight-chain N-acylglycine substrates. Docking orientation of these substrates displayed increased dissociation energy proportional to hydrophobic pocket interaction. The decrease in KIE with hydrophobicity was attributed to a preorganization event which decreased reorganization energy by decreasing the conformational sampling associated with ground state substrate binding. This is the first example of preorganization in the family of noncoupled copper monooxygenases.

Amidation of bioactive peptides: the structure of the lyase domain of the amidating enzyme

Many neuropeptides and peptide hormones require amidation of their carboxy terminal for full biological activity. The enzyme peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL; EC 4.3.2.5) catalyzes the second and last step of this reaction, N-dealkylation of the peptidyl-alpha-hydroxyglycine to generate the alpha-amidated peptide and glyoxylate. Here we report the X-ray crystal structure of the PAL catalytic core (PALcc) alone and in complex with the nonpeptidic substrate alpha-hydroxyhippuric acid. The structures show that PAL folds as a six-bladed beta-propeller. The active site is formed by a Zn(II) ion coordinated by three histidine residues; the substrate binds to this site with its alpha-hydroxyl group coordinated to the Zn(II) ion. The structures also reveal a tyrosine residue (Tyr(654)) at the active site as the catalytic base for hydroxyl deprotonation, an unusual role for tyrosine. A reaction mechanism is proposed based on this structural data and validated by biochemical analysis of site-directed PALcc mutants.

Imino-oxy acetic acid dealkylation as evidence for an inner-sphere alcohol intermediate in the reaction catalyzed by peptidylglycine alpha-hydroxylating monooxygenase

Peptidylglycine alpha-hydroxylating monooxygenase (PHM, EC 1.14.17.3) catalyzes the stereospecific hydroxylation of a glycyl alpha-carbon in a reaction that requires O(2) and ascorbate. Subsequent dealkylation of the alpha-hydroxyglycine by another enzyme, peptidylamidoglycolate lyase (PAL. EC 4.3.2.5), yields a bioactive amide and glyoxylate. PHM is a noncoupled, type II dicopper monooxygenase which activates O(2) at only a single copper atom, Cu(M). In this study, the PHM mechanism was probed using a non-natural substrate, benzaldehyde imino-oxy acetic acid (BIAA). PHM catalyzes the O-oxidative dealkylation of BIAA to benzaldoxime and glyoxylate with no involvement of PAL. The minimal kinetic mechanism for BIAA was shown to be steady-state ordered using primary deuterium kinetic isotope effects. The (D)(V/K)(APPARENT, BIAA) decreased from 14.7 +/- 1.0 as [O(2)] --> 0 to 1.0 +/- 0.2 as [O(2)] --> infinity suggesting the dissociation rate constant from the PHM x BIAA complex decreases as [O(2)] increases; thereby, reducing the steady-state concentration of [PHM](free). BIAA was further used to differentiate between potential oxidative Cu/O species using a QM/MM reaction coordinate simulation to determine which species could yield product O-dealkylation that matched our experimental data. The results of this study provided compelling evidence for the presence of a covalently linked Cu(II)-alkoxide intermediate with a quartet spin state responsible BIAA oxidation.

