Bacteriorhodopsin photocycle at cryogenic temperatures reveals distributed barriers of conformational substates

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 104; no. 23; pp. 9621 - 9626
• Main Authors: Dioumaev, Andrei K, Lanyi, Janos K
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 05-06-2007; National Acad Sciences
• Subjects: Activation energy; Ambient temperature; Bacteria; Bacteriorhodopsins; Bacteriorhodopsins - chemistry; Bacteriorhodopsins - radiation effects; Biochemistry; Biological Sciences; Biophysical Phenomena; Biophysics; Cryogenic temperature; Heat; High temperature; Infrared radiation; Kinetics; Light; Low temperature; Photochemistry; Pigments; Protein Conformation; Reaction kinetics; Spectral bands; Spectroscopy, Fourier Transform Infrared; Spectrum analysis; Substrates; Temperature
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0703859104

The time course of thermal reactions after illumination of 100% humidified bacteriorhodopsin films was followed with FTIR spectroscopy between 125 and 195 K. We monitored the conversion of the initial photoproduct, K, to the next, L intermediate, and a shunt reaction of the L state directly back to the initial BR state. Both reactions can be described by either multiexponential kinetics, which would lead to apparent end-state mixtures that contain increasing amounts of the product, i.e., L or BR, with increasing temperature, or distributed kinetics. Conventional kinetic schemes that could account for the partial conversion require reversible reactions, branching, or parallel cycles. These possibilities were tested by producing K or L and monitoring their interconversion at a single temperature and by shifting the temperature upward or downward after an initial incubation and after their redistribution. The results are inconsistent with any conventional scheme. Instead, we attribute the partial conversions to the other alternative, distributed kinetics, observed previously in myoglobin, which arise from an ensemble of frozen conformational substates at the cryogenic temperatures. In this case, the time course of the reactions reflects the progressive depletion of distinct microscopic substates in the order of their increasing activation barriers, with a distribution width for K to L reaction of [almost equal to]7 kJ/mol.