Crystal Structures of the Met148Leu and Ser86Asp Mutants of Rusticyanin from Thiobacillus ferrooxidans: Insights into the Structural Relationship with the Cupredoxins and the Multi Copper Proteins

The crystal structures of the Met148Leu and Ser86Asp mutants of rusticyanin are presented at 1.82 and 1.65 Å resolution, respectively. Both of these structures have two molecules in the asymmetric unit compared to the one present in the crystal form of the native protein. This provides an opportunity to investigate intramolecular electron transfer pathways in rusticyanin. The redox potential of the Met148Leu mutant (∼800 mV) is elevated compared to that of the native protein (∼670 mV at pH 3.2) while that of the Ser86Asp mutant (∼623 mV at pH 3.2) is decreased. The effect of the Ser86Asp mutation on the hydrogen bonding near the type 1 Cu site is discussed and hence its role in determining acid stability is examined. The type 1 Cu site of Met148Leu mimics the structural and biochemical characteristics of those found in domain II of ceruloplasmin and fungal laccase. Moreover, the native rusticyaninʹs cupredoxin core and the type 1 Cu site closely resemble those found in ascorbate oxidase and nitrite reductase. Structure based phylogenetic trees have been re-examined in view of the additional structural data on rusticyanin and fungal laccase. We confirm that rusticyanin is in the same class as nitrite reductase domain 2, laccase domain 3 and ceruloplasmin domains 2, 4 and 6.