Redox thermodynamics, acid-base equilibria and salt-induced effects for the cucumber basic protein. General implications for blue-copper proteins |
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Authors: | Gianantonio Battistuzzi Marco Borsari Lodovica Loschi M Sola |
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Institution: | (1) Institute of Agricultural Chemistry, University of Bologna, Viale Berti Pichat 10, I-40127 Bologna, Italy Tel.: +39-51-259790; Fax: +39-51-243362; e-mail: sola@c220.unimo.it, IT;(2) Department of Chemistry, University of Modena, Via Campi 183, I-41100 Modena, Italy, IT |
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Abstract: | The reduction potential of the basic blue-copper protein from cucumber peels (CBP) was determined through voltammetric techniques
in different conditions of temperature, pH and ionic composition of the medium. The most notable properties of CBP include
a positive entropy change upon reduction, a low-pH protonation and detachment of a metal-binding histidine in the reduced
protein, and specific binding interactions with a number of anions present in common laboratory buffers, which influence to
some extent the redox thermodynamics. The enthalpy and entropy changes accompanying reduction of the Cu(II) center were compared
with those for other blue-copper proteins and correlated with spectroscopic data, structural properties and theoretical calculations.
This allows some general considerations to be offered regarding the determinants of the reduction potential in this protein
class. It emerges, in line with previous studies of the electronic structure of blue-copper sites, that the enthalpic contribution
to the reduction potential is mainly modulated by the metal-binding interactions in the trigonal N2S ligand set, and particularly by the Cu-cysteinate bond, while the entropy term is mainly affected by solvation properties
and possibly by the weak axial bond to copper. The role of solvent exposure of the metal site in the active-site protonations
in reduced blue-copper proteins is discussed. Finally, it is shown that the Nernst-Debye-Huckel model qualitatively accounts
for the ionic strength dependence of the reduction potential.
Received: 20 December 1996 / Accepted: 26 March 1997 |
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Keywords: | Blue copper proteins Cucumber basic protein Electrochemistry Redox potential Redox thermodynamics |
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