Reaction intermediates discovered in crystal structures of enzymes

Crystal structures of enzymes have provided valuable information for the reaction mechanisms. Structures of the enzyme complex with different reaction intermediates are particularly valuable. In several cases, these structures of intermediates were discovered accidently, presumably by trapping in the crystal during freezing prior to X-ray data collection. High to atomic resolution structures reveal the detailed geometry of the reaction intermediate and its interactions within the enzyme active site. In other cases, the protein can be crystallized with its substrate, including examples of protease precursors that represent their own substrates. Examples are described of an FAD-dependent dehydrogenase, HIV protease and caspases, where the structures provide snapshots of steps in the reaction and the conformational changes occurring during the reaction. Complementary techniques such as computational chemistry, neutron crystallography, Laue crystallography, and time-resolved spectroscopy can give a more complete picture of the reaction.