A Dual Role for Ca²⁺ in Expanding the Spectral Diversity and Stability of Light-Harvesting 1 Reaction Center Photocomplexes of Purple Phototrophic Bacteria

Abstract

The light-harvesting 1 reaction center (LH1-RC) complex in the purple sulfur bacterium Thiorhodovibrio (Trv.) strain 970 cells exhibits its LH1 Q y transition at 973 nm, the lowest-energy Q y absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Q y transition. Growth of Trv. strain 970 did not occur in cultures free of Ca2+, and elemental analysis of Ca2+-grown cells confirmed that purified Trv. strain 970 LH1-RC complexes contained Ca2+. The LH1 Q y band of Trv. strain 970 was blue-shifted from 959 to 875 nm upon Ca2+ depletion, but the original spectral properties were restored upon Ca2+ reconstitution, which also occurs with the thermophilic purple bacterium Thermochromatium (Tch.) tepidum. The amino acid sequences of the LH1 α- and β-polypeptides from Trv. strain 970 closely resemble those of Tch. tepidum; however, Ca2+ binding in the Trv. strain 970 LH1-RC occurred more selectively than in Tch. tepidum LH1-RC and with a reduced affinity. Ultraviolet resonance Raman analysis indicated that the number of hydrogen-bonding interactions between BChl a and LH1 proteins of Trv. strain 970 was significantly greater than for Tch. tepidum and that Ca2+ was indispensable for maintaining these bonds. Furthermore, perfusion-induced Fourier transform infrared analyses detected Ca2+-induced conformational changes in the binding site closely related to the unique spectral properties of Trv. strain 970. Collectively, our results reveal an ecological strategy employed by Trv. strain 970 of integrating Ca2+ into its LH1-RC complex to extend its light-harvesting capacity to regions of the near-infrared spectrum unused by other purple bacteria.

Cryo-EM structure of a Ca2+-bound photosynthetic LH1-RC complex containing multiple αβ-polypeptides

“…5 and 6 ). The pigment and Ca 2+ stoichiometry are consistent with those determined by biochemical analysis 16 . The long and short dimensions of the outer LH1 β-polypeptide ring are 114 Å and 107 Å (distances between the outer edges of opposite helices), respectively.…”

“…This structure is in good agreement with resonance Raman results on the Trv . strain 970 LH1-RC complex 16 that showed a markedly red-shifted C3-acetyl C=O stretching band and indicate even stronger hydrogen bonding for the carbonyl oxygen than occurs in Tch. tepidum LH1.…”

“…tepidum LH1-RC 13 . The LH1 subunits of strain 970 are uniformly distributed around the RC forming a closed, slightly elliptical double cylinder composed of 16 pairs of helical α(inner)β(outer)-polypeptides (αβ-dimer), 32 BChls a , 16 carotenoids (3,4,3′,4′-tetrahydrospirilloxanthin) 16 , and 16 Ca ions (Fig. 1 and Supplementary Figs.…”

“…strain 970 than in Tch. tepidum 16 . Much of the insight gained from the thoroughly investigated Tch.…”

“…As for the thermophilic Tch. tepidum , calcium ions were responsible for this ultra-redshift; removal of Ca 2+ resulted in a blue-shift of the LH1- Q y to 875 nm, whereas the subsequent addition of Ca 2+ restored 960-nm absorbance 16 . Although this process required Ca 2+ and was freely reversible, the structural basis behind these Ca 2+ -induced spectral changes remained unsolved.…”