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ユビセミキノンの機能(2) Complex 3中のUQ

2014-04-05 14:23:24 | ラジカル

 

800pxcomplex_iii_reaction_svg

 
図1 複合体IIIの模式図

 

複合体IIIはシトクロム b を起源に Fe-S タンパク質およびシトクロム c が付加されてできたとされている。ユビセミキノンは、呼吸鎖複合体(シトクロムbc1複合体)III内のプロトンキノンサイクル機構(スカラー反応)に関与している。

複合体IIIはユビキノールからシトクロム cに電子伝達を行い、「ユビキノール:シトクロムc 酸化還元酵素」とも呼ばれる。好気呼吸を行う真核生物はすべてミトコンドリア内膜に複合体 III を所持している。現在、ウシシトクロム bc1 複合体の立体構造が明らかになっている。複合体 III の構成は:

1)シトクロム b(ユビキノールの酸化を行う)

2)リスケ鉄硫黄タンパク質

3)シトクロム c1(シトクロム c に電子伝達を行う)

電子伝達は以下の手順で行われる。

4)ユビキノール → リスケ Fe-S タンパク質 → シトクロム cFe2+

ただし、シトクロム b でのスカラー反応により、以下の電子伝達も行われる。

5)ユビキノール → ヘム bL → ヘム bH → リスケ Fe-S タンパク質 → シトクロム cFe2+

複合体IIIにおいては、複合体Iや複合体IVとは異なる機構でプロトンが膜外に輸送される。複合体I、IVにおいてはプロトンポンプ機構と言う、輸送を受けるプロトンが膜内から膜外に輸送されるのみである。しかしながら複合体IIIにおいては『プロトンキノンサイクル機構』という独自の輸送機構を用いている。プロトンキノンサイクル機構とは、膜内部においてプロトンが消費され、その還元力を使用して膜外側でのプロトンの放出が見られる現象である(この反応をスカラー反応と言う)。実際輸送を受けるプロトンは膜内から放出されるわけではなく、見かけ上そのように見えるだけなのでプロトンポンプ機構とはことなる機構であることが理解できる。その素反応の詳細は、以下の反応ステップからなる。

1)複合体IIIのシトクロムbの膜外側に存在するユビキノール酸化部位 (QP部位あるいはQO)でユビキノールが酸化される。

 2)具体的には、ユビキノールから2電子が抜き取られ、同時に膜外側へプロトンが2分子放出される3)その2電子は異なる方向にそれぞれ伝達される。その内約は以下の通りである。1個目の電子は、可動性リスケ鉄硫黄タンパク質を経て、シトクロムc1、シトクロムcの順番に電子伝達される。 2個目の電子は、シトクロムbに存在する2つのヘム(ヘムbL, bH)を経て膜内側に存在するユビキノン還元部位(QN部位あるいはQI部位)に電子伝達される。つぎに、QN部位に電子が二電子伝達されることにより、ユビキノンは二電子還元を受けてユビキノールとなり、再びプロトンキノンサイクル機構に組み込まれる。

以上が、プロトンキノンサイクル機構の反応であるが、この中でも特に優れた機構は可動性リスケ鉄硫黄タンパク質の関与する、電子伝達の方向性を変化させる過程である。

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