Zooming in on enzyme kinetics

An enzyme is a biological catalyst: It binds to a substrate, the resulting complex forms a biologically relevant product, and the enzyme is then regenerated to do its job all over again. In bulk, the ensemble-averaged kinetics of that process was well characterized back in 1913 by the Michaelis–Menten equation. Taught to medical students ever since, the MM equation serves as a role model for quantitative biology. A Harvard University group led by Sunney Xie has now explored the equation’s microscopic underpinnings. The researchers tethered a single bacterial enzyme (β-galactosidase, essential for using sugar) and recorded waiting times between individual product molecules of the catalyzed reaction, each product giving a burst of fluorescence. Xie and coworkers found that at high substrate concentrations, short waiting times tend to be followed by short ones, and long by long. Those results revealed that the enzyme molecule is a dynamic entity with large fluctuations of its activity rate due to conformational (molecular shape) fluctuations at a broad range of time scales. Somewhat surprisingly, the MM equation still holds at the single-molecule level but with a new interpretation: Its apparent rate constant is an average over the ever-fluctuating conformations, otherwise hidden in previous experiments done on large ensembles of molecules. (B. P. English et al. Nat. Chem. Biol. 2 , 87, 2006 http://dx.doi.org/10.1038/nchembio759 .)