Ultrafast Interconversion between Protein Conformational Substates: Directly Observed by 2D IR Vibrational Echo Spectroscopy

Abstract

Conformational dynamics of flexible biomolecules play an important role in the function and stability of proteins. Folded proteins can exist in multiple conformational substates, where each substate has a distinct structure and corresponds to a local minimum on the free energy landscape. Transitions from one minimum to another correspond to dynamical changes in the structure of the protein. By using 2D IR spectroscopy, conformational interconversion between these well defined substates of a myoglobin double mutant is observed on picoseconds timescale. The conformational dynamics are directly measured through the evolution of cross peaks in the 2D IR spectra of CO bound to the heme active site. The conformational switching changes the CO frequency, as detected by the waiting time dependence of the 2D IR vibrational echo spectrum. This result is an example where conformational switching between protein substates occurs on very fast time scales. Moreover, this myoglobin mutant shows enzymatic activity upon substrate binding, which makes it an excellent system to study the influence of substrate binding on structural dynamics.