Crystal structures of photosynthetic reaction center and high-potential iron-sulfur protein from Thermochromatium tepidum: thermostability and electron transfer

Abstract

The reaction center (RC) of photosynthetic bacteria is a membrane protein complex that promotes a light-induced charge separation during the primary process of photosynthesis. In the photosynthetic electron transfer chain, the soluble electron carrier proteins transport electrons to the RC and reduce the photo-oxidized special-pair of bacteriochlorophyll. The high-potential iron-sulfur protein (HiPIP) is known to serve as an electron donor to the RC in some species, where the c-type cytochrome subunit, the peripheral subunit of the RC, directly accepts electrons from the HiPIP. Here we report the crystal structures of the RC and the HiPIP from Thermochromatium (Tch.) tepidum, at 2.2-A and 1.5-A resolution, respectively. Tch. tepidum can grow at the highest temperature of all known purple bacteria, and the Tch. tepidum RC shows some degree of stability to high temperature. Comparison with the RCs of mesophiles, such as Blastochloris viridis, has shown that the Tch. tepidum RC possesses more Arg residues at the membrane surface, which might contribute to the stability of this membrane protein. The RC and the HiPIP both possess hydrophobic patches on their respective surfaces, and the HiPIP is expected to interact with the cytochrome subunit by hydrophobic interactions near the heme-1, the most distal heme to the special-pair.

Figure 1

Overall structures of the RC from Tch. tepidum. (a) The ribbon models represent four polypeptide chains: Cyt (red), L (green), M (blue), and H (yellow). The dotted circles indicate two regions where the loop conformations are different from those of the Blc. viridis RC (Left: Gln-C161–Asn-C162; Right: Gly-L56–Leu-L64). (b) The ball-and-stick models represent the configuration of the prosthetic groups (D_A and D_B: special-pair; BCh_A and BCh_B: bacteriochlorophyll a monomers; BPh_A and BPh_B: bacteriopheophytin a molecules; Q_A: menaquinone-8; Fe: nonheme iron; Spx: spirilloxanthin; Heme-1, -2, -3, and -4: heme c groups). The secondary quinone (Q_B) site (blue dotted circle) is empty because Q_B was not observed in the electron density maps. (c) The space-filling models represent the detergent and lipid molecules observed on the molecular surface. Six β-OG molecules are represented by cyan. One LDAO molecule and one lipid molecule (P) are represented by green and magenta, respectively. The dotted circle indicates the binding site of the cardiolipin in the Rb. sphaeroides RC structure (49).

Figure 2

Three Arg residues located at the membrane surface of the RC from Tch. tepidum. In the crystal structure, all three additional Arg residues (drawn by the space-filling model) are found to be located at the membrane surface. Each subunit is colored red (Cyt), green (L), blue (M), or yellow (H). The model of the membrane is placed on the ribbon model of the RC complex, and the head groups of the phospholipids are represented by the red circles. All three Arg residues are exposed to the solvent and located in a position from which they can interact with the head groups of the phospholipids.

Figure 3

Overall structure (a and b) and charge distribution of the molecular surface (c and d) of the HiPIP from Tch. tepidum. The molecule is viewed from the side proximal to the [4Fe-4S] cluster (a and c) and from the side distal to the [4Fe-4S] cluster (b and d). (a and b) Ribbon model with the [4Fe-4S] cluster shown in green. The dotted circles indicate the two strands of β-sheet whose conformation is different from that of the Ach. vinosum HiPIP. (c and d) Molecular surface with the charge distribution (negative charge: red; positive charge: blue). The molecular surface proximal to the [4Fe-4S] cluster forms a large hydrophobic patch, as shown in the dotted circle, whereas that of the distal side is hydrophilic and mostly negative.

Figure 4

Structures of the Cyt subunits of Tch. tepidum RC (a–d) and Blc. viridis RC (e and f). (a and b) Charge distribution of the Cyt subunit of Tch. tepidum around the heme-1 (a) and viewed from the backside of the heme-1 (b). The negatively charged surfaces are shown in red, and the positively charged surfaces in blue. The molecular surface around the heme-1 is mostly hydrophobic, which is shown in the black dotted circle. The protruding part of the heme-1 is shown in the green dotted circle. V and W indicate the Val-C65 and Trp-C94, respectively, which affect the molecular recognition of the RC and HiPIP. The yellow dotted circles indicate the two loop regions whose conformations are different from those of the Blc. viridis RC. (c and d) Ribbon model of the Cyt subunit of Tch. tepidum viewed from the same angle as that for a and b, respectively. Each heme is shown in green. (e and f) Charge distribution of the Cyt subunit of Blc. viridis viewed from the same angle as that for a and b, respectively. The molecular surface is generated by using the atomic coordinates of the RC from Blc. viridis (PDB code: 1PRC). In Blc. viridis, only the molecular surface around the heme-1 is acidic, which is shown in the black dotted circle, and most of the other surfaces are basic.