Purple photosynthetic bacteria have LH2 as a light-harvesting protein. This protein possesses two types of bacteriochlorophyll (BChl) a, termed B800 and B850, which exhibit Qy absorption bands at around 800 and 850 nm, respectively. Light energy captured by B800 BChl a is efficiently transferred to B850 BChl a in LH2. BChl a in the LH2 protein play crucial roles not only in photosynthetic functions but also in folding and maintaining the protein structure. However, little information is available about effects of BChl a on the structural property of the LH2 protein.
In this study, we analyzed an LH2 variant denoted as B800-free LH2, in which B800 BChl a was removed from native LH2, in order to clarify the effect of B800 BChl a on the LH2 structure and spectral feature. B800-free LH2 exhibited unique spectral change under neutral pH conditions, in which the Qy absorption band of B800 BChl a was automatically recovered. This spectral change accompanied a decrease of the Qy absorption band at 850 nm and tended to stop when the absorbance of B800/B850 ratio was close to that of native LH2. B800-recovered LH2 was purified and its structural and spectroscopic features were characterized. As a result, the features of B800-recovered LH2 was quite similar to those of native LH2. These results indicate that BChl a released by decomposition of B800-free LH2 inserts into the B800 cavity of survived B800-free LH2. This study will be helpful for understanding the structural role of B800 BChl a in LH2 protein.