Dioxygen activation by enzymes containing binuclear non-heme iron clusters

ing species placed symmetrically between the endo and exo hydrogens could result in a G value of I, with subsequent asymmetric placement of the oxygen atom selectively at the exo face. The specificity of oxygen atom transfer from the exo face of the camphor skeleton is further demonstrated by the epoxidation of 5 ,6-dehydrocamphor.211 High-resolution NMR and 180rlabeling studies indicate that the product is completely the exo isomer and one atom from molecular oxygen is inserted to form the epoxide. This further emphasizes the apparent geometry of the activated oxygen species as being more closely situated at the exo face of camphor. As for hydroxylation of the natural substrate, the stereochemistry of the epoxidation reaction is the same for the NADH-, iodosobenzene-, and HzOz-driven turnovers. Epoxidations in model systems have been well documented. 234 As with the hydroxylation reactions, differences in the stereochemistry of epoxidation by Mn(III)TPP versus Fe(IIl)TPP may reflect differences in the lifetime of the radical species generated. The absolute stereoselectivity of oxygen insertion at the exo face of camphor is very reminiscent of the rigorous stereochemical integrity of product formation observed with the P-450scc enzyme. Here, cholesterol is hydroxylated at the 22and 20-position to afford (20R, 22R)-dihydroxyBACfERIAL P-450 ENZYMES 477 cholesterol as an intermediate in the production of pregnenolone.235-237 No other stereoisomeric products are obtained with those steroid-metabolizing activities. Nevertheless, the stereochemical parallel to P-450cam cannot be extended to the hydrogen abstraction step since evidence for such a chemical step is tenuous at best in these systems. When 22R-[223H]cholesterol was incubated with a side-chain cleavage preparation, the 22R-[22-3H]-22-hydroxycholesterol intermediate had negligible loss of radioactivity. This suggests selectivity in the hydrogen abstraction if it occurs. On the other hand, the loss of even a small amount of tritium may in fact suggest possible abstraction of either enantiotopic hydrogen in the natural cholesterol, especially when one consdiers the large isotopic selectivity expected for hydrogen rather than tritium. So, whereas, the insertion of the oxygens into the cholesterol side chain demonstrates strict stereochemical selectivity, a systematic investigation of the stereochemistry of the putative hydrogen abstraction steps has not been performed. As mentioned previously, when norbornane is hydroxylated by P-450LM20 both the 2-exoand 2-endo-alcohols are produced. 193 Also, the four exo-hydrogens were replaced by deuterium, and the deuterium-containing product alcohols were examined. It was found that the exo-2norborneol contained four deuteriums, whereas the endo-2-norborneol contained three deuteriums. This demonstrates an interesting contrast to the P-450cam stereochemistry, assuming both P-450LM2 and P-450cam proceed via a similar hydrogen-abstracted substrate species. These norborneol products demonstrate endo abstraction followed by exo rebound, and exo abstraction followed by endo rebound. Thus, both P-450LM2 and P-450cam are capable of endo or exo hydrogen abstraction, although P450cam displays absolute stereochemical control in the oxygen transfer step. This lack of complete stereochemical integrity of oxygen insertion is perhaps explained by a great degree of substrate motion at the active site of P-450LM2 as compared to P-450cam. A comparison of substrate regioselection has been made directly between these two isozymes using camphor, adamantanone, and adamantane as substrates. 154 As expected, P-450LM2 gave both 5-exoand 5-endo-hydroxycamphor, a result analogous to what was observed with norbornane. Also evident were 3-exoand 3-endo-hydroxycamphor. The greater stereoselectivity of oxygen insertion by P-450cam was observed with the other substrates as well (Table V). With P-450cam as the catalyst, 5-hydroxyadamantanone and 1-adamantanol were produced from the nonhydroxylated substrates. As the 5and 1-carbons of these substrates are tertiary, only one isomeric alcohol is possible for these products. With P-450LM2 , adamantanone was processed to 4-antiand 5-hydroxyadamantanone, while adamantane was hydroxylated at both the 1and 2-positions. The authors predict which

