Single-Molecule Spectroscopy Studies of the Conformational Dynamics of Enzymes

Conformational motions of enzymes are highly dynamic and intrinsically stochastic. Obtaining molecular level insights into conformational dynamics of enzymes is critical for unraveling the complex intimate structure-to-function relationship. This dissertation describes the investigation of conformational dynamics of HPPK (6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase) and T4 lysozyme by single-molecule FRET (Forster/fluorescence resonance energy transfer) spectroscopy and photon stamping spectroscopy. This dissertation also demonstrates the developments of corresponding single-molecule spectroscopic approaches to serve scientifically experimental demands. Multiple conformational intermediate states and multi-dimensional conformational motions of T4 lysozyme have been observed. The Markov process has successfully reproduced the experimental observations, suggesting that T4 lysozyme hinge-bending open-close conformational changes follow multiple pathways involving multiple intermediate states. The combination of lifetime and anisotropy results presents a whole picture of multi-dimensional conformational dynamics in the process of T4 lysozyme open-close hinge-bending. The non-exponential features of both lifetime and anisotropy autocorrelation functions reveal dynamic and static inhomogeneity/complexity of multi-dimensional conformational fluctuations. The investigations of probing and manipulating HPPK conformational dynamics has been described. The consistency between the decay rate of donor lifetime and rising rate of acceptor lifetime gives a direct observation of FRET dynamic process at single-molecule level. The autocorrelation analysis of donor lifetimes have revealed intermittent conformational coherence of multiple HPPK Loop3-active site conformational states, regulated by substrate-enzyme interactions. Mechanically manipulating a targeted dye-labeled single HPPK in pinpoint nano-scale precision and simultaneously monitoring the conformational changes during the AFM pulling event has been achieved. The observed results of different lifetime fluctuations, distinct anisotropy fluctuations and various dynamic rates have suggested the existence of function-inert and function-active scenarios of HPPK Loop 3-active site conformational dynamic motions. The developments of single-molecule spectroscopic approaches have been demonstrated, including 1) single molecule photon stamping FRET spectroscopy, on the basis of only measuring the donor\u27s lifetime trajectory; 2) single-molecule AFM-FRET nanoscopy, capable of effectively pinpointing and mechanically manipulating a targeted dye-labeled single protein in a large sampling area; and 3) single-molecule multi-parameter photon stamping spectroscopy system, integrating fluorescence anisotropy-FRET-lifetime and capable of observing single-molecule multi-dimensional conformational motions