New Experiments May Resolve Doubts About Enzyme's Mechanism

¹⁹F nuclear magnetic resonance spectroscopy experiments seem to have resolved uncertainties about the transition-state intermediate of a key process in sugar metabolism—the β-phosphoglucomutase-catalyzed transfer of a phosphoryl group to a substrate. A few years ago, Karen N. Allen of the Boston University School of Medicine, Debra Dunaway-Mariano of the University of New Mexico, Albuquerque, and coworkers obtained the first crystallographic view of β-phosphoglucomutase’s transition state, finding that an unprecedented trigonal bipyramidal oxyphosphorane intermediate was involved in the transfer (Science 2003, 299, 2067). But a team including G. Michael Blackburn of the University of Sheffield claimed that other experimental evidence indicated the intermediate is MgF₃–, not oxyphosphorane. ¹⁹F NMR studies by Blackburn, his Sheffield colleague Jonathan P. Waltho, and coworkers now confirm that MgF₃– is the intermediate (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0910333106). The new evidence resolves the issue, Blackburn notes, because X-ray crystallography cannot differentiate fluorine from oxygen at resolutions common for protein crystals, whereas ¹⁹F NMR can. Allen comments, “It would be exciting if the group can demonstrate generality by showing the existence of MgF₃– in other phosphoryl-transfer enzymes.”