Ligand loop effects on the free energy change of redox and pH-dependent equilibria in cupredoxins probed on amicyanin variants.
In this work, we have determined the thermodynamic parameters of the reduction of four different variants of Thiobacillus versutus amicyanin by electrochemical techniques. In addition, the thermodynamic parameters were determined of the low-pH conformational change involving protonation of the C-terminal histidine ligand and the concomitant dissociation of this histidine from the Cu(I) ion. In these variants, the native C-terminal loop containing the Cys, His, and Met copper ligands has been replaced with the corresponding polypeptide segments of Pseudomonas aeruginosa azurin, Populus nigra plastocyanin, Alcaligenes faecalis S-6 pseudoazurin, and Thiobacillus ferrooxidans rusticyanin. For the reduction reaction, each loop invariably holds an entropic "memory" of the mother protein. The thermodynamics of the low-pH transition vary in a fashion that is species-dependent. When present, the memory effect again shows a large entropic component. In particular, loop elongation tends to favor the formation of the Cu(I)-His bond (hence disfavors His protonation, yielding lower pK(a) values) probably due to an increased flexibility of the loop in the reduced state. Overall, it appears that both reduction and low-pH transition are loop-responsive processes. The spacing between the ligands mostly affects the change in the conformational freedom that accompanies the reaction.
Battistuzzi
Gianantonio
G
Department of Chemistry and SCS Center, University of Modena and Reggio Emilia, Via Campi 183, 41100 Modena, Italy.
Borsari
Marco
M
Canters
Gerard W
GW
di Rocco
Giulia
G
de Waal
Ellen
E
Arendsen
Yvonne
Y
Leonardi
Alan
A
Ranieri
Antonio
A
Sola
Marco
M
D019943:Amino Acid Substitution / Q000235:genetics*; D001400:Azurin / Q000737:chemistry*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D003300:Copper / Q000737:chemistry; D019277:Entropy; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D010084:Oxidation-Reduction; D011487:Protein Conformation; D017434:Protein Structure, Tertiary / Q000235:genetics; D012997:Solvents; D055672:Static Electricity; D013816:Thermodynamics
D001426:Bacterial Proteins; D008024:Ligands; D012997:Solvents; C050107:cupredoxin; C017079:mauC protein, Methylobacterium extorquens; D001400:Azurin; D003300:Copper