四级结构Protein quaternary structure

亚基之间的接触面通常富含疏水残基，并辅以氢键和盐桥。由相同亚基组成的称为同源寡聚体（如四聚体 α₄），由不同亚基组成的称为异源寡聚体（如血红蛋白 α₂β₂）。对称排列常见，可用循环（Cₙ）、二面体（Dₙ）或二十面体对称描述，病毒衣壳就是由数十至数千个亚基组成的二十面体结构。

Subunit interfaces are usually rich in hydrophobic residues, supported by hydrogen bonds and salt bridges. Assemblies of identical subunits are homo-oligomers (such as an α₄ tetramer) and of different subunits hetero-oligomers (such as haemoglobin, α₂β₂). Symmetric arrangements are common and are described by cyclic (Cₙ), dihedral (Dₙ) or icosahedral symmetry; virus capsids are icosahedral shells of tens to thousands of subunits.

1950 年代伯纳尔提出“四级结构”一词以描述亚基聚集。1959 年佩鲁茨解析血红蛋白结构，揭示了四个亚基的排布及其在氧合时的相对运动。1965 年莫诺、怀曼和尚热提出别构效应的协同模型（MWC 模型），1966 年科什兰等提出序变模型，用以解释多亚基蛋白的协同调节。

In the 1950s Bernal introduced the term quaternary structure for subunit assembly. Perutz's 1959 haemoglobin structure revealed the arrangement of its four subunits and how they shift on oxygenation. In 1965 Monod, Wyman and Changeux proposed the concerted (MWC) model of allostery, and in 1966 Koshland and colleagues the sequential model, to explain cooperative regulation in multi-subunit proteins.

四级结构使蛋白质能实现协同结合（如血红蛋白的 S 形氧合曲线）和别构调节（如天冬氨酸转氨甲酰酶受 CTP 反馈抑制）。多亚基复合物还能把连续的酶促反应组织在一起（如丙酮酸脱氢酶复合体），或构成分子机器（如 ATP 合酶、核糖体、蛋白酶体）。

Quaternary structure enables cooperative binding, as in haemoglobin's S-shaped oxygen curve, and allosteric regulation, as in feedback inhibition of aspartate transcarbamoylase by CTP. Multi-subunit complexes can also channel consecutive reactions (the pyruvate dehydrogenase complex) or form molecular machines such as ATP synthase, the ribosome and the proteasome.