Jp. Germanas et al., SITE SATURATION OF THE HISTIDINE-46 POSITION IN PSEUDOMONAS-AERUGINOSA AZURIN - CHARACTERIZATION OF THE HIS46ASP COPPER AND COBALT PROTEINS, Biochemistry, 32(30), 1993, pp. 7698-7702
Cassette mutagenesis has been used to replace the copper ligand His46
of Pseudomonas aeruginosa azurin with 19 other amino acids and a stop
codon. Several mutant proteins were expressed in Escherichia coli and
isolated; however, only the variant in which His was replaced by Asp e
xhibited the spectral characteristics of a blue (type 1) center. The s
pectroscopic and electrochemical properties of this mutant protein sho
w that the copper site is perturbed relative to wild-type azurin. The
absorption spectrum of Cu(II)(His46Asp) azurin exhibits a S(Cys)-->Cu(
II) band at 612 nm, as well as weaker features at approximately 300, 4
54, and approximately 850 nm; its EPR spectrum is rhombic (g(parallel-
to) = 2.327(1), g(x) almost-equal-to 2.03, and g(y) almost-equal-to 2.
07; A(parallel-to) = 22(2) x 10(-4), A(x) almost-equal-to 46 x 10(-4),
and A(y) almost-equal-to 22 x 10(-4) cm-1). The reduction potential o
f the mutant (260 mV vs NHE at pH 8.5; 297 mV at pH 5.0) is lower than
that of wild-type azurin (288 mV at pH 8.5; 349 mV at pH 5.0). The S(
Cys)-->Co(II) absorption bands (approximately 300 and 362 nm) in Co(II
)(His46Asp) azurin are strongly blue-shifted relative to those (330 an
d 375 nm) in the spectrum of the Co(II)(His46) protein, whereas the in
tensities of the ligand-field bands in the 500-650-nm region (epsilon
almost-equal-to 100 M-1 cm-1) indicate a five-coordinate Co(II) enviro
nment.