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The Bioinorganic Chemistry of Copper

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The Bioinorganic Chemistry of Copper Indian Journal of Chemislry Vol. 42A, September 2003, pp. 2175-2184 The bioinorganic chemistry of copper R N Mukheljee Department of Chemistry, lndian In stitute of Technology. Kanpur 208016, India Reccivcd 2 1mlumy 2003 Many enzymes and proteins have copper at their active sites, which plays a key role in biology. An important goal of bioinorganic chemistry is the development of small in organic coordination complexes that reproduce structural, spectroscopic features and functiollal aspects in a manner similar to th eir natural counterparts. To provide an overview of the activities in this field. some results Oil synthetic modelling of a selected number of copper proteins/enzymes are described in this article. Copper is one of the transItIon elements frequently Correlated with the enzymatic acrivity, the copper found at the active site of proteins. The copper­ proteins exhibit unique spectroscopic properties and, conta111mg enzymes and proteins constitute an accordingly, the proteins are divided in mainly three important class of biologically active compounds. The types. biological functions of copper protei ns/enzymes Type I copper proteins (also called "blue" copper include electron transfer. dioxygcn transport, proteins) are known to have one copper ion in the oxygenation. oxidation, reduction and active site. This copper ion shows some remarkable disproportionation 1.2 . spectroscopic features: an intense absorption around In nature, a variety of copper proteins are essential 600 nm, with an extinction coefficient of about 3000 constituents of aerobic organisms, including ~I em-I. Another characteristic feature of the Type I hemocyanins (arthropodal and molluskan O2 carriers) copper proteins is the extremely small hyperfine 4 I and enzymes that "activate" O2, promoting oxygen splitting in the EPR spectra (A II"" 40-90 x 10- cm- ). atom incorporation into biological sllbstrates. The Type II copper proteins have no distinct unique latter include tyrosinase (a monooxygenase, prop~rties . The spectroscopic data of these proteins incorporating one oxygen atom to the substrate and are comparable to those of "normai" copper reducing th e other to water) and dopamine ~­ compounds. hydroxylase (a mOllooxygenase). "Blue" multicopper Type III copper proteins contain oxidases [e .g., laccase (phenol and diamine antiferromagnetically coupled copper dimers. These oxidation), ascorbate oxidase (oxidation of [­ proteins are diamagnetic and therefore are EPR silent. ascorbate) and ceruloplasmin] promote substrate one­ In some proteins, all three types of copper sites are electron oxidation while reducing O2 to water. present. Such proteins were proposed to classify as Ceruloplasmin may be involved in copper Type IV. In ascorbate oxidase one of the copper ions metabolism. Cytochrome c oxidase transduce energy is found in a distorted tetrahedral (trigonal pyramidal) from the same 4e-/4H+ reduction of O2 occurring at a coordination with two histidines, a methionine and a heme-Cu binuclear centre, and couple this to cysteine. This resembles the active site of the blue membrane proton translocation, utilized in ATP copper protein plastocyanin. Also, a trinuclear copper synthesis. Amine oxidases and galactose oxidase site was found consisting of a Type III copper pair effect amine -7 aldehyde oxidative deaminations and and a "normal" Type II copper ion. alcohol -7 aldehyde oxidative dehydrogenations, Reactions that copper proteins carry out have long respectively. Copper ion reactions with reduced interested inorganic chemists. Copper is an important dioxygen derivatives (e.g., sLlperoxide (02'), hydrogen element in oxidation catalysts for laboratory and peroxide) are essential in Cu-Zn superoxide industrial use. Interest in the copper-dioxygen dismutase, and may be involved in copper-mediated complexes stems from the diverse occurrence of oxidative damage in biological media, including copper proteins which function as highly efficient possibly in Alzheimer's disease. biooxidation catalysts. Copper-dioxygen adducts are 2176 INDIAN J CHEM, SEC A, SEPTEMBER 2003 suggested as key reaction intermediates in these compound. However, when the structure of the biosite enzymatic reactions. The differentiation in the is known, then the complex that it reproduces has, as function of these proteins is attributed primarily to the far as possible, the known structure. A different coordination structure of the copper-dioxygen emphasis is obtained when the action of the metal in intermediate formed in the protein matrices, the protein is reproduced by a model compound and depending on the ligand donors, the geometry, and the the mechanism of a particular reaction is elucidated or