Galactose oxidase as a model for reactivity at a copper superoxide center

The mononuclear copper enzyme, galactose oxidase, has been investigated under steady-state conditions via O(2)-consumption assays using 1-O-methyl-alpha-D-galactopyranoside as the sugar substrate to produce an aldehyde at the C-6 position. The rate-determining step of the oxidative half-reaction was probed through the measurement of substrate and solvent deuterium and O-18 isotope effects on k(cat)/K(m)(O(2)). The reaction conforms to a ping-pong mechanism with the kinetic parameters for the reductive half, k(cat)/K(m)(S) = 8.3 x 10(3) M(-1) s(-1) at 10 degrees C and pH 7.0, comparing favorably to literature values. The oxidative half-reaction yielded a value of k(cat)/K(m)(O(2)) = 2.5 x 10(6) M(-1) s(-1). A substrate deuterium isotope effect of 32 was measured for the k(cat)/K(m)(S), while a smaller, but significant value of 1.6-1.9 was observed on k(cat)/K(m)(O(2)). O-18 isotope effects of 1.0185 with either protiated or deuterated sugar, together with the absence of any solvent isotope effect, lead to the conclusion that hydrogen atom transfer from reduced cofactor to a Cu(II)-superoxo intermediate is fully rate-determining for k(cat)/K(m)(O(2)). The measured O-18 isotope effects provide corroborative evidence for the reactive superoxo species in the dopamine beta-monooxygenase/peptidylglycine alpha-hydroxylating monooxygenase family, as well as providing a frame of reference for copper-superoxo reactivity. The combination of solvent and substrate deuterium isotope effects rules out solvent deuterium exchange into reduced enzyme as the origin of the relatively small substrate deuterium isotope effect on k(cat)/K(m)(O(2)). These data indicate fundamental differences in the hydrogen transfer step from the carbon of substrate vs the oxygen of reduced cofactor during the reductive and oxidative half-reactions of galactose oxidase.

Substituted hippurates and hippurate analogs as substrates and inhibitors of peptidylglycine alpha-hydroxylating monooxygenase (PHM)

Peptidyl alpha-hydroxylating monooxygenase (PHM) functions in vivo towards the biosynthesis of alpha-amidated peptide hormones in mammals and insects. PHM is a potential target for the development of inhibitors as drugs for the treatment of human disease and as insecticides for the management of insect pests. We show here that relatively simple ground state analogs of the PHM substrate hippuric acid (C(6)H(5)-CO-NH-CH(2)-COOH) inhibit the enzyme with K(i) values as low as 0.5microM. Substitution of sulfur atom(s) into the hippuric acid analog increases the affinity of PHM for the inhibitor. Replacement of the acetylglycine moiety, -CO-NH-CH(2)-COOH with an S-(thioacetyl)thioglycolic acid moiety, -CS-S-CH(2)-COOH, yields compounds with the highest PHM affinity. Both S-(2-phenylthioacetyl)thioglycolate and S-(4-ethylthiobenzoyl)thioglycolic acid inhibit the proliferation of cultured human prostate cancer cells at concentrations >100-fold excess of their respective K(i) values. Comparison of K(i) values between mammalian PHM and insect PHM shows differences in potency suggesting that a PHM-based insecticide with limited human toxicity can be developed.

Mechanism of the insect enzyme, tyramine beta-monooxygenase, reveals differences from the mammalian enzyme, dopamine beta-monooxygenase

Tyramine beta-monooxygenase (TbetaM) catalyzes the synthesis of the neurotransmitter, octopamine, in insects. Kinetic and isotope effect studies have been carried out to determine the kinetic mechanism of TbetaM for comparison with the homologous mammalian enzymes, dopamine beta-monooxygenase and peptidylglycine alpha-hydroxylating monooxygenase. A new and distinctive feature of TbetaM is very strong substrate inhibition that is dependent on the level of the co-substrate, O(2), and reductant as well as substrate deuteration. This has led to a model in which tyramine can bind to either the Cu(I) or Cu(II) forms of TbetaM, with substrate inhibition ameliorated at very high ascorbate levels. The rate of ascorbate reduction of the E-Cu(II) form of TbetaM is also reduced at high tyramine, leading us to propose the existence of a binding site for ascorbate to this class of enzymes. These findings may be relevant to the control of octopamine production in insect cells.

