Crystal Structure of the Electron Carrier Domain of the Reaction Center Cytochrome cz Subunit from Green Photosynthetic Bacterium Chlorobium tepidum

Abstract

In green sulfur photosynthetic bacteria, the cytochrome c(z) (cyt c(z)) subunit in the reaction center complex mediates electron transfer mainly from menaquinol/cytochrome c oxidoreductase to the special pair (P840) of the reaction center. The cyt c(z) subunit consists of an N-terminal transmembrane domain and a C-terminal soluble domain that binds a single heme group. The periplasmic soluble domain has been proposed to be highly mobile and to fluctuate between oxidoreductase and P840 during photosynthetic electron transfer. We have determined the crystal structure of the oxidized form of the C-terminal functional domain of the cyt c(z) subunit (C-cyt c(z)) from thermophilic green sulfur bacterium Chlorobium tepidum at 1.3-A resolution. The overall fold of C-cyt c(z) consists of four alpha-helices and is similar to that of class I cytochrome c proteins despite the low similarity in their amino acid sequences. The N-terminal structure of C-cyt c(z) supports the swinging mechanism previously proposed in relation with electron transfer, and the surface properties provide useful information on possible interaction sites with its electron transfer partners. Several characteristic features are observed for the heme environment: These include orientation of the axial ligands with respect to the heme plane, surface-exposed area of the heme, positions of water molecules, and hydrogen-bond network involving heme propionate groups. These structural features are essential for elucidating the mechanism for regulating the redox state of cyt c(z).

Overexpression, characterization, and crystallization of the functional domain of cytochrome c z from Chlorobium tepidum

“…The measured molecular mass of 10,745 Da indicates that it is composed of the 90 amino acid C-cyt c z (Ala117-Phe206) with two residues (Ala-Met) attached at the N-terminus. The mid-point redox potential was measured to be 188 mV (Hirano et al 2009), in agreement with that of the cyt c z in native membrane (Okumura et al 1994). Under the optimized condition, the final yield was determined to be approximately 17 mg l -1 of E. coli medium.…”

“…The expanded area shows the high-resolution region RC special pair, an understanding of the molecular mechanism has long been elusive. This study has paved the way for a detailed structural analysis of the electron carrier domain at atomic level, and the results will be given in an accompanying paper (Hirano et al 2009). …”

“…In addition, some residues beyond the axial methionine fit the consensus for the C-terminal helical region of Class I cyt c. For example, a phenylalanine, three residues after the methionine, matches an aromatic residue in the algal cyt c 6 and the sequence IXXWL is approximately the same distance from the methionine as in Class I cyt c (Meyer 1996). If the C-cyt c z is related to Class I cyt c, it has experienced an approximately 15-residues deletion between the heme-binding site and the axial methionine relative to other small bacterial cyt c (Hirano et al 2009). Figure 5 shows RR spectra of (a) oxidized and (b) reduced C-cyt c z .…”

“…Detailed characterization of the product has been described and shows that the C-cyt c z expressed is fully functional. The C-cyt c z has been crystallized, and results of the structural analysis will be described elsewhere (Hirano et al 2009). …”

Structural Analysis of the Homodimeric Reaction Center Complex from the Photosynthetic Green Sulfur Bacterium Chlorobaculum tepidum

“…The PscC protein, which mediates the transfer of an electron from the menaquinol/cytochrome c oxidoreductase to P840, has three membrane-spanning regions at the N-terminal end and a soluble domain that binds a single heme group at the C-terminal end on the periplasmic side of the membrane. 9 The PscD subunit of RCC shows some similarities in the amino acid sequences with PsaD in PS I of plants and cyanobacteria. 10 PscD is loosely bound to the RCC and is not essential for photosynthetic growth.…”