It is well known that proteins play essential roles in the proper functioning of all biological systems. Protein hydration dynamics are of fundamental importance to a protein’s structure and function as well as its recognition and interaction with substrates. It has become a consensus in the field of biochemistry that protein function, structure and dynamics are closely related, among which the dynamics holds the key to understanding how protein structure ultimately determines its biological functions. Here, we will discuss a thorough investigation of protein conformation dynamics and local hydration dynamics at the most fundamental level by integrating state-of-the-art femtosecond laser spectroscopy technology and molecular biology techniques (such as site-directed mutagenesis).
Protein conformational fluctuations can occur on many timescales, but it is the ultrafast dynamics that are still not well understood. By using an intrinsic tryptophan as the local optical probe, this group has developed optical techniques that would allow one to see, in real time, ultrafast protein hydration dynamics. The understanding of these dynamics has far-reaching implications relating to protein plasticity and recognition, protein folding and aggregation, and enzyme catalysis. With site-directed mutagenesis, tryptophan was chosen as a local optical probe, and was placed at desired positions on the peptide or protein surfaces to detect environmental response using the femtosecond-resolved fluorescence up-conversion technique. This revealed hydration dynamics with single-residue resolution. Systematic studies were first performed on small model systems, such as α-helix, β-hairpin, and a 20 residue globular protein - termed tryptophan-cage. These systems clearly show that an ordered, rigid water network has been formed even for simple secondary structures. Next, a large and flexible globular protein, calmodulin, was chosen to map the global surface hydration dynamics. For comparison, the protein was studied in its calcium free (apoCaM), calcium binding (Ca2+-CaM) and, Ca2+-CaM in complex with a target protein (CaM:Target) states. A series of water-network relaxations and coupled protein structural fluctuations were observed. The results paint a clear picture of distinct behaviors of hydration water around various protein environments, revealing that hydration water is an integral part of proteins and directly controls their structure, dynamics, and function.