coordination mode of the dioxygen. However, the partially explained. correlation between these structural factors and the The purpose of models is not necessarily to function/catalysis of the enzymes remains to be duplicate natural properties but to sharpen or focus elucidated. certain questions. The goal is to elucidate fundamental aspects of structure, spectroscopy, magnetic and electronic structure, reactivity and Importance of inorganic model chemistry chemical mechanism. A synergistic approach to the Investigations of metallobiomolecules have study of metalloenzymes can and has yielded crucial increased markedly during the last two decades. High­ information because synthetic analogues can be used resolution X-ray crystallographic results, in particular, to investigate the effects of systematic variations in have facilitated detailed considerations of structural, coordination chemistry, ligation, local environment electronic and reactivity properties at the molecular and other factors, often providing insights that cannot level. These metallobiomolecules are highly be easily attained from protein studies (Fig. 1). elaborated coordination complexes whose metal­ Reproducing complex biological reactivity within a containing sites (coordination units), termed as simple synthetic molecule is a challenging endeavour "active sites", comprising one or more metal ions and with both intellectual and aesthetic goals. their ligands, are usually the loci of electron transfer, 2 Several researchers ,4 have endeavoured to binding of exogenous molecules and catalysis. The understand the structure and function of copper demonstrated or potential relation between the proteins involved in copper(l)/02 interactions by properties of these sites and those of synthetic studying inorganic models, i.e., synthetically derived coordination complexes has contributed significantly copper(I) complexes, and their O reactivity. Such to~ the emergence of the interdisciplinary field of 2 biomimetic approaches can lead to fundamental bioinorganic chemistry. The complexity of biological insights into the copper-based chemistry. One might systems renders a detailed study of their mechanism also envision the development of reagents or catalysts very difficult. An increasingly popular method of 4f for use in practical oxidation processes • elucidating structures and mechanisms is the use of a 3 It is the purpose of this article to highlight recent simple chemical compound or system . advances in bioinorganic model (structurally Interest in elucidating or mimicking the physico­ characterized) studies on· some selected copper chemical properties of metalloproteins has led to proteins/enzymes, including some results from spurring activity in the synthesis of numerous author's laboratory. interesting coordination complexes. However, recently there has been an increased emphasis upon Protein(s) and synthetic models functional modelling of proteins. While the structural Blue copper proteins: Type I copper and spectroscopic modelling of metalloprotein active "Blue" copper proteins (azurin, plastocyanin)5, sites is an important and ongoing endeavour, the realization that coordination chemists can and should make significant contributions to reactivity studies and mechanism has become apparent. The value of SynthetiC models Metalloprotelns models for metalloproteins will always be relative. One of the difficulties encount~red in simulating a biosite is that, as time p"sses, the objective may I f'---__~I \ Establish relevant Active site change with advancing knowledge. If the structure of coordination chemistry the metal ion environment in the metalloprotein is unknown, the objective may be to reproduce some Fig. I-The synergistic relationship between studies involving property of the system in a similar model coordination metalloprotein biochemistry and inorganic modelling. MUKHERJEE: THE BIOINORGANIC CHEMISTRY OF COPPER 2177 which function as electron transport agents in a (methionine j ~. number of biochemical systems, gain their colour • 0 from an intense electronic absorption band that arises o '.3.12A 2 \ from a charge transfer transition to the Cu + ion at the 2.01A~ 1090 active site from the cysteine thiolate ligand. The (histidine)N'11'ft ~•. '0• unusually low energy of the transition results from the 101\_.'·~cu'f...l.12 A 2 (histidine) "" S (thiolate) coordination geometry about the Cu +, which involves N~"-..Io a nearly trigonal arrangement of two imidazole N 2.081..-.....-/ 135 atoms and the thiolate S atom, as shown in Fig. 2. The (a) methionine S atom is found along the trigonal axis at a long distance, 2.6-3.1 A, reflecting a weak bonding interaction; sometimes a peptide carbonyl oxygen atom is located on the other side of the trigonal plane, at an even longer distance. This bonding arrangement, together
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