Selenocysteine positional variants reveal contributions to copper binding from cysteine residues in domains 2 and 3 of human copper chaperone for superoxide dismutase

Trapping intermediates in metal transfer reactions of the CusCBAF export pump of Escherichia coli

Escherichia coli CusCBAF represents an important class of bacterial efflux pump exhibiting selectivity towards Cu(I) and Ag(I). The complex is comprised of three proteins: the CusA transmembrane pump, the CusB soluble adaptor protein, and the CusC outer-membrane pore, and additionally requires the periplasmic metallochaperone CusF. Here we used spectroscopic and kinetic tools to probe the mechanism of copper transfer between CusF and CusB using selenomethionine labeling of the metal-binding Met residues coupled to RFQ-XAS at the Se and Cu edges. The results indicate fast formation of a protein-protein complex followed by slower intra-complex metal transfer. An intermediate coordinated by ligands from each protein forms in 100 ms. Stopped-flow fluorescence of the capping CusF-W44 tryptophan that is quenched by metal transfer also supports this mechanism. The rate constants validate a process in which shared-ligand complex formation assists protein association, providing a driving force that raises the rate into the diffusion-limited regime.

Copper in Eukaryotes

Copper is essential for normal growth and development of eukaryotic organisms. Numerous physiological processes rely on sufficient availability of copper: from indispensable reactions such as mitochondrial respiration to more highly specialized processes such as pigment development in a skin. Copper misbalance has been linked to a variety of metabolic and neurodegenerative disorders in humans. Complex cellular machinery has evolved to mediate copper uptake, compartmentalization and incorporation into target proteins. Extensive studies revealed a predominant utilization of methionines and histidines by copper handling molecules for copper capture at the extracellular surface and delivery to cuproenzymes in the lumen of cellular compartments, respectively. Cu(I) is a predominant form within the cell, and copper binding and distribution inside the cell at the cytosolic sites relies heavily on cysteines. The selectivity and directionality of copper transfer reactions is determined by thermodynamic and kinetic factors as well as spatial distribution of copper donors and acceptors. In this chapter, we review current structural and mechanistic data on copper transport and distribution in yeast and mammalian cells and highlight important issues and questions for future studies.

Stable Cu(II) and Cu(I) mononuclear intermediates in the assembly of the CuA center of Thermus thermophilus cytochrome oxidase

CuA is a dinuclear mixed-valence center located in subunit 2 of the ba(3)-type cytochrome oxidase from Thermus thermophilus. The assembly of this site within the periplasmic membrane is believed to be mediated by the copper chaperones Sco and/or PCuAC, but the biological mechanisms are still poorly understood, thereby stimulating interest in the mechanisms of CuA formation from inorganic ions. The formulation of the CuA center as an electron-delocalized Cu(1.5)-Cu(1.5) system implicates both Cu(II) and Cu(I) states in the metalation process. In earlier work we showed that selenomethionine (SeM) substitution of the coordinated M160 residue provided a ligand-directed probe for studying the copper coordination environment via the Se XAS signal, which was particularly useful for interrogating the Cu(I) states where other spectroscopic probes are absent. In the present study we have investigated the formation of mixed-valence CuA and its M160SeM derivative by stopped-flow UV-vis, EPR, and XAS at both Cu and Se edges, while the formation of fully reduced di-Cu(I) CuA has been studied by XAS alone. Our results establish the presence of previously undetected mononuclear intermediates and show important differences from the metalation reactions of purple CuA azurin. XAS spectroscopy at Cu and Se edges has allowed us to extend mechanistic inferences to formation of the di-Cu(I) state which may be more relevant to biological CuA assembly. In particular, we find that T. thermophilus CuA assembles more rapidly than reported for other CuA systems and that the dominant intermediate along the pathway to mixed-valence is a new green species with λ(max) = 460 nm. This intermediate has been isolated in a homogeneous state and shown to be a mononuclear Cu(II)-(His)(Cys)(2) species with no observable Cu(II)-(Met) interaction. Reduction with dithionite generates its Cu(I) homologue which is again mononuclear but now shows a strong interaction with the Met160 thioether. The results are discussed within the framework of the "coupled distortion" model for Cu(II) thiolates and their relevance to biological metalation reactions of the CuA center.

Cu(I) affinities of the domain 1 and 3 sites in the human metallochaperone for Cu,Zn-superoxide dismutase

The delivery of copper by the human metallochaperone CCS is a key step in the activation of Cu,Zn-superoxide dismutase (SOD1). CCS is a three-domain protein with Cu(I)-binding CXXC and CXC motifs in domains 1 and 3, respectively. A detailed analysis of the binding of copper to CCS, including variants in which the Cys residues from domains 1 and 3 have been mutated to Ser, and also using separate domain 1 and 3 constructs, demonstrates that CCS is able to bind 1 equiv of Cu(I) in both of these domains. The Cu(I) affinity of domain 1 is approximately 5 × 10(17) M(-1) at pH 7.5, while that of domain 3 is at least 1 order of magnitude weaker. The CXXC site will therefore be preferentially loaded with Cu(I), suggesting that domain 1 plays a role in the acquisition of the metal. The delivery of copper to the target occurs via domain 3 whose structural flexibility and ability to be transiently metalated during copper delivery appear to be more important than the Cu(I) affinity of its CXC motif. The Cu(I) affinity of domain 1 of CCS is comparable to that of HAH1, another cytosolic copper metallochaperone. CCS and HAH1 readily exchange Cu(I), providing a mechanism whereby cross-talk can occur between copper trafficking pathways.

