Conservation and Diversity in the Primary Reverse Photodynamics of the Canonical Red/Green Cyanobacteriochrome Family

Abstract

In this report, we compare the femtosecond to nanosecond primary reverse photodynamics (15E Pg → 15Z Pr) of eight tetrapyrrole binding photoswitching cyanobacteriochromes in the canonical red/green family from the cyanobacterium Nostoc punctiforme. Three characteristic classes were identified on the basis of the diversity of excited-state and ground-state properties, including the lifetime, photocycle initiation quantum yield, photointermediate stability, spectra, and temporal properties. We observed a correlation between the excited-state lifetime and peak wavelength of the electronic absorption spectrum with higher-energy-absorbing representatives exhibiting both faster excited-state decay times and higher photoisomerization quantum yields. The latter was attributed to both an increased number of structural restraints and differences in H-bonding networks that facilitate photoisomerization. All three classes exhibited primary Lumi-Go intermediates, with class II and III representatives evolving to a secondary Meta-G photointermediate. Class II Meta-GR intermediates were orange absorbing, whereas class III Meta-G had structurally relaxed, red-absorbing chromophores that resemble their dark-adapted 15Z Pr states. Differences in the reverse and forward reaction mechanisms are discussed within the context of structural constraints.

Comparison of the Forward and Reverse Photocycle Dynamics of Two Highly Similar Canonical Red/Green Cyanobacteriochromes Reveals Unexpected Differences

“…A unique feature of the reverse dynamics of AnPixJg2 is the presence of the Meta-G f intermediate red-shifted from the 15 Z P r photoproduct (Figure B). Photointermediates red-shifted from the photoproduct absorption have been observed at later stages of the reverse reaction for NpR5113g2 (red/green), NpF2164g7 (orange/green; B2J668), and Slr1393 (P73184). The presence of comparable intermediate populations in the reverse dynamics of canonical red/green CBCRs and a noncanonical orange/green CBCR suggests that this population may be more ubiquitous in the reverse pathway of CBCRs.…”

“…PCC6803 and the dark-adapted state of AnPixJg2 . That said, a mechanistic role of this type of structural heterogeneity on the reaction dynamic heterogeneity observed more broadly across the extended red/green (XRG) family of CBCRs ,− remains an open question.…”

“…Both of the remaining classes resolved Lumi-G o intermediates that more rapidly evolved to secondary intermediates. Class II R (NpAF142g2, NpF2164g4, NpF2854g3, and NpR6012g4) resolved blue-shifted Meta-G y intermediates, whereas class III R (NpR1597g4, NpR4776g3, and NpR5113g2) resolved red-shifted Meta-G r intermediates on the nanosecond time scale …”

“…Class II R (NpAF142g2, NpF2164g4, NpF2854g3, and NpR6012g4) resolved blue-shifted Meta-G y intermediates, whereas class III R (NpR1597g4, NpR4776g3, and NpR5113g2) resolved red-shifted Meta-G r intermediates on the nanosecond time scale. 31 This study was undertaken to compare the static properties and dynamic photoactivity of the GAF2 photosensory domain of the AnPixJ cyanobacteriochrome (AnPixJg2) from Anabaena sp. PCC 7120 and GAF4 of the NpR6012 cyanobacteriochrome (NpR6012g4) from Nostoc punctiforme, two red/green CBCRs that share ∼68% sequence identity (Figure S1).…”