Structural and functional studies on the tetraheme cytochrome subunit and its electron donor proteins: the possible docking mechanisms during the electron transfer reaction

Abstract

The photosynthetic reaction centers (RCs) classified as the group II possess a peripheral cytochrome (Cyt) subunit, which serves as the electron mediator to the special-pair. In the cycle of the photosynthetic electron transfer reactions, the Cyt subunit accepts electrons from soluble electron carrier proteins, and re-reduces the photo-oxidized special-pair of the bacteriochlorophyll. Physiologically, high-potential cytochromes such as the cytochrome c2 and the high-potential iron-sulfur protein (HiPIP) function as the electron donors to the Cyt subunit. Most of the Cyt subunits possess four heme c groups, and it was unclear which heme group first accepts the electron from the electron donor. The most distal heme to the special-pair, the heme-1, has a lower redox potential than the electron donors, which makes it difficult to understand the electron transfer mechanism mediated by the Cyt subunit. Extensive mutagenesis combined with kinetic studies has made a great contribution to our understanding of the molecular interaction mechanisms, and has demonstrated the importance of the region close to the heme-1 in the electron transfer. Moreover, crystallographic studies have elucidated two high-resolution three-dimensional structures for the RCs containing the Cyt subunit, the Blastochloris viridis and Thermochromatium tepidum RCs, as well as the structures of their electron donors. An examination of the structural data also suggested that the binding sites for both the cytochrome c2 and the HiPIP are located adjacent to the solvent-accessible edge of the heme-1. In addition, it is also indicated by the structural and biochemical data that the cytochrome c2 and the HiPIP dock with the Cyt subunit by c2 is recognized through electrostatic interactions while hydrophobic interactions are important in the HiPIP docking.

Genomics of a phototrophic nitrite oxidizer: insights into the evolution of photosynthesis and nitrification

“…c protein (PufC) that is bound on the periplasmic side of the RCII complex and serves as a wire connecting the soluble electron donor with the special pair. The redox potentials of these hemes vary, with the direct donor to the special pair (heme c 559 in Figure 2) having a potential of~+380 mV (Nogi et al, 2005). The majority of electron donors for anoxygenic phototrophy have redox potentials 0 mV or lower, providing a significant thermodynamic driving force for the overall electron transfer reaction (Supplementary Figure S1).…”

Simulation of the Electron Transfer between the Tetraheme Subunit and the Special Pair of the Photosynthetic Reaction Center Using a Microstate Description

“…The RC is an integral membrane protein, which couples the oxidation of a soluble cytochrome c or an iron sulfur protein on the periplasmic side of the membrane to the reduction of a quinone at the cytoplasmic side. , Structures are available for various systems, − and the equilibrium energetics as well as transfer kinetics have been accessed in numerous studies. − The core of all reaction centers is formed by three subunits labeled H, M, and L . Following the photoinduced excitation of the SP an electron is transferred via several redox cofactors to the quinone Q B .…”

Unfolding pathway and intermolecular interactions of the cytochrome subunit in the bacterial photosynthetic reaction center

“…In some purple photosynthetic bacteria that lack the 4Hcyt subunit in the RC complex, the soluble electron carrier Cyt c 2 docks directly to the RC complex through electrostatic interactions [ 34 ]. The specific binding has allowed the recognition of the Rba.…”