# Proton-Coupled Chromophore and Protein Structural Changes Control Phytochrome Activation

## TL;DR

This paper explains how proton movements in phytochromes trigger structural changes that control their activation.

## Contribution

The study reveals that proton transfer, not chromophore relaxation, initiates structural transitions in phytochromes.

## Key findings

- Meta-Rc formation involves enolization and deprotonation of chromophore rings.
- pH changes induce β-sheet and α-helix interconversion in the tongue region.
- Proton transfer is essential for secondary structure transitions in phytochromes.

## Abstract

Phytochromes are sensory photoreceptors in eukaryotes
and prokaryotes
that control physiological processes. In prototypical phytochromes,
photoisomerization of the methine-bridged tetrapyrrole of the Pr state
is the first step in (de)­activating the photoreceptor. The underlying
reaction sequence runs through a series of intermediate states. Among
them, the Meta-Rc state plays a critical role since it precedes the
formation of the Pfr state, which is linked to the functional secondary
structure transition of the tongue, a phytochrome-specific peptide
segment. In this work, we have studied the structure and reactions
of Meta-Rc of the bacterial phytochrome Agp1 (Agrobacterium
fabrum) by IR difference and resonance Raman spectroscopy.
It is shown that the formation of Meta-Rc is associated with the enolization
of the terminal ring D and the deprotonation of ring B or C, whereas
reprotonation of the chromophore occurs with the decay of Meta-Rc.
Proton migration represents the essential trigger for the secondary
structure transition of the tongue since the β-sheet and α-helix
structures can be interconverted by changing the pH. The pH-dependent
conformational equilibrium is observed in Meta-Rc at 250 K and in
Pfr at 290 K, albeit with different pK
A values. The results show that the secondary structure transition
is induced by chromophore-linked proton transfer steps rather than
by conformational relaxations of the chromophore itself. In view of
previous findings on the proton dependence of the reverse process
in bathy phytochromes, we conclude that intramolecular proton transfer
is an indispensable prerequisite for the secondary structure transition
in phytochromes in general.