Electrochemistry measures enzyme rate acceleration in nanodroplets

Enzymes are typically studied in bulk solution, but confined environments such as droplets made by electrospray ionization may more closely mimic the natural environment inside cells. Electrospray forms droplets of various sizes, which makes it difficult to quantify how droplet size affects reaction rates. Jeffrey E. Dick and coworkers at the University of North Carolina at Chapel Hill have used gold microelectrodes to electrochemically measure reaction rates of an enzyme in aqueous nanodroplets, enabling them to see how the rates are affected by droplet size (Proc. Natl. Acad. Sci. U.S.A. 2021, DOI: 10.1073/pnas.2025726118). As a model system, they used a flavin adenine dinucleotide–dependent glucose dehydrogenase in a 1,2-dichloroethane emulsion. Individual nanodroplets adsorb to the microelectrode, whose electric field drives the enzymatic reaction via the oxidation of ferrocyanide to ferricyanide, which in turn delivers electrons for enzymatic glucose oxidation and gets reduced back to ferrocyanide. The cycle generates a current until the glucose is used up. The current can be used to determine the enzyme turnover rate, which increases to as much as 100 times the rate in bulk solution as the size of the droplets decreases. The method will allow droplet-by-droplet quantitation of other reactions to see how generalized the phenomenon is. The researchers plan to study whether and how life uses this confinement-based acceleration.