Secondary Structure and Pd(II) Coordination in S-Layer Proteins from Bacillus sphaericus Studied by Infrared and X-Ray Absorption Spectroscopy

摘要

The S-layer of Bacillus sphaericus strain JG-A12, isolated from a uranium-mining site, exhibits a high metal-binding capacity, indicating that it may provide a protective function by preventing the cellular uptake of heavy metals and radionuclides. This property has allowed the use of this and other S-layers as self-assembling organic templates for the synthesis of nanosized heavy metal cluster arrays. However, little is known about the molecular basis of the metal-protein interactions and their impact on secondary structure. We have studied the secondary structure, protein stability, and Pd^(II) coordination in S-layers from the B. sphaericus strains JG-A12 and NCTC 9602 to elucidate the molecular basis of their biological function and of the metal nanocluster growth. Fourier transform infrared spectroscopy reveals similar secondary structures, containing ∼35% β-sheets and little helical structure. pH-induced infrared absorption changes of the side-chain carboxylates evidence a remarkably low pK < 3 in both strains and a structural stabilization when Pd^(II) is bound. The COO⁻-stretching absorptions reveal a predominant Pd^(II) coordination by chelation/bridging by Asp and Glu residues. This agrees with XANES and EXAFS data revealing oxygens as coordinating atoms to Pd^(II). The additional participation of nitrogen is assigned to side chains rather than to the peptide backbone. The topology of nitrogen- and carboxyl-bearing side chains appears to mediate heavy metal binding to the large number of Asp and Glu in both S-layers at particularly low pH as an adaptation to the environment from which the strain JG-A12 has been isolated. These side chains are thus prime targets for the design of engineered S-layer-based nanoclusters.