Biliverdin's Propionic Chains Influence Oligomerization in Sandercyanin.

Area of Science:

• Biophysics
• Structural Biology
• Spectroscopy

Background:

• Sandercyanin is a tetrameric biliprotein with a non-covalently bound biliverdin (BV) chromophore.
• It exhibits mild fluorescence and a large Stokes shift, making it a candidate for bioimaging.
• Current limitations include its tetrameric structure and moderate fluorescence quantum yield.

Purpose of the Study:

• To investigate the role of biliverdin propionic acid tail protonation states on sandercyanin structure and spectroscopy.
• To identify key residues involved in oligomerization and their response to chromophore protonation.
• To provide mechanistic insights for engineering monomeric sandercyanin mutants with enhanced fluorescence for bioimaging.

Main Methods:

• Utilized a microstate model for biliverdin titration.
• Employed constant pH molecular dynamics simulations.
• Analyzed protonation states in apo protein, monomeric mutant, and tetrameric sandercyanin.

Main Results:

• Identified specific residues potentially mediating oligomerization in response to BV presence and tail protonation.
• Demonstrated that BV tail protonation impacts chromophore geometry.
• Observed that absorption properties are less affected by tails compared to fluorescence.

Conclusions:

• The protonation state of BV propionic tails influences sandercyanin chromophore geometry and likely fluorescence.
• Specific residues play a crucial role in sandercyanin oligomerization.
• Understanding these mechanisms is key to engineering improved fluorescent proteins for bioimaging applications.