Crystallographic Noise Characterizes Enzyme

A new method to identify and analyze the contributions of minor conformational states of enzymes to catalysis takes advantage of crystallographic data normally discarded as noise, reports a research team led by Dorothee Kern of Brandeis University and Tom Alber of the University of California, Berkeley (Nature 2009, 462, 669). The researchers dug into high-resolution, room-temperature X-ray crystallography data on the human proline isomerase enzyme cyclophilin A to look at electron density below the normal noise threshold and identify secondary peaks indicative of minor conformations. They then mutated a residue far from the enzyme’s active site, switching out a serine for threonine, to sterically stabilize a minor conformational form. NMR studies of the altered enzyme revealed that the mutation shifted the equilibrium between the major and minor conformers to favor the minor form, with a corresponding 300-fold slowing of conformational dynamics and reduction in catalytic turnover. The results directly demonstrate that both chemical interactions and interconversion of conformational states are necessary for catalysis, the researchers say. They add that the approach may prove to be a general strategy to identify and evaluate the roles of hidden, higher energy conformations in other enzymes.