Cytochrome P-450

Molecular imaging is the visualization, characterization and measurement of biological processes at the molecular and cellular levels in humans and other living systems. Molecular imaging agents are probes used to visualize, characterize and measure biological processes in living systems. These two definitions were put forth by the Sociey of Nuclear Medicine (SNM) in 2007 as a way to capture the interdisciplinary nature of this relatively new field. The emergence of molecular imaging as a scientific discipline is a result of advances in chemistry, biology, physics and engineering, and the application of imaging probes and technologies has reshaped the philosophy of drug discovery in the pharmaceutical sciences by providing more cost effective ways to evaluate the efficacy of a drug candidate and allowing pharmaceutical companies to reduce the time it takes to introduce new therapeutics to the marketplace. Finally the impact of molecular imaging on clinical medicine has been extensive since it allows a physician to diagnose a patient’s illness, prescribe treatment and monitor the efficacy of that treatment non-invasively.

Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) were the first molecular imaging modalities used clinically. SPECT requires the use of a contrast agent labeled with a gamma emitting radionuclide, which should have an ideal gamma energy of 100-250 keV. These gamma rays are recorded by the detectors of a dedicated gamma camera or SPECT instrument and after signal processing can be converted into an image indentifying the localization of the radiotracer. PET requires the injected radiopharmaceutical to be labeled with a positron emitting radionuclide. As the radionuclide decays it ejects a positron from its nucleus which travels a short distance before being annihilated with an electron to release two 511 keV gamma rays 180° apart that are detected by the PET scanner (Figure 1). After sufficient acquisition time the data are reconstructed using computer based algorithms to yield images of the radiotracer’s location within the organism. When compared to SPECT, PET has greater advantages with respect to sensitivity and resolution and has been gaining in clinical popularity, with the number of PET-based studies expected to reach 3.2 million by 2010.1 While SPECT and PET technology has been around for decades, its use remained limited because of the limited availability of relevant isotopes which had to be produced in nuclear reactors or particle accelerators. However, the introduction of the small biomedical cyclotron, the self-contained radionuclide generator and the dedicated small animal or clinical SPECT and PET scanners to hospitals and research facilities has increased the demand for SPECT and PET isotopes.



Figure 1

Cartoon depicting the fundamental principle of Positron Emission Tomography (PET). As the targeting group interacts with the cell surface receptor, the positron emitting radio-metal decays by ejecting β+ particles from its nucleus. After traveling ...



Traditional PET isotopes such as 18F, 15O, 13N and 11C have been developed for incorporation into small molecules, but due to their often lengthy radio-syntheses, short half-lives and rapid clearance, only early time points were available for imaging, leaving the investigation of biological processes, which occur over the duration of hours or days, difficult to explore. With the continuing development of biological targeting agents such as proteins, peptides, antibodies and nanoparticles, which demonstrate a range of biological half-lives, a need arose to produce new radionuclides with half-lives complementary with their biological properties. As a result, the production and radiochemistry of radiometals such as Zr, Y, In, Ga and Cu have been investigated as radionuclide labels for biomolecules since they have the potential to combine their favorable decay characteristics with the biological characteristics of the targeting molecule to become a useful radiopharmaceutical (Tables ​(Tables11 and ​and22).2



Table 1

Gamma- and Beta-Emitting Radiometals





Table 2

Positron-Emitting Radiometals



The number of papers published describing the production or use of these radiometals continues to expand rapidly, and in recognition of this fact, the authors have attempted to present a comprehensive review of this literature as it relates to the production, ligand development and radiopharmaceutical applications of radiometals (excluding 99mTc) since 1999. While numerous reviews have appeared describing certain aspects of the production, coordination chemistry or application of these radiometals,2-18 very few exhaustive reviews have been published.10,12 Additionally, this review has been written to be used as an individual resource or as a companion resource to the review written by Anderson and Welch in 1999.12 Together, they provide a literature survey spanning 50 years of scientific discovery. To accomplish this goal, this review has been organized into three sections: the first section discusses the coordination chemistry of the metal ions Zr, Y, In, Ga and Cu and their chelators in the context of radiopharmaceutical development; the second section describes the methods used to produce Zr, Y, In, Ga and Cu radioisotopes; and the final section describes the application of these radiometals in diagnostic imaging and radiotherapy.

