DESCRIPTION AND AIM
The ability to observe single molecules by fluorescence detection, first realized by Orrit et al., was a momentous achievement that has revolutionized the field of molecular optical spectroscopy and fluorescence imaging. Ensuing discoveries, innovations and applications are still proliferating at a rapid rate.
Already at an early stage, the potential of single-molecule optical detection as a tool to investigate the dynamics of enzyme catalysis was recognized. Enzymes are the cell’s workhorses, hence understanding their mechanism and function at the single-molecule level is key to understanding life’s machinery. Early investigations were pioneered by Sunney Xie and co-workers (Harvard Univ.). Instead of monitoring the concentration change of the substrate or the product to measure the reaction rate, a single-molecule experiment follows individual catalytic turnovers in real time and records the waiting times (τ) for completing individual reactions. Single-molecule kinetic theories are being developed to explore underlying catalytic mechanisms and the associated kinetic models in unprecedented detail by analysing probability distributions and statistical properties of the waiting time parameters. The molecular interpretation of such single-enzyme studies provides a new paradigm for the (phenomenological) Michaelis-Menten formalism of enzyme kinetics. These studies raise a number of fundamental questions:
1. Is the observation of conformational and catalytic heterogeneity in single-enzyme experiments a universal phenomenon?
2. Can it be proven that the conformational and catalytic heterogeneity are biologically relevant? If so, can the biological relevance be identified?
3. Is it possible to establish the molecular basis for catalytic heterogeneity in terms of the underlying conformational differences from single-enzyme behavior?
4. What are the challenges for research aimed at answering these questions?
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