Pro219 is an electrostatic color determinant in the light-driven sodium pump KR2

Area of Science:

• Biophysics
• Structural Biology
• Photochemistry

Background:

• Microbial rhodopsins exhibit color tuning, modulated by amino acid residues in the chromophore cavity.
• Specific amino acid residues, termed "color switches," are critical for tuning rhodopsin color.
• A recently identified G/P switch in the F helix influences spectral properties of light-driven sodium pumps.

Purpose of the Study:

• To investigate the molecular mechanism of the G/P "color switch" in the KR2 rhodopsin.
• To analyze the impact of mutations at position 219 (P219X) on KR2's spectral properties.
• To elucidate the interplay of electrostatic and steric factors in rhodopsin color tuning.

Main Methods:

• Comprehensive site-directed mutagenesis of the P219 position in KR2 rhodopsin.
• Spectroscopic analysis of absorption spectra for all 19 possible P219X mutants.
• Hybrid quantum mechanics/molecular mechanics (QM/MM) modeling to analyze structural and electronic changes.

Main Results:

• Most P219X mutants exhibited red-shifted light absorption, with P219R being a notable exception (blue-shifted).
• QM/MM analysis indicated electrostatic interactions are the primary drivers of red-shifting induced by the G/P switch.
• The blue-shifted P219R variant's spectral shift resulted from a complex interplay of electrostatic, steric, protonation, and hydrogen bonding changes.

Conclusions:

• The G/P switch primarily utilizes electrostatic interactions to tune microbial rhodopsin color.
• Steric effects contribute less significantly to spectral shifts, except in complex cases like P219R.
• Predicting color switch mechanisms requires detailed atomistic analysis, considering environmental consequences of mutations.