Saga Yoshitaka, Hirota Keiya, Asakawa Hitoshi, Takao Kazufumi, Fukuma Takeshi
Department of Chemistry, Faculty of Science and Engineering, Kindai University , Higashi-Osaka, Osaka 577-8502, Japan.
Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency , Kawaguchi, Saitama 332-0012, Japan.
Biochemistry. 2017 Jul 11;56(27):3484-3491. doi: 10.1021/acs.biochem.7b00267. Epub 2017 Jun 28.
Light-harvesting complex 2 (LH2) is an integral membrane protein in purple photosynthetic bacteria. This protein possesses two types of bacteriochlorophyll (BChl) a, termed B800 and B850, which exhibit lowest-energy absorption bands (Q bands) around 800 and 850 nm. These BChl a pigments in the LH2 protein play crucial roles not only in photosynthetic functions but also in folding and maintaining its protein structure. We report herein the reversible structural changes in the LH2 protein derived from a purple photosynthetic bacterium, Rhodoblastus acidophilus, induced by the removal of B800 BChl a (denoted as B800-free LH2) and the reconstitution of exogenous BChl a. Atomic force microscopy observation clearly visualized the nonameric ring structure of the B800-free LH2 with almost the same diameter as the native LH2. Size exclusion chromatography measurements indicated a considerable decrease in the size of the protein induced by the removal of B800 BChl a. The protein size was almost recovered by the insertion of BChl a pigments into the B800 binding sites. The decrease in the LH2 size would mainly originate from the shrinkage of the B800 binding sites perpendicular to the macrocycle of B800 BChl a without deformation of the circular arrangement. The reversible changes in the LH2 structure induced by the removal and reconstitution of B800 BChl a will be helpful for understanding the structural principle and the folding mechanism of photosynthetic pigment-protein complexes.
光捕获复合物2（LH2）是紫色光合细菌中的一种整合膜蛋白。该蛋白含有两种类型的细菌叶绿素（BChl）a，分别称为B800和B850，它们在800和850 nm左右表现出最低能量吸收带（Q带）。LH2蛋白中的这些BChl a色素不仅在光合功能中起关键作用，而且在其蛋白质结构的折叠和维持中也起关键作用。我们在此报告了由嗜酸红假单胞菌这种紫色光合细菌衍生的LH2蛋白中，因去除B800 BChl a（记为无B800的LH2）和重新构建外源BChl a而引起的可逆结构变化。原子力显微镜观察清楚地显示了无B800的LH2的九聚体环结构，其直径与天然LH2几乎相同。尺寸排阻色谱测量表明，去除B800 BChl a会导致蛋白质尺寸显著减小。通过将BChl a色素插入B800结合位点，蛋白质尺寸几乎恢复。LH2尺寸的减小主要源于垂直于B800 BChl a大环的B800结合位点的收缩，而圆形排列未变形。去除和重新构建B800 BChl a引起的LH2结构的可逆变化将有助于理解光合色素 - 蛋白质复合物的结构原理和折叠机制。
