αヘリックス、β-シート、三次構造、四次構造――よく聞くけど、結局なんなの？ なんで生物を学ぶのに化学式が必要なの？ って思ったことのある人もいるかもしれません。この記事では生物を学ぶ人が知っておきたいタンパク質の構造や性質などをについて、簡単にわかりやすく解説しました。
タンパク質の構造
タンパク質はアミノ酸がたくさん連なった鎖です。ネックレスがタンパク質だとすると、アミノ酸はネックレスのビーズに相当します。(下図の①)
ちなみに、このアミノ酸という名の「ビーズ」は、DNAがコードしている情報です。なので、DNAにはビーズの種類を決める情報が刻まれていて、そのビーズが集まってタンパク質ができている。そして、みなさんもご存じのとおり、タンパク質は動物を構成する主な成分です。
約20種類のビーズがDNAによってコードされていますが、そのビーズはそれぞれ違う性質を持っています。
あるビーズはプラスの電気を帯びており、またあるビーズはマイナスの電気を脅えています。電荷を脅えているので、引き合ったり、退け合ったりします。
そんなビーズの性質から、ネックレスはよじれたり、ジグザグしたりするのです。こうしてよくできるのがαヘリックスと呼ばれるらせん状のネックレスの鎖と、β-シートと呼ばれるジグザグ状のネックレスの鎖です。(右図②）
ちなみに①の構造をタンパク質の一次構造、②の構造をタンパク質の二次構造といいます。これは一次元(点)、二次元(線)、三次元(平面)というときの「次」ではなく、第一次テスト、第二次テスト――というときの「次」になります。なので、「平面」、「立体」を指すわけではありません。
では次に、このタンパク質の「ネックレス」を手のひらに乗せてみましょう。ネックレスを手のひらにのせるときに、長くのばしたままでは手のひらに収まりきりませんよね。そのときは鎖はくしゃっとして手に乗せるはずです。
しかし、鎖のビーズの性質によって、曲がりやすい方向があったり、水を嫌う部分があったり、逆に水とくっつきやすい部分があったりします。細胞の中は水分で満たされているので、水を嫌う部分は内側に入り込もうとしますし、水と仲良しの部分は外側でもオッケーです。こうして、タンパク質のネックレスはある一定のかたちに折れ曲がるのです。これがタンパク質の三次構造(上図③)です。
そして、これがタンパク質の一個のユニット(分子)だとしたら、同じものがいくつも組み合わさって、目に見える大きさのタンパク質となります。分子ひとつでは目にも見えないし、身体を構成することもできませんね。
しかし、乱雑にただ固まっているのでは、砂のようにぽろぽろと崩れてしまいます。タンパク質の塊がしっかりと固まっているためには、きまった位置に決まった角度にタンパク分子が組み合わされる必要があります。これがタンパク質の四次構造(上図④)です。
タンパク質は酵素になる
このようにタンパク質にはいくつもの構造があります。その立体構造の中には活性部位となり、酵素としてはたらくものがあります。
リガンド という小分子と結合すると、タンパク質の構造(コンフォメーション)が変わることがあります。こうなると、酵素としてタンパク質がしっかりと働けなくなることもあります。
リガンドにはこのような性質があるため、製薬の分野においては、特定のタンパク質と結びつくリガンドについて様々なが研究が行われています。
また、タンパク質は温度や水素イオン濃度(pH)などの影響を受けて変形してしまいます。そうすると、タンパク質は酵素として働かなくなってしまいます。酵素それぞれに特定の最適pHがあります。
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