Copper in biological systems. A report from the 7th Manziana Conference, held at Santa Severa, September 11-15, 1995

Walking the seven lines: binuclear copper A in cytochrome c oxidase and nitrous oxide reductase

The enzymes nitrous oxide reductase (N2OR) and cytochrome c oxidase (COX) are constituents of important biological processes. N2OR is the terminal reductase in a respiratory chain converting N₂O to N₂ in denitrifying bacteria; COX is the terminal oxidase of the aerobic respiratory chain of certain bacteria and eukaryotic organisms transforming O₂ to H₂O accompanied by proton pumping. Different spectroscopies including magnetic resonance techniques, were applied to show that N2OR has a mixed-valent Cys-bridged [Cu^1.5+(CyS)₂Cu^1.5+] copper site, and that such a binuclear center, called CuA, does also exist in COX. A sequence motif shared between the CuA center of N2OR and the subunit II of COX raises the issue of a putative evolutionary relationship of the two enzymes. The suggestion of a binuclear CuA in COX, with one unpaired electron delocalized between two equivalent Cu nuclei, was difficult to accept originally, even though regarded as a clever solution to many experimental observations. This minireview in honor of Helmut Sigel traces several of the critical steps forward in understanding the nature of CuA in N2OR and COX, and discusses its unique electronic features to some extent including the contributions made by the development of methodology and the discovery of a novel multi-copper enzyme. Left: X-band (9.130 GHz) and C-band (4.530 GHz, 1st harmonic display of experimental spectrum) EPR spectra of bovine heart cytochrome c oxidase, recorded at 20K. Right: Ribbon presentation of the CuA domain in cytochrome c oxidase and nitrous oxide reductase.

Respiratory transformation of nitrous oxide (N2O) to dinitrogen by Bacteria and Archaea

N2O is a potent greenhouse gas and stratospheric reactant that has been steadily on the rise since the beginning of industrialization. It is an obligatory inorganic metabolite of denitrifying bacteria, and some production of N2O is also found in nitrifying and methanotrophic bacteria. We focus this review on the respiratory aspect of N2O transformation catalysed by the multicopper enzyme nitrous oxide reductase (N2OR) that provides the bacterial cell with an electron sink for anaerobic growth. Two types of Cu centres discovered in N2OR were both novel structures among the Cu proteins: the mixed-valent dinuclear Cu(A) species at the electron entry site of the enzyme, and the tetranuclear Cu(Z) centre as the first catalytically active Cu-sulfur complex known. Several accessory proteins function as Cu chaperone and ABC transporter systems for the biogenesis of the catalytic centre. We describe here the paradigm of Z-type N2OR, whose characteristics have been studied in most detail in the genera Pseudomonas and Paracoccus. Sequenced bacterial genomes now provide an invaluable additional source of information. New strains harbouring nos genes and capability of N2O utilization are being uncovered. This reveals previously unknown relationships and allows pattern recognition and predictions. The core nos genes, nosZDFYL, share a common phylogeny. Most principal taxonomic lineages follow the same biochemical and genetic pattern and share the Z-type enzyme. A modified N2OR is found in Wolinella succinogenes, and circumstantial evidence also indicates for certain Archaea another type of N2OR. The current picture supports the view of evolution of N2O respiration prior to the separation of the domains Bacteria and Archaea. Lateral nos gene transfer from an epsilon-proteobacterium as donor is suggested for Magnetospirillum magnetotacticum and Dechloromonas aromatica. In a few cases, nos gene clusters are plasmid borne. Inorganic N2O metabolism is associated with a diversity of physiological traits and biochemically challenging metabolic modes or habitats, including halorespiration, diazotrophy, symbiosis, pathogenicity, psychrophily, thermophily, extreme halophily and the marine habitat down to the greatest depth. Components for N2O respiration cover topologically the periplasm and the inner and outer membranes. The Sec and Tat translocons share the task of exporting Nos components to their functional sites. Electron donation to N2OR follows pathways with modifications depending on the host organism. A short chronology of the field is also presented.