Long Distance Electron-Transfer Mechanism in Peptidylglycine α-Hydroxylating Monooxygenase: A Perfect Fitting for a Water Bridge

The active sites of copper enzymes have been the subject of many theoretical and experimental investigations from a number of years. Such studies have embraced topics devoted to the modeling of the first coordination sphere at the metallic cations up to the development of biomimetic, or bioinspired, catalytic systems. At least from the theoretical viewpoint, fewer efforts have been dedicated to elucidate how the two copper cations act concertedly in noncoupled dicopper enzymes such as peptidylglycine alpha-hydroxylating monooxygenase (PHM) and dopamine beta-monooxygenase (DbetaM). In these metalloenzymes, an electronic transfer is assumed between the two distant copper cations (11 A). Recent experimental results suggest that this transfer occurs through water molecules, a phenomenon which has been theoretically evidenced to be of high efficiency in the case of cytochrome b5 (Science, 2005, 310, 1311). In the present contribution dedicated to PHM, we overpass the common theoretical approaches dedicated to the electronic and geometrical structures of sites CuM or CuH restricted to their first coordination spheres and aim at directly comparing theoretical results to the experimentally measured activity of the PHM enzyme. To achieve this goal, molecular dynamics simulations were performed on wild-type and various mutants of PHM. More precisely, we provide an estimate of the electron-transfer efficiency between the CuM and CuH sites by means of such molecular dynamics simulations coupled to Marcus theory joined to the Beratan model to approximate the required coupling matrix elements. The theoretical results are compared to the kinetics measurements performed on wild and mutated PHM. The present work, the dynamic aspects of which are essential, accounts for the experimental results issued from mutagenesis. It supports the conclusion that an electronic transfer can occur between two copper(I) sites along a bridge involving a set of hydrogen and chemical bonds. Residue Gln170 is evidenced to be the keystone of this water-mediated pathway.

Enzymatic C–H activation by metal–superoxo intermediates

The mechanisms of four enzymes that initiate oxidation of their substrates by using mid-valent metal-superoxo intermediates, rather than the more frequently described high-valent iron-oxo complexes, to cleave relatively strong C-H bonds have come into focus in the past several years. In two of these reactions, the alternative manifold for O2 and C-H activation enables unique four-electron oxidation reactions, thus significantly augmenting Nature's arsenal for transformation of aliphatic carbon compounds. General principles of this alternative manifold, including common kinetic characteristics and thermodynamic limitations, are emerging. Recent, combined experimental and computational studies on other systems have shown how a more thorough understanding of the structures of the metal-superoxo intermediates and the mechanisms by which they cleave C-H bonds might be achieved.

The Hydrogen Peroxide Reactivity of Peptidylglycine Monooxygenase Supports a Cu(II)-Superoxo Catalytic Intermediate

We have investigated the reaction of peptidylglycine monooxygenase with hydrogen peroxide to determine whether Cu(II)-peroxo is a likely intermediate. When the oxidized enzyme was reacted with the dansyl-YVG substrate and H(2)O(2), the alpha-hydroxyglycine product was formed. The reaction was catalytic and did not require the presence of additional reductant. When (18)O-labeled H(2)O(2) was reacted with peptidylglycine monooxygenase and substrate anaerobically, oxygen in the product was labeled with (18)O and must therefore be derived from H(2)O(2). However, when the reaction was carried out with H (16)(2)O(2) in the presence of (18)O(2), 60% of the product contained the (18)O label. Therefore, the reaction must proceed via an intermediate that can react directly with dioxygen and thus scramble the label. Under strictly anaerobic conditions (in the presence of glucose and glucose oxidase, where no oxygen was released into the medium from nonenzymatic peroxide decomposition), product formation and peroxide consumption were tightly coupled, and the rate of product formation was identical to that measured under aerobic conditions. Peroxide reactivity was eliminated by a mutation at the Cu(H) center, which should not be involved in the peroxide shunt. Our data lend support to recent proposals that Cu(II)-superoxide is the active species.

Mononuclear copper(ii)–hydroperoxo complex derived from reaction of copper(i) complex with dioxygen as a model of DβM and PHM

A mononuclear copper(II)-hydroperoxo species has been generated by the reaction of Cu(I)-H2BPPA complex with dioxygen, which illustrates the enzymatic reaction process of the CuB site in the DbetaM and PHM.

