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.