Interactions of Cu(I) with selenium-containing amino acids determined by NMR, XAS, and DFT studies

Cu(I) coordination by organoselenium compounds was recently reported as a mechanism for their prevention of copper-mediated DNA damage. To establish whether direct Se-Cu coordination may be involved in selenium antioxidant activity, Cu(I) coordination of the selenoamino acids methyl-Se-cysteine (MeSeCys) and selenomethionine (SeMet) was investigated. NMR results in D(2)O indicate that Cu(I) binds to the Se atom of both MeSeCys and SeMet as well as the carboxylic acid oxygen atom(s) or amine nitrogen atoms. X-ray absorption spectroscopy (XAS) and density functional theory (DFT) results confirm Se-Cu coordination, with the identification of a 2.4 Å Se-Cu vector in both the Se- and Cu-EXAFS data. XAS studies also show Cu(I) in an unusual three-coordinate environment with the additional two ligands arising from O/N (2.0 Å). DFT models of 1:1 Cu-selenoamino acid complexes suggest that both selenoamino acids coordinate Cu(I) through the selenium and amino groups, with the third ligand assumed to be water. These compounds represent the first structurally characterized copper(I) complexes with sulfur- or selenium-containing amino acids.

A copper-hydrogen peroxide redox system induces dityrosine cross-links and chemokine oligomerisation

The activity of the chemoattractant cytokines, the chemokines, in vivo is enhanced by oligomerisation and aggregation on glycosaminoglycan (GAG), particularly heparan sulphate, side chains of proteoglycans. The chemokine RANTES (CCL5) is a T-lymphocyte and monocyte chemoattractant, which has a minimum tetrameric structure for in vivo activity and a propensity to form higher order oligomers. RANTES is unusual among the chemokines in having five tyrosine residues, an amino acid susceptible to oxidative cross-linking. Using fluorescence emission spectroscopy, Western blot analysis and LCMS-MS, we show that a copper/H2O2 redox system induces the formation of covalent dityrosine cross-links and RANTES oligomerisation with the formation of tetramers, as well as higher order oligomers. Amongst the transition metals tested, namely copper, nickel, mercury, iron and zinc, copper appeared unique in this respect. At high (400 μM) concentrations of H2O2, RANTES monomers, dimers and oligomers are destroyed, but heparan sulphate protects the chemokine from oxidative damage, promoting dityrosine cross-links and multimer formation under oxidative conditions. Low levels of dityrosine cross-links were detected in copper/H2O2-treated IL-8 (CXCL8), which has one tyrosine residue, and none were detected in ENA-78 (CXCL5), which has none. Redox-treated RANTES was fully functional in Boyden chamber assays of T-cell migration and receptor usage on activated T-cells following RANTES oligomerisation was not altered. Our results point to a protective, anti-oxidant, role for heparan sulphate and a previously unrecognised role for copper in chemokine oligomerisation that may offer an explanation for the known anti-inflammatory effect of copper-chelators such as penicillamine and tobramycin.

Cellular copper distribution: a mechanistic systems biology approach

Copper is an essential but potentially harmful trace element required in many enzymatic processes involving redox chemistry. Cellular copper homeostasis in mammals is predominantly maintained by regulating copper transport through the copper import CTR proteins and the copper exporters ATP7A and ATP7B. Once copper is imported into the cell, several pathways involving a number of copper proteins are responsible for trafficking it specifically where it is required for cellular life, thus avoiding the release of harmful free copper ions. In this study we review recent progress made in understanding the molecular mechanisms of copper transport in cells by analyzing structural features of copper proteins, their mode of interaction, and their thermodynamic and kinetic parameters, thus contributing to systems biology of copper within the cell.

Structural and biophysical properties of the pathogenic SOD1 variant H46R/H48Q

Over 100 mutations in the gene encoding human copper-zinc superoxide dismutase (SOD1) cause an inherited form of the fatal neurodegenerative disease amyotrophic lateral sclerosis (ALS). Two pathogenic SOD1 mutations, His46Arg (H46R) and His48Gln (H48Q), affect residues that act as copper ligands in the wild type enzyme. Transgenic mice expressing a human SOD1 variant containing both mutations develop paralytic disease akin to ALS. Here we show that H46R/H48Q SOD1 possesses multiple characteristics that distinguish it from the wild type. These properties include the following: (1) an ablated copper-binding site, (2) a substantially weakened affinity for zinc, (3) a binding site for a calcium ion, (4) the ability to form stable heterocomplexes with the copper chaperone for SOD1 (CCS), and (5) compromised CCS-mediated oxidation of the intrasubunit disulfide bond in vivo. The results presented here, together with data on pathogenic SOD1 proteins coming from cell culture and transgenic mice, suggest that incomplete posttranslational modification of nascent SOD1 polypeptides via CCS may be a characteristic shared by familial ALS SOD1 mutants, leading to a population of destabilized, off-pathway folding intermediates that are toxic to motor neurons.