Role of ligand substitution on long-range electron transfer in azurins.
Azurin contains two potential redox sites, a copper centre and, at the opposite end of the molecule, a cystine disulfide (RSSR). Intramolecular electron transfer between a pulse radiolytically produced RSSR- radical anion and the blue Cu(II) ion was studied in a series of azurins in which single-site mutations were introduced into the copper ligand sphere. In the Met121His mutant, the rate constant for intramolecular electron transfer is half that of the corresponding wild-type azurin. In the His46Gly and His117Gly mutants, a water molecule is co-ordinated to the copper ion when no external ligands are added. Both these mutants also exhibit slower intramolecular electron transfer than the corresponding wild-type azurin. However, for the His117Gly mutant in the presence of excess imidazole, an azurin-imidazole complex is formed and the intramolecular electron-transfer rate increases considerably, becoming threefold faster than that observed in the native protein. Activation parameters for all these electron-transfer processes were determined and combined with data from earlier studies on intramolecular electron transfer in wild-type and single-site-mutated azurins. A linear relationship between activation enthalpy and activation entropy was observed. These results are discussed in terms of reorganization energies, driving force and possible electron-transfer pathways.
Farver
O
O
Institute of Analytical and Pharmaceutical Chemistry, The Royal Danish School of Pharmacy, Copenhagen, Denmark. of@dfh.dk
Jeuken
L J
LJ
Canters
G W
GW
Pecht
I
I
D000421:Alcaligenes / Q000235:genetics / Q000378:metabolism; D001400:Azurin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D003300:Copper / Q000737:chemistry; D003553:Cystine / Q000737:chemistry; D004579:Electron Transport; D005609:Free Radicals; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D011550:Pseudomonas aeruginosa / Q000235:genetics / Q000378:metabolism; D013816:Thermodynamics
D005609:Free Radicals; D001400:Azurin; D003553:Cystine; D003300:Copper