X-ray absorption spectroscopy of molybdenum enzymes

One of the major goals of bioinorganic chemistry is a quantitative understanding of the structure-function relationships of metalloproteins. X-ray absorption spectroscopy (XAS) is one of the few experimental techniques that can provide structural details on active sites under almost any experimental condition. In the last few years, the field of molybdenum enzymes has been revolutionized by several protein crystal structures (see [1, 2] and references therein). Protein crystallography provides detailed information on protein folding and on the inter-relationship of various redox-active sites within the molecule. Unfortunately, the precise metrical details of the molybdenum site which are required for an adequate understanding of structure-function relationships can be lacking. This commentary will provide some perspective on the usefulness and limitations of X-ray absorption spectroscopy in providing an accurate picture of the active site of molybdenum enzymes.