Incharoensakdi A, Takabe T, Akazawa T
Arch Biochem Biophys. 1986 Jul;248(1):62-70. doi: 10.1016/0003-9861(86)90401-7.
The large (A) and small (B) subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) from the cyanobacterium Aphanothece halophytica and from the purple sulfur photosynthetic bacterium Chromatium vinosum (strain D) were separated by sucrose density gradient centrifugation at low ionic strength and alkaline pH (9.3), respectively. It was found that subunit B enhances the extent of activation by CO2 and Mg2+ at equilibrium of the two homologous enzymes consisting of Aphanothece large subunit and its own small subunit (AaBa) and the Chromatium large subunit and its own small subunit (AcBc). The extent of activation induced by saturating amounts of subunit B was larger with AcBc than AaBa, amounting to 3.7- and 1.8-fold of that by each catalytic core alone, respectively. Subunit B stimulated both the extent of activation at equilibrium and catalysis in a parallel and simultaneous manner with respect to the concentration of B in both homologous enzymes. These results suggest that subunit B interacts with both activation and catalytic sites simultaneously. On the other hand, Chromatium subunit B only slightly stimulated the extent of activation in the hybrid enzyme AaBc. The role of subunit B in enhancing the extent of activation at equilibrium can be substituted by the effect exerted by 6-phosphogluconate. Both homologous enzymes AaBa and AcBc showed a faster deactivation rate when the enzyme was activated in the absence of subunit B. The mechanism by which subunit B promotes activation seems to involve its effect on stabilizing the activated enzyme molecule. From studies on the Km for substrate CO2 in the hybrid enzyme AaBc a major involvement of subunit B in influencing Km (CO2) seems unlikely.
通过在低离子强度和碱性pH（9.3）条件下进行蔗糖密度梯度离心，分别分离了来自嗜盐隐球藻的1,5 - 二磷酸核酮糖羧化酶/加氧酶（EC 4.1.1.39）的大亚基（A）和小亚基（B）以及来自紫色硫光合细菌嗜硫红假单胞菌（菌株D）的该酶的大亚基和小亚基。研究发现，亚基B增强了由嗜盐隐球藻大亚基及其自身小亚基组成的同源酶（AaBa）以及由嗜硫红假单胞菌大亚基及其自身小亚基组成的同源酶（AcBc）在CO₂和Mg²⁺平衡时的激活程度。饱和量的亚基B诱导的激活程度，AcBc比AaBa更大，分别相当于各自单独催化核心激活程度的3.7倍和1.8倍。对于两种同源酶中B的浓度，亚基B以平行且同时的方式刺激了平衡时的激活程度和催化作用。这些结果表明亚基B同时与激活位点和催化位点相互作用。另一方面，嗜硫红假单胞菌亚基B仅略微刺激了杂合酶AaBc的激活程度。亚基B在增强平衡时激活程度方面的作用可被6 - 磷酸葡萄糖酸的作用所替代。当在没有亚基B的情况下激活时，两种同源酶AaBa和AcBc均显示出更快的失活速率。亚基B促进激活的机制似乎涉及其对稳定激活的酶分子的作用。从对杂合酶AaBc中底物CO₂的米氏常数的研究来看，亚基B似乎不太可能主要参与影响米氏常数（CO₂）。
