The structure of many proteins is mostly described in three levels, but a quaternary structural level must be considered if the protein consists of more than one polypeptide chain.
许多蛋白质结构主要用三种标准描述，但是如果蛋白质包含一个以上的多肽链，就必需考虑四级结构标准。
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Some proteins consist of more than one polypeptide chain, and the association of these subunits in the protein gives rise to the quaternarystructure.