LH1 and LH2

Purple bacteria contain two membrane integral antenna proteins - the LH1 and LH2 - which form ring structures. These structures are described in detail in Chapter 14.

The LH2 complex serves as a peripheral antenna for the purple bacterial reaction center. AFM images of native purple bacterial membranes have shown that the cells can increase the ratio of LH2 rings to RC cores in low light. The LH2 rings can form quasicrystalline lattices in the native membrane [45].

The structure of LH2 has been determined at atomic resolution )46] by X) ray crystallography (Figure 1.3 right shows the structures of the LH2 ring and the reaction center LH1 supercomplex). LH2 consists of two subunits, a and P, which contain a single transmembrane spanning a-helix each. The subunits form a ring, which is composed of eight or nine pairs of a,P-dimers. The stoichiometry ofthe ring is species dependent. The outer wall of the ring is formed by the a-helices of the P-proteins; the a-helices from the a-apoproteins form the inner wall of the ring. All of the pigments are arranged in two rings between these rings of a -helices.. The first ring of pigments, located closer to the cytoplasmic (stromal) side of the complex, consists of monomeric Bchl a molecules, which are oriented parallel to the plane of the membrane. The pigments absorb at 800 nm (B800 molecules). Each a,P-dimer binds one B800 Bchl a. The second ring consists of 16 or 18 tightly interacting bacteriochlorophyll molecules, which are oriented nearly at right angles to the membrane plane. Due to the stronger coupling between the pigments, these chlorophylls absorb light at a wavelength of 850 nm and are named B850 molecules. In addition to the chlorophylls, each a,P-dimer in the LH2 complex contains one carotenoid, which is essential for the assembly and stabilization of the LH2.

LH1 has been crystallized in the form of an elliptical open ring surrounding the reaction center [16, 47]. The structure is shown in Figure 1.3. The LH1 ring consists of 15 a,P-dimers that surround the RC. The ring is open, interrupted by a single transmembrane helix, which has been called Protein W and may be analogous to PufX. This ring opening is located opposite the QB binding site in the RC. This has led to the suggestion that the ring opening may be essential for the function as it would allow the reduced ubiquinol molecule to leave the complex to reach the cytochrome bc complex. Each a,P-dimer coordinates two bacteriochlorophyll molecules that strongly interact, leading to an absorption maximum of 880 nm (B880 molecules). The arrangement of the ring of pigments resembles the B850 ring of the LH2 complex, but the LH1 ring is interrupted. It should also be noted that the LH1 does not contain pigments that resemble the B800 ring in the LH2 complex.

The spectral distribution of the pigments is highly optimized and allows a very efficient excitation energy transfer from the periphery to the RC core. This involves the transfer of energy from the highest energy pigment B800, to the B850 ring in LH2, to the B880 ring in LH1, to the primary donor P in the core of the reaction center.

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