Determination of Protein Surface Hydration by Systematic Charge Mutations

Area of Science:

• Biophysics
• Protein Dynamics
• Computational Biology

Background:

• Protein surface hydration is crucial for protein stability, flexibility, dynamics, and function.
• Picosecond timescale surface solvation motions have sparked debate regarding their origin: hydration water or protein charged side chains.
• Molecular dynamics simulations are often used to study these phenomena.

Purpose of the Study:

• To differentiate the contributions of hydration water and charged side chains to protein surface dynamics.
• To investigate the role of charged residues near a tryptophan probe in protein surface motion.
• To understand the mechanisms behind picosecond timescale protein surface dynamics.

Main Methods:

• Utilized a unique nuclease with a single tryptophan as a local probe.
• Systematically mutated three neighboring charged residues to assess their impact on tryptophan Stokes shifts.
• Analyzed relaxation patterns and dynamics in relation to hydration water and charged side chain environments.

Main Results:

• Observed slight increases in total tryptophan Stokes shifts with reduced neighboring charged residues.
• Found that charged side chains were insensitive to the observed relaxation patterns.
• Demonstrated a correlation between protein dynamics and hydration water relaxation, with varying timescales based on the local environment (dense charged vs. hydrophobic).

Conclusions:

• Protein surface motion on picosecond timescales is restricted.
• Total Stokes shifts are predominantly influenced by hydration water relaxation.
• Slow dynamics observed are water-driven, coupled to local protein fluctuations, rather than solely from charged side chains.