/pdf/coordinating-radiometals-of-copper-gallium-indium-yttrium-3clup681fj.pdf

Coordinating Radiometals of Copper, Gallium, Indium, Yttrium and Zirconium for PET and SPECT Imaging of Disease

Radiometals comprise many useful radioactive isotopes of various metallic elements When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, eg67Ga, 99mTc, 111In, 177Lu) and positron emission tomography (PET, eg68Ga, 64Cu, 44Sc, 86Y, 89Zr), as well as therapeutic applications (eg47Sc, 114mIn, 177Lu, 90Y, 212/213Bi, 212Pb, 225Ac, 186/188Re) A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents

Matching chelators to radiometals for radiopharmaceuticals.

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and trans...

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

Convenient synthesis of bifunctional tetraaza macrocycles.

Oxygen Activation by Metalloporphyrins. Formation and Decomposition of an Acylperoxymanganese(III) Complex

Mossbauer spectra of a polycrystalline form of oxidized chloro‐5,10,15,20‐tetra(mesityl)porphyrin‐ atoiron(III) [Fe(TMP)Cl], compound A, were recorded over a range of temperatures (4.2–195 K) and magnetic fields (0–6 T). These spectra of compound A exhibit magnetic features which are markedly different from those of the analogous protein complexes, horse radish peroxidase compound I (HRP‐I) and compound ES of cytochrome c peroxidase, even though chemical evidence and optical spectroscopy indicate that compound A is similar to the others in comprising a Fe(IV) complex within a porphyrin cation radical. We interpret the data by employing a spin Hamiltonian model in which the central Fe(IV) complex, with S=1, is tightly coupled to a S=1/2 system of the oxidized porphyrin to yield a net S=3/2 system as suggested by the susceptibility measurements. The theoretical treatment yields information on the d‐electron energies which is similar to that more directly available in the peroxidase spectra. The strength of ...

/pdf/mossbauer-effect-study-of-tight-spin-coupling-in-oxidized-2x4lcken20.pdf

Mössbauer effect study of tight spin coupling in oxidized chloro-5,10,15,20-tetra(mesityl)porphyrinatoiron(III)

The elucidation of the molecular mechanisms of biological oxygen activation has been the focus of sustained attention for over a decade.1 In this time, cytochrome P-450 has become a Rosetta stone among the hemecontaining monooxygenases.2,3 The wide variety of oxygenations mediated by P-450 and the significance of these processes in steroid metabolism, drug detoxification, and the carcinogenic activation of polycyclic aromatic hydrocarbons have stimulated an effort to understand these processes. The selective hydroxylation of unactivated alkanes, in particular, lacks a classical paradigm in organic chemistry. Accordingly, there has been an effort to develop synthetic models of P-450 which might be used for the oxyfunctionalization of hydrocarbons. An understanding of the mechanism of these simple cases has begun to provide a conceptual base for the understanding of the enzymatic pathway.

Metalloporphyrin Models for Cytochrome P-450

The ligand N-[2-amino-3-(p-isothiocyanatophenyl)propyl]-(±)-trans-1,2-diaminocyclohexane-N,N′,N″-pentaacetic acid has been synthesized and linked to IgG and to monoclonal antibody B72.3, and labelled with 206Bi and 212Bi to demonstrate the in vivo stability of the label and its utility for 212Bi-radioimmunotherapy.

Thomas J. McMurry

Papers

Convenient synthesis of bifunctional tetraaza macrocycles.

Oxygen Activation by Metalloporphyrins. Formation and Decomposition of an Acylperoxymanganese(III) Complex

Mössbauer effect study of tight spin coupling in oxidized chloro-5,10,15,20-tetra(mesityl)porphyrinatoiron(III)

Metalloporphyrin Models for Cytochrome P-450

An effective chelating agent for labelling of monoclonal antibody with 212Bi for α-particle mediated radioimmunotherapy