The impact of chromophore choice on the assembly kinetics and primary photochemistry of a red/green cyanobacteriochrome

Cyanobacteriochromes (CBCRs) are bi-stable photoreceptor proteins with high potential for biotechnological applications. Most of these proteins utilize phycocyanobilin (PCB) as a light-sensing co-factor, which is unique to cyanobacteria, but some variants also incorporate biliverdin (BV). The latter are of particular interest for biotechnology due to the natural abundance and red-shifted absorption of BV. Here, AmI-g2 was investigated, a CBCR capable of binding both PCB and BV. The assembly kinetics and primary photochemistry of AmI-g2 with both chromophores were studied in vitro. The assembly reaction with PCB is roughly 10x faster than BV, and the formation of a non-covalent intermediate was identified as the rate-limiting step in the case of BV. This step is fast for PCB, where the formation of the covalent thioether bond between AmI-g2 and PCB becomes rate-limiting. The photochemical quantum yields of the forward and backward reactions of AmI-g2 were estimated and discussed in the context of homologous CBCRs.

Automated summary

Cyanobacteriochromes (CBCRs) are photoreceptor proteins with potential for biotechnological applications, some of which can bind both phycocyanobilin (PCB) and biliverdin (BV). The assembly kinetics of CBCR AmI-g2 with both chromophores were studied, revealing that the assembly reaction with PCB is faster than with BV, where the formation of a non-covalent intermediate is the rate-limiting step. Photochemical quantum yields of forward and backward reactions of AmI-g2 were estimated and discussed in comparison with homologous CBCRs.