Synthesis, Structure, and Electrochemical Properties of Copper(I) Complexes with S/N Homoscorpionate and Heteroscorpionate Ligands

Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(Tr(Me,o)(-)(Py)) (1) and Li(Tr(Mes,Me)) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(Br(Mes)pz(o)(-)(Py))] (3) were synthesized and crystallographically characterized. The complexes [Cu(Tr(Mes,Me))](2) (4) and [Cu(Tr(Me,o)(-)(Py))](2) (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H...Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S(3)H coordination for each metal). The complex [Cu(Br(Mes)pz(o)(-)(Py))](2) (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S(2)N,N' coordination for each metal). [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl(3)) and 3/2 for 6 (CD(2)Cl(2))] are distinguishable in the (1)H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation (DeltaG()(Tc)) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH(2)Cl(2) (3 and 6) or CH(2)Cl(2)/CH(3)CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with E(pa) = +719 mV and E(pc) = +538 mV for 4, E(pa) = +636 mV and E(pc) = -316 mV for 5, and E(pa) = +418 mV and E(pc) = -319 mV for 6.

Role of the Tat ransport system in nitrous oxide reductase translocation and cytochrome cd1 biosynthesis in Pseudomonas stutzeri

By transforming N2O to N2, the multicopper enzyme nitrous oxide reductase provides a periplasmic electron sink for a respiratory chain that is part of denitrification. The signal sequence of the enzyme carries the heptameric twin-arginine consensus motif characteristic of the Tat pathway. We have identified tat genes of Pseudomonas stutzeri and functionally analyzed the unlinked tatC and tatE loci. A tatC mutant retained N2O reductase in the cytoplasm in the unprocessed form and lacking the metal cofactors. This is contrary to viewing the Tat system as specific only for fully assembled proteins. A C618V exchange in the electron transfer center CuA rendered the enzyme largely incompetent for transport. The location of the mutation in the C-terminal domain of N(2)O reductase implies that the Tat system acts on a completely synthesized protein and is sensitive to a late structural variation in folding. By generating a tatE mutant and a reductase-overproducing strain, we show a function for TatE in N2O reductase translocation. Further, we have found that the Tat and Sec pathways have to cooperate to produce a functional nitrite reductase system. The cytochrome cd1 nitrite reductase was found in the periplasm of the tatC mutant, suggesting export by the Sec pathway; however, the enzyme lacked the heme D1 macrocycle. The NirD protein as part of a complex required for heme D1 synthesis or processing carries a putative Tat signal peptide. Since NO reduction was also inhibited in the tatC mutant, the Tat protein translocation system is necessary in multiple ways for establishing anaerobic nitrite denitrification.

Interactions between carbonic anhydrase and some decarboxylating enzymes as studied by a new bioelectrochemical approach

This work presents the results of a study, carried out by recently developed amperometric bioelectrodes, on the interactions between carbonic anhydrase (CA) and the decarboxylating enzymes arginine decarboxylase (ADC), L-lysine decarboxylase (LDC), and L-ornithine decarboxylase (ODC). These are all pyridoxal-phosphate dependent enzymes and catalyze the decarboxylation reaction of the respective amino acids, to give carbon dioxide and the corresponding diamine (agmatine, cadaverine, and putrescine, respectively). The rate of each decarboxylase catalyzed reaction was measured by monitoring the production of the respective diamine by a plant tissue diamino oxidase (DAO) based bioelectrode. DAO is the enzyme which catalyzes the oxidation of agmatine, cadaverine, and putrescine with the production of NH and H2O2. DAO-based bioelectrodes consist of an amperometric H2O2 electrode, coupled to the biocatalytic membrane formed by a whole plant tissue (lentil cotyledon) containing the enzyme DAO, immobilized on a dialysis membrane by polyazetidine prepolymer (PAP). The bioelectrodes were calibrated and characterized in standard solutions of agmatine, cadaverine, and putrescine. Kinetic studies to measure decarboxylase activity were performed in the presence of different concentrations of ADC, LDC, and ODC, resulting in a lowest detection limit of 10, 25, and 10 U l(-1), respectively. The effect of bovine CA II (bCAII) was evaluated in the presence of 500 U l(-1) of each decarboxylase, showing a marked increase of the rate of the decarboxylation reaction. These results suggest that (i) CA can be used to enhance the performance of decarboxylase-based biosensors, and (ii) it possibly plays further physiological roles, acting synergistically, at specific cellular and subcellular sites, with low-activity decarboxylating enzymes.