Abstract
Flavin cofactor is known to perform diverse biological functions. Recently, its role as a photoreceptor has been identified. So far, three classes of photoactive flavoproteins have been recognized: phototropin with LOV (Light, Oxygen and Voltage) domain, blue light sensory protein with BLUF (Blue Light sensing Using Flavin adenine dinucleotide) domain and photolyase/cryptochrome protein with PHR (Photolyase Homology Region) domain. Photochemistry of flavin is the key to unravel the reaction mechanisms of photoactive flavoproteins in their biological functions such as DNA repair or signal transduction. Vibrational (Infrared and Raman) spectroscopy is a useful and sensitive tool to investigate the photochemistry of flavin in protein environments and has significantly contributed to elucidate the reaction mechanisms of these photoactive proteins. This study will survey recent advances in vibrational spectroscopic studies on this topic and remaining questions to be answered.
Highlights
Flavin, which constitutes of a group of compounds having the heteroaromatic ring system of 7,8dimethylisoalloxazine, is contained in protein as a cofactor and plays critical roles in various important processes in cells [36,47]
The structure of flavin and its atomic numbering are illustrated in Fig. 1(a), where riboflavin, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) reside
The formation of flavin-thiol adduct was engineered by introducing a cysteine residue into the BLUF domain, in which the occurrence of a LOV-like photoreaction of flavin was identified by Fourier Transform Infrared (FTIR) and UV-vis spectroscopy, suggesting that the cysteine works as the key factor in determining the reaction of flavin in LOV domain [50]
Summary
Flavin, which constitutes of a group of compounds having the heteroaromatic ring system of 7,8dimethylisoalloxazine, is contained in protein as a cofactor and plays critical roles in various important processes in cells [36,47]. The traditional biological function of flavin is of catalysis in redox reactions It was found recently, that this cofactor works as a photoreceptor to initiate the light-induced DNA repairs and signal transductions. FAD was first identified as a catalytical cofactor of photolyase in 1987 – a repair enzyme of photo-damaged DNA [20]. It was noted that CRY works as an important player in rapid light perception like rhodopsin [8]. In both photolyase and CRY, FAD is buried in a PHR domain. Photochemistry of flavin in these photoactive proteins has been studied extensively with vibrational spectroscopy including Infrared (IR) and Raman spectra. We summarize the information provided to date on these photoactive flavoproteins by vibrational spectroscopy and discuss the remaining questions to be answered
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