The human peripheral subunit-binding domain folds rapidly while overcoming repulsive Coulomb forces

Area of Science:

• Biochemistry
• Structural Biology
• Protein Folding

Background:

• Peripheral subunit binding domains (PSBDs) are key components of multienzyme complexes in carbohydrate metabolism.
• PSBDs are crucial for prosthetic group transfer between catalytic subunits.
• A high density of positive charges on the PSBD surface is essential for subunit binding.

Purpose of the Study:

• To investigate the folding thermodynamics and kinetics of the human PSBD (HSBD).
• To understand the role of electrostatic interactions in HSBD stability and folding.
• To explore the evolutionary trade-offs between protein function and stability.

Main Methods:

• Circular dichroism spectroscopy to study protein folding.
• Tryptophan fluorescence experiments to monitor protein dynamics.
• Analysis of ionic-strength dependent folding behavior.

Main Results:

• HSBD exhibits marginal stability under physiological conditions.
• Folding occurs rapidly (microseconds) via a barrier-limited, apparent two-state transition.
• High positive surface charge density creates repulsive forces affecting stability and kinetics.
• Electrostatic strain is mitigated by Debye-Hückel screening at high ionic strength.

Conclusions:

• HSBD folding kinetics and stability are significantly modulated by electrostatic forces.
• Differences in surface charge distribution explain varying ionic-strength dependencies among PSBDs.
• Protein evolution balances functional requirements with stability constraints.