The photoactivated adenylyl cyclase from the spirochete bacterium Leptonema illini, abbreviated LiPAC, was synthesized and characterized by absorption and fluorescence spectroscopic methods. LiPAC consists of a BLUF (Blue Light sensor Using Flavin) domain and an adenylyl cyclase homology domain (CHD). Photo-excitation of fully oxidized flavin Flox in LiPAC resulted in a typical primary (dark-adapted) BLUF domain photo-cycle dynamics. The quantum efficiency of BLUF domain signaling state formation was determined to be Φs ≈ 0.60. Continued blue-light-excitation of LiPAC in the light-adapted state caused irreversible photo-degradation of non-covalently bound Flox to covalently bound fully reduced flavin Flred with a quantum efficiency of ΦD ≈ 1.1 × 10-5. At 20 °C the time constant of signaling state recovery to the receptor state after excitation light switch-off was τrec ≈ 2.6 s. The protein thermal stability was studied by stepwise sample heating and cooling. An apparent LiPAC melting temperature of 54 °C was determined. Schemes of the primary BLUF domain photo-cycling dynamics and the secondary BLUF domain photo-degradation in the signaling state are presented.
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