PubMed · 13328557
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A K APATENKO, B N ABRASHENKOV. 1956. [Universal light-microprojector for microscopy].. https://pubmed.ncbi.nlm.nih.gov/13328557/
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Light-harvesting complex II (LHC-II) regulates the light energy distribution between photosystem I and II in plants. This process is mediated by phosphorylation of the LHC-II protein, which depends on the oxidation state of photosynthetic electron carriers. In addition to this regulatory mechanism, it has recently been proposed that light can directly induce a conformational change in isolated LHC-II. To provide biophysical evidence for such a conformational change in the protein, we studied infrared absorbance changes in isolated LHC-II upon exposure to light flashes. Compared to the signals obtained with other proteins that exhibit well-characterized conformational changes, the signal in the LHC-II difference spectra is very weak. The position of the difference bands coincides with the main IR absorption bands of chlorophyll. We conclude that there are no detectable light-induced changes in the LHC protein structure and attribute the observed IR signals to light-induced chlorophyll degradation.
The unusual temperature-dependent excited-state dynamics of a stilbene-antibody complex reported by Simeonov et al. are explored using theoretical methods. The anomalous temperature-dependent fluorescence emission and lifetime are shown to be the result of interplay among temperature-modulated protein flexibility, the excited-state potential surface for the stilbene central twist, and changes in the stilbene charge distribution upon excitation. Stilbene is found to possess a similar geometry and orientation within the antibody binding pocket at all temperatures in the ground state and at low temperatures (approximately 240 K) in the excited state. At higher temperatures (approximately 260 K), the excited-state conformation twists around the central double bond and adopts an alternate orientation within the binding pocket. These changes result from protein side chain and loop motions that are frozen out at lower temperatures and account for the observed red shift of the fluorescence emission spectrum (a calculated shift of 3.8 kcal mol-1 compares favorably with the approximately 5 kcal.mol-1 observed experimentally). Approximately 3.0 kcal mol-1 of this stabilization is global in nature and is not attributable to specific local interactions. Local interactions between stilbene and Tyr-B39 contribute approximately 0.8 kcal mol-1 to the fluorescence shift. The primary structural change in simulations of the high temperature excited state involves a decrease in the stilbene-tyrosine distance and a relative change in orientation of the aromatic rings. We identify several nearby charged residues that contribute to the fluorescence emission shift and provide targets for mutagenesis to probe the temperature-dependent dynamics of protein-chromophore interactions.
We have examined scavenging of a superoxide by various prenyllipids occurring in thylakoid membranes, such as plastoquinone-9, alpha-tocopherolquinone, their reduced forms, and alpha-tocopherol, measuring oxygen uptake in hexane-extracted and untreated spinach thylakoids with a fast oxygen electrode under flash-light illumination. The obtained results demonstrated that all the investigated prenyllipids showed the superoxide scavenging properties, and plastoquinol-9 was the most active in this respect. Plastoquinol-9 formed in thylakoids as a result of enzymatic reduction of plastoquinone-9 by ferredoxin-plastoquinone reductase was even more active than the externally added plastoquinol-9 in the investigated reaction. Scavenging of superoxide by plastoquinol-9 and other prenyllipids could be important for protecting membrane components against the toxic action of superoxide. Moreover, our results indicate that vitamin K(1) is probably the most active redox component of photosystem I in the generation of superoxide within thylakoid membranes.