Blue Light Responses: Phenomena and Occurrence in Plants and Microorganisms II. [antikvár]

Blue Light Responses I. INTRODUCTION A sensory or developmental response of an organism to blue-wavelength light (400 to 500 nm) is triggered by the absorption of quanta by photoreceptors. The light-energized photoreceptor then initiates primary molecular reaction(s) that lead to the subsequent transduction chain; in analogy to the enzyme reaction catalyzing a substrate reaction, photoreceptor and quanta can be thought of as catalyst and substrate, respectively. One of the essential prerequisites for the efficient operation of a photobiological process in light-responsive organisms is that the photoreceptor in its excited state undergoes fast or ultrafast primary reaction(s) that lead to the sensory transduction chain (Figure 1). Such primary processes, fast enough to compete effectively with other relaxation modes of the excited state, would ensure the high photosensitivity of photobiologically responsive organisms. Figure 2 illustrates the role of primary photoreceptors in light-signal perception with flavin as the hypothetical chromophore of blue light receptors. Unlike the photoreceptors of vision and photomorphogenesis, blue light receptors remain unidentified. The question as to the possible identity of the chromophore(s) of blue light receptors has centered around the choice between flavins and carotenoids. More than 10 years ago the question as to the possible identity of the chromophore(s) of blue light receptors was examined in terms of the photophysical and photochemical reactivities of flavins and carotenoids.'-^ The main arguments were that flavins are intrinsically more likely to be the blue light photoreceptors than are carotenoids. Subsequently, it was argued that flavins could act as the primary photoreceptor for a number of responses of organisms to blue light and that the short fluorescence lifetime of flavins bound to the com coleoptile plasmalemma could serve as a photophysical assay for the photoreceptor flavins (cf. Figure 2).' " On the other hand, the photobiological function of carotenoids primarily involves the absorption of blue light as the light-harvesting antenna. The antenna function of carotenoids is facilitated by the binding of carotenoids to proteins and/or membranes and by chromophore-chrom-ophore dipolar (exciton) interactions between and/or among the bound carotenoid molecules. In the present review, spectroscopic and photochemical properties of flavins and carotenoids will be examined to ascertain whether these chromophores can act as primary blue light receptors. More recently, relative merits of flavin vs. carotenoid as blue light receptors have been reviewed by Presti,^ suggesting that carotenoids perhaps function as photoreceptors for several cases of blue light-regulated responses and flavins acting as photoreceptors more prominently in blue light phenomena. Schmidt"^ has reviewed various flavin systems as a workable model for blue light effects. The reader is referred to these reviews for extensive literature coverage of the topic discussed in this chapter. In the present chapter, flavins and carotenoids are reexamined as blue light receptors in terms of their spectroscopic and photochemical properties. Rhodopsin-like photoreceptors are treated here as a separate category from long-chain carotenoids, since their role as visual and energy-transducing photoreceptors for higher animals and Halobacterium halobium, respectively, is well established. II. FLAVINS A. Spectroscopy Flavins show three major absorption bands at about 450, 370, and 260 to 270 nm. These bands are attributable to tt-^tt* type transitions with oscillator strengths of 0.19, 0.20, and 0.66, respectively, although other types of transitions such as n-^ir* may contribute to some of these band intensities either directly by overlapping band or via vibronic mixing between n-^ir* and ii-^tt* transitions. A number of blue light action spectra in prokaryotic and eukaryotic organisms can be qualitatively matched with the absorption spectra of flavins.