Permeability and ammonia selectivity in aquaporin TIP2;1: linking structure to function.

Area of Science:

• Biophysics
• Structural Biology
• Computational Biology

Background:

• Aquaporin TIP2;1 facilitates water and ammonia transport.
• The structural basis for ammonia selectivity is not well understood.
• Previous experiments showed a double mutation impacts ammonia permeability.

Purpose of the Study:

• To computationally investigate the structural and functional effects of a double mutation in aquaporin TIP2;1.
• To explain the observed changes in water and ammonia permeability.
• To explore potential gating mechanisms in the aquaporin channel.

Main Methods:

• Microsecond molecular dynamics simulations were performed.
• Two force fields (CHARMM36 and Amber ff99SB-ILDN) were utilized.
• Permeability and free energy calculations for water and ammonia were conducted.

Main Results:

• One force field showed a 2.5-fold decrease in water permeability and a 4-fold decrease in ammonia permeability for the double mutant.
• Water selectivity increased by a factor of 1.6.
• Increased pore-water interactions and a narrower pore were observed, leading to decreased water entropy.
• Spontaneous pore opening and closing events suggested a gating mechanism.

Conclusions:

• The study successfully reproduced and explained the effects of the double mutation on aquaporin TIP2;1 function.
• Altered pore dimensions and water-protein interactions contribute to enhanced water selectivity.
• Molecular dynamics simulations are effective for studying subtle mutation effects and long-timescale events.