Rapid freeze-quench 57Fe Mossbauer spectroscopy:: Monitoring changes of an iron-containing active site during a biochemical reaction

Nuclear gamma resonance spectroscopy, also known as Mossbauer spectroscopy, is a technique that probes transitions between the nuclear ground state and a low-lying nuclear excited state. The nucleus most amenable to Mossbauer spectroscopy is Fe-57, and Fe-57 Mossbauer spectroscopy provides detailed information about the chemical environment and electronic structure of iron. Iron is by far the most structurally and functionally diverse metal ion in biology, and Fe-57 Mossbauer spectroscopy has played an important role in the elucidation of its biochemistry. In this article, we give a brief introduction to the technique and then focus on two recent exciting developments pertaining to the application of Fe-57 Mossbauer spectroscopy in biochemistry. The first is the use of the rapid freeze-quench method in conjunction with Mossbauer spectroscopy to monitor changes at the Fe site during a biochemical reaction. This method has allowed for trapping and subsequent detailed spectroscopic characterization of reactive intermediates and thus has provided unique insight into the reaction mechanisms of Fe-containing enzymes. We outline the methodology using two examples: (1) oxygen activation by the non-heme diiron enzymes and (2) oxygen activation by taurine: alpha-ketoglutarate dioxygenase (TauD). The second development concerns the calculation of Mossbauer parameters using density functional theory (DFT) methods. By using the example of TauD, we show that comparison of experimental Mossbauer parameters with those obtained from calculations on model systems can be used to provide insight into the structure of a reaction intermediate.