Site-directed mutagenesis of pseudoazurin from Alcaligenes faecalis S-6; Pro80Ala mutant exhibits marked increase in reduction potential.

Abstract

Pseudoazurin (a blue copper protein or cupredoxin) of a denitrifying bacterium Alcaligenes faecalis S-6 is a direct electron carrier for a Cu-containing nitrite reductase (NIR) of the same organism. Site-directed mutagenesis of the pseudoazurin was carried out using an Escherichia coli expression system. Replacement of Tyr74 by Phe to remove an internal hydrogen bond in the beta-barrel caused a slight decrease in heat stability as well as a requirement for a higher concentration of Cu2+ for production in the E. coli host. Exchange of Ala for Pro80 adjacent to His81, one of the four ligands binding a type I Cu atom, caused a marked increase in reduction potential by 139 mV without change in the optical absorption spectrum. The ability of the pseudoazurin to transfer electrons to NIR was markedly diminished but the apparent Km of NIR for pseudoazurin was not affected by the mutation. X-ray diffraction data collected on the oxidized and reduced forms of the Pro80Ala mutant show that a water molecule occupies the pocket created by the absent side chain. This observation suggests that the increase in reduction potential may be caused due to the increased solvent accessibility to the Cu atom. The electron density difference maps on these structures (at 2.0 A) show that this water moves during the change in oxidation state, and that there are small, but localized, conformational changes greater than 6.5 A from the copper site, as well as movement of both the Cu2+ and the cysteinate sulfur.