McEvily A J, Harrison J H
J Biol Chem. 1986 Feb 25;261(6):2593-8.
Porcine heart citrate synthase, a dimeric protein of Mr = 100,000 composed of two identical subunits, is shown to undergo a monomer-dimer equilibrium. The extent of dimerization is found to be dependent on the concentration of citrate synthase, pH, ionic strength, and the specific buffer system employed. Oxaloacetate and citrate, substrates for the forward and reverse reaction catalyzed by citrate synthase, affect dimerization at concentrations of the protein which exists as monomer in their absence. The dissociation of citrate synthase dimers has been demonstrated utilizing the techniques of gel permeation chromatography, fluorescence polarization, fluorescence energy transfer, and heat denaturation. Earlier studies of citrate synthase quarternary structure found the protein to be nondissociable except under denaturing conditions or extensive modification; however, most former studies were performed at relatively high protein concentration, ionic strength, and pH, conditions which stabilize the dimer. In light of recent evidence derived from x-ray crystallographic studies showing amino acid residues from one subunit contributing to the citrate and CoA binding sites of the other, the dissociation into monomers would be expected to have profound effects on citrate synthase activity and regulation, as well as overall tricarboxylic acid cycle activity.
猪心脏柠檬酸合酶是一种分子量为100,000的二聚体蛋白质，由两个相同的亚基组成，已证明其存在单体 - 二聚体平衡。发现二聚化程度取决于柠檬酸合酶的浓度、pH值、离子强度以及所采用的特定缓冲体系。草酰乙酸和柠檬酸分别是柠檬酸合酶催化的正向和逆向反应的底物，在不存在它们时以单体形式存在的蛋白质浓度下，它们会影响二聚化。利用凝胶渗透色谱、荧光偏振、荧光能量转移和热变性等技术已证明了柠檬酸合酶二聚体的解离。早期对柠檬酸合酶四级结构的研究发现，该蛋白质除了在变性条件或广泛修饰下外不可解离；然而，大多数以前的研究是在相对较高的蛋白质浓度、离子强度和pH值（这些条件会稳定二聚体）下进行的。鉴于最近来自X射线晶体学研究的证据表明，一个亚基的氨基酸残基对另一个亚基的柠檬酸和辅酶A结合位点有贡献，预计解离成单体将对柠檬酸合酶的活性和调节以及整个三羧酸循环活性产生深远影响。
