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ItemUnification of the Copper(I) Binding Affinities of the Metallo-chaperones Atx1, Atox1, and Related Proteins DETECTION PROBES AND AFFINITY STANDARDSXiao, Z ; Brose, J ; Schimo, S ; Ackland, SM ; La Fontaine, S ; Wedd, AG (AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC, 2011-04-01)Literature estimates of metal-protein affinities are widely scattered for many systems, as highlighted by the class of metallo-chaperone proteins, which includes human Atox1. The discrepancies may be attributed to unreliable detection probes and/or inconsistent affinity standards. In this study, application of the four Cu(I) ligand probes bicinchoninate, bathocuproine disulfonate, dithiothreitol (Dtt), and glutathione (GSH) is reviewed, and their Cu(I) affinities are re-estimated and unified. Excess bicinchoninate or bathocuproine disulfonate reacts with Cu(I) to yield distinct 1:2 chromatophoric complexes [Cu(I)L(2)](3-) with formation constants β(2) = 10(17.2) and 10(19.8) m(-2), respectively. These constants do not depend on proton concentration for pH ≥7.0. Consequently, they are a pair of complementary and stable probes capable of detecting free Cu(+) concentrations from 10(-12) to 10(-19) m. Dtt binds Cu(I) with K(D) ∼10(-15) m at pH 7, but it is air-sensitive, and its Cu(I) affinity varies with pH. The Cu(I) binding properties of Atox1 and related proteins (including the fifth and sixth domains at the N terminus of the Wilson protein ATP7B) were assessed with these probes. The results demonstrate the following: (i) their use permits the stoichiometry of high affinity Cu(I) binding and the individual quantitative affinities (K(D) values) to be determined reliably via noncompetitive and competitive reactions, respectively; (ii) the scattered literature values are unified by using reliable probes on a unified scale; and (iii) Atox1-type proteins bind Cu(I) with sub-femtomolar affinities, consistent with tight control of labile Cu(+) concentrations in living cells.
ItemMetallo-oxidase Enzymes: Design of their Active SitesXiao, Z ; Wedd, AG (CSIRO PUBLISHING, 2011-01-01)Multi-copper oxidases are a large family of enzymes prevalent in all three domains of life. They couple the one-electron oxidation of substrate to the four-electron reduction of dioxygen to water and feature at least four Cu atoms, traditionally divided into three sites: T1, T2, and (binuclear) T3. The T1 site catalyzes substrate oxidation while a trinuclear cluster (comprising combined T2 and T3 centres) catalyzes the reduction of dioxygen. Substrate oxidation at the T1 Cu site occurs via an outer-sphere mechanism and consequently substrate specificities are determined primarily by the nature of a substrate docking/oxidation (SDO) site associated with the T1 Cu centre. Many of these enzymes ‘moonlight’, i.e. display broad specificities towards many different substrates and may have multiple cellular functions. A sub-set are robust catalysts for the oxidation of low-valent transition metal ions such as FeII, CuI, and MnII and are termed ‘metallo-oxidases’. They play essential roles in nutrient metal uptake and homeostasis, with the ferroxidase ceruloplasmin being a prominent member. Their SDO sites are tailored to facilitate specific binding and facile oxidation of these low-valent metal ions and this is the focus of this review.