Removal OF B800 Bacteriochlorophyll A from Two Structure-Determined Light-Harvesting Proteins 2 in Purple Photosynthetic Bacteria

Abstract

The light-harvesting complex 2 (LH2) is a peripheral antenna protein in purple photosynthetic bacteria. This protein possesses two types of bacteriochlorophyll (BChl) a termed B800 and B850. These BChl a pigments in the LH2 protein play important roles not only in the photosynthetic antenna functions but also in folding and maintaining its protein structure. We report herein removal kinetics of B800 BChl a from two structure-determined LH2 proteins derived from purple photosynthetic bacteria Rhodoblastus (Rbl.) acidophilus and Phaeospirillum (Phs.) molischianum (denoted acidophilus-LH2 and molischianum-LH2, respectively) as well as physicochemical properties of LH2 proteins without B800 BChl a. B800 BChl a was released from molischianum-LH2 much slower than acidophilus-LH2 under acidic conditions. The difference in the removal kinetics of B800 BChl a can be qualitatively explained by the difference in interactions of B800 BChl a with surrounding amino acids in the B800 binding sites between two types of LH2. Based on the results of removal kinetics of B800 BChl a, we successfully prepared LH2 proteins that selectively lacked B800 BChl a from native LH2 proteins derived from Rbl. acidophilus and Phs. molischianum. CD spectroscopy indicated that the local structures around B850 BChl a and the contents of α-helices were barely changed even if B800 BChl a was absent. Exogenous BChl a was reconstituted into both the LH2 proteins that selectively lacked B800 BChl a, and transfer the excitation energy to B850 BChl a in these proteins.