Time-Resolved Single Molecule Fluorescence Spectroscopy of an α‑Chymotrypsin Catalyzed Reaction

Single molecule fluorescence spectroscopy offers great potential for studying enzyme kinetics. A number of fluorescence reporter systems allow for monitoring the sequence of individual reaction events with a confocal microscope. When using a time-correlated single photon counting (TCSPC) detection scheme, additional information about the fluorescence lifetimes of the fluorophores can be obtained. We have applied a TCSPC detection scheme for studying the kinetics of α-chymotrypsin hydrolyzing a double-substituted rhodamine 110-based fluorogenic substrate in a two-step reaction. On the basis of the lifetime information, it was possible to discriminate the intermediate and the final product. At the high substrate concentration used, only the formation of the intermediate was observed. No rebinding of the intermediate followed by rhodamine 110 formation occurred at these high concentrations. We have further found no alterations in the fluorescence lifetime of this intermediate that would indicate changes in the local environment of the fluorophore originating from strong interactions with the enzyme. Our results clearly show the power of using lifetime-resolved measurements for investigating enzymatic reactions at the single molecule level

Dynamic Disorder in Single-Enzyme Experiments: Facts and Artifacts

Using a single-molecule fluorescence approach, the time series of catalytic events of an enzymatic reaction can be monitored, yielding a sequence of fluorescent “on”- and “off”-states. An accurate on/off-assignment is complicated by the intrinsic and extrinsic noise in every single-molecule fluorescence experiment. Using simulated data, the performance of the most widely employed binning and thresholding approach was systematically compared to change point analysis. It is shown that the underlying on- and off-histograms as well as the off-autocorrelation are not necessarily extracted from the “signal’’ buried in noise. The shapes of the on- and off-histograms are affected by artifacts introduced by the analysis procedure and depend on the signal-to-noise ratio and the overall fluorescence intensity. For experimental data where the background intensity is not constant over time we consider change point analysis to be more accurate. When using change point analysis for data of the enzyme α-chymotrypsin, no characteristics of dynamic disorder was found. In light of these results, dynamic disorder might not be a general sign of enzymatic reactions