Selective oxidation of B800 bacteriochlorophyll a in photosynthetic light-harvesting protein LH2.

Engineering chlorophyll (Chl) pigments that are bound to photosynthetic light-harvesting proteins is one promising strategy to regulate spectral coverage for photon capture and to improve the photosynthetic efficiency of these proteins. Conversion from the bacteriochlorophyll (BChl) skeleton (7,8,17,18-tetrahydroporphyrin) to the Chl skeleton (17,18-dihydroporphyrin) produces the most drastic change of the spectral range of absorption by light-harvesting proteins. We demonstrated in situ selective oxidation of B800 BChl a in light-harvesting protein LH2 from a purple bacterium Rhodoblastus acidophilus by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. The newly formed pigment, 3-acetyl Chl a, interacted with the LH2 polypeptides in the same manner as native B800. B850 BChl a was not oxidized in this reaction. CD spectroscopy indicated that the B850 orientation and the content of the α-helices were unchanged by the B800 oxidation. The nonameric circular arrangement of the oxidized LH2 protein was visualized by AFM; its diameter was almost the same as that of native LH2. The in situ oxidation of B800 BChl a in LH2 protein with no structural change will be useful not only for manipulation of the photofunctional properties of photosynthetic pigment-protein complexes but also for understanding the substitution of BChl to Chl pigments in the evolution from bacterial to oxygenic photosynthesis.