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Message definition (Protein ) The chief characteristic of proteins that also allows their diverse set of functions is their ability to bind other molecules specifically and tightly. The region of the protein responsible for binding another molecule is known as the [[binding site]] and is often a depression or "pocket" on the molecular surface. This binding ability is mediated by the tertiary structure of the protein, which defines the binding site pocket, and by the chemical properties of the surrounding amino acids' side chains. Protein binding can be extraordinarily tight and specific; for example, the [[ribonuclease inhibitor]] protein binds to human [[angiogenin]] with a sub-femtomolar [[dissociation constant]] (<10<sup>−15</sup> M) but does not bind at all to its amphibian homolog [[onconase]] (>1 M). Extremely minor chemical changes such as the addition of a single methyl group to a binding partner can sometimes suffice to nearly eliminate binding; for example, the [[aminoacyl tRNA synthetase]] specific to the amino acid [[valine]] discriminates against the very similar side chain of the amino acid [[isoleucine]].
タンパク質の最大の特徴は、他の分子と特異的かつ強固に結合できることである。他の分子と結合するタンパク質の領域は結合部位 と呼ばれ、多くの場合、分子表面の窪みや「ポケット」である。この結合能力は、結合部位ポケットを規定するタンパク質の三次構造と、周囲のアミノ酸側鎖の化学的性質によって媒介される。例えば、リボヌクレアーゼ阻害剤 タンパク質はヒトのアンジオジェニン とはフェムトモル以下の解離定数 （<10-15 M）で結合するが、両生類のホモログであるオンコナーゼ （>1 M）とは全く結合しない。例えば、アミノ酸バリン に特異的なアミノアシルtRNA合成酵素 は、アミノ酸イソロイシン の非常によく似た側鎖を識別する。