Memory landscapes of single-enzyme molecules

Immobilized single horseradish peroxidase enzymes were observed by confocal fluorescence spectroscopy during catalysis of the oxidation reaction of th e nonfluorescent dihydrorhodamine 6G substrate into the highly fluorescent product rhodamine 6G. By extracting only the non-Markovian behavior of the spectroscopic two-state process of enzyme-product complex formation and rel ease, memory landscapes were generated for single-enzyme molecules. The mem ory landscapes can be used to discriminate between different origins of str etched exponential kinetics that are found in the first-order correlation a nalysis. Memory landscapes of single-enzyme data shows oscillations that ar e expected in a single-enzyme system that possesses a set of transient stat es. Alternative origins of the oscillations may not, however, be ruled out. The data and analysis indicate that substrate interaction with the enzyme selects a set of conformational substates for which the enzyme is active.