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

Yoshitaka Saga,Michie Imanishi,Yukihiro Kimura,Sayaka Matsui,Yuji Otsuka,Hitoshi Asakawa,Kiyoshiro Kawano

doi:10.1038/s41598-019-40082-y

Yoshitaka Saga, Michie Imanishi + Show 5 more

Open Access

https://doi.org/10.1038/s41598-019-40082-y

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Abstract

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.

Highlights

Light-harvesting antenna proteins play crucial roles in the primary process in photosynthesis
No information is available about the isolation and the detailed characterization of Light-harvesting complex 2 (LH2) proteins whose B800 BChl a is selectively converted to Chl pigments by in situ oxidation, BChl oxidation has been reported in studies on photoprotection in light-harvesting proteins in purple bacteria[18,19,20,21,22,23,24] and the functional roles of BChl pigments in proteins in green sulfur bacteria[25] and heliobacteria[26]
We first demonstrated the selective conversion of B800 BChl a to 3-acetyl Chl a in LH2 protein by in situ oxidation with DDQ and characterized the oxidized protein by spectroscopic and atomic force microscopy (AFM) measurements

Summary

Introduction

Light-harvesting antenna proteins play crucial roles in the primary process in photosynthesis. Changing BChl a to Chl-type pigments in bacterial photosynthetic proteins is one excellent strategy to control light-harvesting and energy transfer abilities. This is helpful for the elucidation of the mechanisms of energy transfer and photoprotection in the target photosynthetic proteins and for the development of novel photofunctional proteins. Bacteriochlorin pigments (7,8,17,18-tetrahydroporphyrins) are chemically oxidized by 2,3-dichloro-5,6 -dicyano-1,4-benzoquinone (DDQ) to give corresponding chlorins (17,18-dihydroporphyrins)[27] We apply this reaction to in situ conversion of B800 BChl a in LH2 as a novel methodology to regulate the photofunctional abilities. Circular dichroism (CD) spectroscopy and atomic force microscopy (AFM) indicated that the protein structure was preserved even if B800 BChl a is converted to 3-acetyl Chl a

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