Locked 5Zs-biliverdin blocks the Meta-RA to Meta-RC transition in the functional cycle of bacteriophytochrome Agp1

The bacteriophytochrome Agp1 was reconstituted with a locked 5Zs-biliverdin in which the C4C5 and C5–C6 bonds of the methine bridge between rings A and B are fixed in the Z and syn configuration/conformation, respectively. In Agp1-5Zs the photoconversion proceeds via the Lumi-R intermediate to Meta-RA, but the following millisecond-transition to Meta-RC is blocked. Consistently, no transient proton release was detected. The photoconversion of Agp1-5Zs is apparently arrested in a Meta-RA-like intermediate, since the subsequent syn to anti rotation around the C5–C6 bond is prevented by the lock. The Meta-RA-like photoproduct was characterized by its distinctive CD spectrum suggesting a reorientation of ring D.