| 概 要 |
このたび、公益信託分子科学研究奨励森野基金の援助を受けて、オランダの Klaas Jan Hellingwerf教授の森野レクチャーを、分子研コロキウムとの共同で開催する事になりました。
Hellingwerf教授は、外界の情報を取り入れる生物の情報伝達機構に興味を持ち、一貫して研究を続けておられます。特に、現在多くの研究者が蛋白 質による情報伝達を調べる上でのモデル蛋白質として注目している、黄色蛋白質（PYP）と呼ばれる蛋白質の研究を強力に推進している研究者として非常に著 名であります。今回は、PYPとAppAという2種類の最近注目されている蛋白質反応を高速分光法で調べられた研究について講演される予定です。
Most of the biological photosensory receptors can be classified into six major families, of which three base their functioning on E/Z isomerization of a (poly)ene containing chromophore (i.e. rhodopsins, phytochromes and xanthopsins), whereas in the other three families (i.e. cryptochromes, and LOV- and BLUF-domains) a photo-activated flavin presumably initiates light-driven electron transfer. As the initial steps that follow the photochemical activation of these receptors take place on the ultra-fast time scale, ultra-fast spectroscopy is the method of choice to study the molecular mechanism of the initiation of their signaling-state formation. Here I will discuss the results of two of these: UV/Vis- and IR-spectroscopy.
The first photosensory receptor that will be discussed is photoactive yellow protein, a p-hydroxy-cinnamic acid containing water-soluble protein with a restricted phylogenetic distribution in the Bacterial domain. Formation of an electronically excited singlet state leads to isomerization of its chromophore from the trans- to a cis ground state configuration. In part of the population of molecules this is followed by disruption of a hydrogen bond between the carbonyl oxygen atom of the chromophore and an N-H group of the polypeptide backbone and successful entrance into the photocycle. In the majority of the molecules, however, the chromophore thermally isomerizes back to the trans ground-state configuration. The stable cis conformer then initiates intra-molecular proton transfer, which in turn leads to considerable conformational change in the protein, the essence of which can be captured experimentally with multinuclear NMR spectroscopy and with computational simulations.
The second photosensory receptor that I will discuss is the BLUF domain of the transcriptional anti-repressor AppA. The first most striking property of this photoreceptor protein is the extremely long lifetime of its signaling state, which correlates with its genetic function. This signaling state is formed at the ultra-fast timescale, due to a re-arrangement of the hydrogen bonding interactions between the flavin and a nearby glutamine. Our most recent evidence suggests that this ‘Q-flip’ is initiated by reversible, flavin-mediated, electron (and hydrogen ion) abstraction from a nearby tyrosine residue.
Surprisingly, for both photosensory receptors mutant forms are easily engineered that show a significant increase in the quantum yield of signaling state formation.
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