The catalytic copper of peptidylglycine alpha-hydroxylating monooxygenase also plays a critical structural role

Many bioactive peptides require amidation of their carboxy terminus to exhibit full biological activity. Peptidylglycine alpha-hydroxylating monooxygenase (PHM; EC 1.14.17.3), the enzyme that catalyzes the first of the two steps of this reaction, is composed of two domains, each of which binds one copper atom (CuH and CuM). The CuM site includes Met(314) and two His residues as ligands. Mutation of Met(314) to Ile inactivates PHM, but has only a minimal effect on the EXAFS spectrum of the oxidized enzyme, implying that it contributes only marginally to stabilization of the CuM site. To characterize the role of Met(314) as a CuM ligand, we determined the structure of the Met(314)Ile-PHM mutant. Since the mutant protein failed to crystallize in the conditions of the original wild-type protein, this structure determination required finding a new crystal form. The Met(314)Ile-PHM mutant structure confirms that the mutation does not abolish CuM binding to the enzyme, but causes other structural perturbations that affect the overall stability of the enzyme and the integrity of the CuH site. To eliminate possible effects of crystal contacts, we redetermined the structure of wt-PHM in the Met(314)Ile-PHM crystal form and showed that it does not differ from the structure of wild-type (wt)-PHM in the original crystals. Met(314)Ile-PHM was also shown to be less stable than wt-PHM by differential scanning calorimetry. Both structural and calorimetric studies point to a structural role for the CuM site, in addition to its established catalytic role.

Dioxygen binds end-on to mononuclear copper in a precatalytic enzyme complex

Copper active sites play a major role in enzymatic activation of dioxygen. We trapped the copper-dioxygen complex in the enzyme peptidylglycine-alphahydroxylating monooxygenase (PHM) by freezing protein crystals that had been soaked with a slow substrate and ascorbate in the presence of oxygen. The x-ray crystal structure of this precatalytic complex, determined to 1.85-angstrom resolution, shows that oxygen binds to one of the coppers in the enzyme with an end-on geometry. Given this structure, it is likely that dioxygen is directly involved in the electron transfer and hydrogen abstraction steps of the PHM reaction. These insights may apply to other copper oxygen-activating enzymes, such as dopamine beta-monooxygenase, and to the design of biomimetic complexes.

Oxygen Activation by the Noncoupled Binuclear Copper Site in Peptidylglycine α-Hydroxylating Monooxygenase. Reaction Mechanism and Role of the Noncoupled Nature of the Active Site

Reaction thermodynamics and potential energy surfaces are calculated using density functional methods to investigate possible reactive Cu/O(2) species for H-atom abstraction in peptidylglycine alpha-hydroxylating monooxygenase (PHM), which has a noncoupled binuclear Cu active site. Two possible mononuclear Cu/O(2) species have been evaluated, the 2-electron reduced Cu(II)(M)-OOH intermediate and the 1-electron reduced side-on Cu(II)(M)-superoxo intermediate, which could form with comparable thermodynamics at the catalytic Cu(M) site. The substrate H-atom abstraction reaction by the Cu(II)(M)-OOH intermediate is found to be thermodynamically accessible due to the contribution of the methionine ligand, but with a high activation barrier ( approximately 37 kcal/mol, at a 3.0-A active site/substrate distance), arguing against the Cu(II)(M)-OOH species as the reactive Cu/O(2) intermediate in PHM. In contrast, H-atom abstraction from substrate by the side-on Cu(II)(M)-superoxo intermediate is a nearly isoenergetic process with a low reaction barrier at a comparable active site/substrate distance ( approximately 14 kcal/mol), suggesting that side-on Cu(II)(M)-superoxo is the reactive species in PHM. The differential reactivities of the Cu(II)(M)-OOH and Cu(II)(M)-superoxo species correlate to their different frontier molecular orbitals involved in the H-atom abstraction reaction. After the H-atom abstraction, a reasonable pathway for substrate hydroxylation involves a "water-assisted" direct OH transfer to the substrate radical, which generates a high-energy Cu(II)(M)-oxyl species. This provides the necessary driving force for intramolecular electron transfer from the Cu(H) site to complete the reaction in PHM. The differential reactivity pattern between the Cu(II)(M)-OOH and Cu(II)(M)-superoxo intermediates provides insight into the role of the noncoupled nature of PHM and dopamine beta-monooxygenase active sites, as compared to the coupled binuclear Cu active sites in hemocyanin, tyrosinase, and catechol oxidase, in O(2) activation.

