分子モーター（Molecular Motor）は、分子機械（分子レベルの機械装置・究極のナノテクノロジー）における素子の一つです。モーター機能を単一分子（もしくは複数分子の複合体）の分子で実現してしまおう、というものです。

人工的にはロタキサン・カテナンなどの超分子骨格を用いて研究されることが多いですが、動力学機構を実際に備えたものはほとんど報告がありません。

　冒頭の図でも示されている、オレフィン架橋型ヘリセン分子は、オランダ・グローニンゲン大学のBen Feringaによって1999年に開発されました。^[1]熱/光のエネルギーを周期的にかけてやることで一方向にだけ回転するという、ユニークな特性を持ちます。

　東北大の原田宜之らは、世界で始めて動力機構を備えた光動力分子モーターの構築に成功しています。これは、オレフィンのシスートランス光異性化を回転の動力源とし、ツメ歯車効果、モーター分子のキラリティの特性を生かしたものです。

　一方で、生物界で知られるたんぱく質・RNAでできた分子の中には、モーター挙動を示すものが知られています。

　ATP合成酵素(ATP synthase)は、アデノシン三リン酸（ATP）を分解・消費して上にくっついたF1部位を回転させることが実験的に観測されています。^[2]ATPを燃料とした分子モーターというわけです。

　では逆に、これを人力的に回してやればどうなるでしょうか？なんと、これによってATPが化学合成されてくるそうです（！）。モーターに対する”発電機”の関係と同じというわけですね。これを実証せしめた^[3]のは日本人研究者です。こんなミクロな世界にまで人工物と自然のアナロジーが観られるというのは、本当に面白いですね。

関連文献

[1] “Light-driven monodirectional molecular rotor”

Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa, B. L. Nature 1999, 401, 152. DOI: 10.1038/43646

Attempts to fabricate mechanical devices on the molecular level^1, 2have yielded analogues of rotors³, gears⁴, switches⁵, shuttles^6, 7, turnstiles⁸ and ratchets⁹. Molecular motors, however, have not yet been made, even though they are common in biological systems¹⁰. Rotary motion as such has been induced in interlocked systems^11, 12, 13 and directly visualized for single molecules¹⁴, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon–carbon double bond in a chiral, helical alkene, with each 360° rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180° rotation around the carbon–carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for ‘molecular machinery’ driven by light.

[2] “Direct observation of the rotation of F1-ATPase”

Noji, H.; Yasuda, R.; Yoshida, M.; Kinoshita, K. Nature 1997, 286, 299. DOI: 10.1038/386299a0

Cells employ a variety of linear motors, such as myosin^1–3, kinesin⁴ and RNA polymerase⁵, which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum⁶ (a complex of about 100 protein molecules⁷). We now show that a single molecule of F₁-ATPase acts as a rotary motor, the smallest known, by direct observation of its motion. A central rotor of radius ~1 nm, formed by its γ-subunit, turns in a stator barrel of radius ~5nm formed by three α– and three β-subunits⁸. F₁ ATPase, together with the membrane-embedded proton-conducting unit F₀, forms the H⁺-ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthetic cells^9,10. It has been suggested that the γ-subunit of F₁-ATPase rotates within the αβ-hexamer¹¹, a conjecture supported by structural⁸, biochemical^12,13 and spectroscopic¹⁴ studies. We attached a fluorescent actin filament to the γ-subunit as a marker, which enabled us to observe this motion directly. In the presence of ATP, the filament rotated for more than 100 revolutions in an anticlockwise direction when viewed from the ‘membrane’ side. The rotary torque produced reached more than 40 pN nm ^−l under high load.

[3] “Mechanically driven ATP synthesis by F1-ATPase”

Itoh, H.; Takahashi, A.; Adachi, K.; Noji, H.; Yasuda, R.; Yoshida, M.; Kinoshita, K. Nature 2004, 427, 465.  DOI:10.1038/nature02212

ATP, the main biological energy currency, is synthesized from ADP and inorganic phosphate by ATP synthase in an energy-requiring reaction^1, 2, 3. The F₁ portion of ATP synthase, also known as F₁-ATPase, functions as a rotary molecular motor: in vitro its γ-subunit rotates⁴against the surrounding α₃β₃ subunits⁵, hydrolysing ATP in three separate catalytic sites on the β-subunits. It is widely believed that reverse rotation of the γ-subunit, driven by proton flow through the associated F_o portion of ATP synthase, leads to ATP synthesis in biological systems^{1, 2, 3, 6, 7}. Here we present direct evidence for the chemical synthesis of ATP driven by mechanical energy. We attached a magnetic bead to the γ-subunit of isolated F₁ on a glass surface, and rotated the bead using electrical magnets. Rotation in the appropriate direction resulted in the appearance of ATP in the medium as detected by the luciferase–luciferin reaction. This shows that a vectorial force (torque) working at one particular point on a protein machine can influence a chemical reaction occurring in physically remote catalytic sites, driving the reaction far from equilibrium.

関連書籍

関連リンク

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• 分子モーター – Wikipedia
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• Synthetic Molecular Motors – Wikipedia
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• 分子モーターの話題（生活環境化学の部屋）
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• ATP synthase – Wikipedia
• ATP合成酵素 – Wikipedia
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