Unusual Photoisomerization Pathway in a Near-Infrared Light Absorbing Enzymerhodopsin.

Microbial and animal rhodopsins possess retinal chromophores which capture light and normally photoisomerize from all- trans to 13- cis and from 11 -cis to all- trans -retinal, respectively. Here, we show that a near-infrared light-absorbing enzymerhodopsin from Obelidium mucronatum (OmNeoR) contains the all- trans form in the dark but isomerizes into the 7- cis form upon illumination. The photoproduct (λ max = 372 nm; P 372 ) possesses a deprotonated Schiff base, and the system exhibits a bistable nature. The photochemistry of OmNeoR was arrested at <270 K, indicating the presence of a potential barrier in the excited state. Formation of P 372 is accompanied by protonation changes of protonated carboxylic acids and peptide backbone changes of an α-helix. Photoisomerization from the all- trans to 7- cis retinal conformation rarely occurs in any solvent and protein environments; thus, the present study reports on a novel photochemistry mediated by a microbial rhodopsin, leading from the all- trans to 7- cis form selectively.