Biological X-ray Absorption Spectroscopy (XAS)

Extended x-ray absorption fine structure (EXAFS) is a short-range technique that can provide structural information for a wide variety of materials and states of matter at extremely high accuracy. For metalloproteins where transition metal atoms are in structurally or functionally important site, EXAFS and x-ray absorption spectroscopy (XAS) can provide detailed information on metal-ligand bond distances and geometry, respectively. This technique can be used independently or to supplement structural information derived from X-ray crystallography or nuclear magnetic resonance spectroscopy.

The X-ray absorption spectra consist of two regions, such as the edge (XANES) and the Extended X-ray Absorption Fine Structure (EXAFS). The edge region spectrum reflects metal ion oxidation state, covalency, geometry, chemical shifts, etc. The role of metal site structure in enzymatic catalysis can be determined by comparing the XAS spectra of the active site with or without substrates or inhibitors. The EXAFS region provides direct structural information about the atomic neighbors of the metal atom such as number and identity of ligand atoms, and their precise bond distances.

Synchrotron x-ray sources with spectral distributions between 2 and 15 keV can investigate elemental transitions (K-edge) for absorber atoms from 2.5 KeV (Sulfur) to elements with higher energy such as th transition metal atoms (5-15 keV). Therefore, XAS and EXAFS using such synchrotron radiation can investigate atomic and molecular structure for a wide range of elements of interest in the fields of biosciences, environmental and soil science, and catalysis.

The Case Center for Synchrotron Bioscience affiliated beamline X3B located at NSLS provides full support for biological XAS research including focused beam and advanced solid state detector capabilities. The main focus of the X3B beamline is to support research that probes the structures and functions of proteins and other biomolecules, and to develop innovative physical and biological approaches to solving critical problems in the realm of bio-medicine. The major goals of this facility are to provide various academic and industrial institutions access to the synchrotron light source, to design experiments and actively participate in solving complex problems in biomedical research.

CSB has recently developed a high-throughput, X-ray absorption spectroscopy and fluorescence based approach to identify and characterize metalloproteins. So far, we have successfully characterized the presence/absence of transition metal contents in over 1000 protein families, which represent 15% of all known genomes. The main objective is to develop a metalloprotein annotation database which includes characterization of at least one protein from the 5000 largest protein families.