A new proposal for the mechanism of glycine hydroxylation as catalyzed by peptidylglycine α-hydroxylating monooxygenase (PHM)

The title enzyme, peptidylglycine alpha-hydroxylating monooxygenase (PHM), is essential to the in vivo generation of a wide variety of physiologically significant alpha-amidated peptide hormones from the corresponding C-terminal glycine-extended prohormones. Over a 20-year period of time a massive amount of experimental information about the enzyme has accumulated, but its mechanism of action has remained obscure. A major stumbling block to proposed mechanisms is the fact that the two copper atoms found in the active site are fixed 11 A apart. A novel mechanism is now proposed which accommodates and, indeed, requires this separation and proceeds through energetically accessible steps. It is proposed that hydroxylation at the terminal glycine residue of the C-terminal glycine-extended prohormone proceeds first by a concerted sequence of single-electron electromeric shifts, whereby both copper atoms are oxidized to Cu(II), oxygen is reduced to peroxide coordinated to Cu(M), and the glycyl group is tautomerized to its enolate coordinately bound to Cu(H). Upon subsequent reversion to the carbonyl tautomer, by a sequence of two-electron shifts, the enolate as nucleophile reacts with peroxide as electrophile, generating product alpha-hydroxyglycine, decoordinated from Cu(H), reopening the mouth of the active-site pocket to egress of product and ingress of substrates.

Evidence that dioxygen and substrate activation are tightly coupled in dopamine beta-monooxygenase. Implications for the reactive oxygen species

Oxygen activation occurs at a wide variety of enzyme active sites. Mechanisms previously proposed for the copper monooxygenase, dopamine beta-monooxygenase (DbetaM), involve the accumulation of an activated oxygen intermediate with the properties of a copper-peroxo or copper-oxo species before substrate activation. These are reminiscent of the mechanism of cytochrome P-450, where a heme iron stabilizes the activated O2 species. Herein, we report two experimental probes of the activated oxygen species in DbetaM. First, we have synthesized the substrate analog, beta,beta-difluorophenethylamine, and examined its capacity to induce reoxidation of the prereduced copper sites of DbetaM upon mixing with O2 under rapid freeze-quench conditions. This experiment fails to give rise to an EPR-detectable copper species, in contrast to a substrate with a C-H active bond. This indicates either that the reoxidation of the enzyme-bound copper sites in the presence of O2 is tightly linked to C-H activation or that a diamagnetic species Cu(II)-O2* has been formed. In the context of the open and fully solvent-accessible active site for the homologous peptidylglycine-alpha-hydroxylating monooxygenase and by analogy to cytochrome P-450, the accumulation of a reduced and activated oxygen species in DbetaM before C-H cleavage would be expected to give some uncoupling of oxygen and substrate consumption. We have, therefore, examined the degree to which O2 and substrate consumption are coupled in DbetaM using both end point and initial rate experimental protocols. With substrates that differ by more than three orders of magnitude in rate, we fail to detect any uncoupling of O2 uptake from product formation. We conclude that there is no accumulation of an activated form of O2 before C-H abstraction in the DbetaM and peptidylglycine-alpha-hydroxylating monooxygenase class of copper monooxygenases, presenting a mechanism in which a diamagnetic Cu(II)-superoxo complex, formed initially at very low levels, abstracts a hydrogen atom from substrate to generate Cu(II)-hydroperoxo and substrate-free radical as intermediates. Subsequent participation of the second copper site per subunit completes the reaction cycle, generating hydroxylated product and water.