Distinct Protochromic Mechanisms Driving Green/Red Absorption in Phycocyanobilin-Binding Proteins

RcaE, a phycocyanobilin
(PCB)-binding protein, undergoes
a reversible
structural conversion, shifting light absorption between red (Pr-state)
and green (Pg-state). Using a quantum mechanical/molecular mechanical
approach combined with a linear Poisson–Boltzmann equation,
we reveal the molecular mechanisms underlying this 130 nm blue shift.
The experimentally measured Pg-RcaE absorption wavelength is reproduced
only when ring B of PCB is deprotonated. While the low-dielectric
chromophore environment remains unchanged during the Pr-to-Pg conversion,
Lys261 deprotonation in Pg-RcaE is driven by the loss of key electrostatic
interactions, specifically the loss of salt bridges with PCB propionic
groups. Unlike Slr1393g3, where a 110 nm blue shift arises from PCB
conformational changes, RcaE employs a distinct mechanism, leveraging
proton-mediated electrostatic changes while maintaining a low-dielectric
environment. This Pr-to-Pg conversion is triggered by ring B deprotonation
via Glu217, facilitated by water molecules forming a Grotthuss-like
proton transfer pathway. This unique strategy achieves efficient photochromic
switching and a large spectral shift without PCB structural rearrangements.