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(A)
Q
Fe-S
RC
RC
Cyanobacteria
Q
LH2 LH1
RC
Chlorosome
Purple bacteria
Fe-S
RC
PR
Green sulfur
bacteria
(B)
X
(C)
307
FIGURE 16.1
Overview of composition of light-harvesting machinery across different organisms. (A) Overview of the basic organization of
the core reaction center (RC) complexes and the light-harvesting subunits (LH). Colors of the core complexes indicate
whether they are composed of homo-dimers or hetero-dimers. Q or Fe S indicate quinones or iron sulfur clusters
(respectively) as the pathway for the transfer of excited electrons from the special pair to the final acceptor (modified from
Bryant et al. 1 ). (B) Schematic representation of the organization of the RC LH1 LH2 complex in R. sphaeroides as is
present in the plane of the membrane. The core L, M, and H subunits (green and yellow) are surrounded by the LH1
complex (purple). The LH2 complex (red) is localized to the periphery of the RC LH1 complex. (C) Top view of a crystal
structure of the core RC LH1 complex from R. capsulatus (PDB# 1PYH). The core L, M, and H subunits are located in the
center surrounded by the αβ-peptides composing the LH1 (purple). Bacteriochlorophyll molecules are shown in green. The
structure illustrates the complexity and precise assembly of the light-capturing machinery.
eventually returned to the special Bchl pair of the RC via cytochrome c 2 . 25 In contrast, type I
RCs found in green-sulfur bacteria can utilize ferrodoxin (Fd) as the final electron acceptor. 2
Since the electrons do not return to the special Bchl pair of the RC, they need to be
regenerated from a different source, frequently from oxidation of sulfur compounds. The
majority of bacterial RCs are surrounded by light-harvesting complexes containing
additional pigments that increase the efficiency of light capture ( Fig. 16.1 ). 1
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