Crystal structure of a photosynthetic LH1-RC in complex with its electron donor HiPIP

Photosynthetic electron transfers occur through multiple components ranging from small soluble proteins to large integral membrane protein complexes. Co-crystallization of a bacterial photosynthetic electron transfer complex that employs weak hydrophobic interactions was achieved by using high-molar-ratio mixtures of a soluble donor protein (high-potential iron-sulfur protein, HiPIP) with a membrane-embedded acceptor protein (reaction center, RC) at acidic pH. The structure of the co-complex offers a snapshot of a transient bioenergetic event and revealed a molecular basis for thermodynamically unfavorable interprotein electron tunneling. HiPIP binds to the surface of the tetraheme cytochrome subunit in the light-harvesting (LH1) complex-associated RC in close proximity to the low-potential heme-1 group. The binding interface between the two proteins is primarily formed by uncharged residues and is characterized by hydrophobic features. This co-crystal structure provides a model for the detailed study of long-range trans-protein electron tunneling pathways in biological systems. The high potential iron-sulfur (HiPIP) proteins are direct electron donors to the light-harvesting-reaction center complexes (LH1-RC) in photosynthetic beta- and gamma -Proteobacteria. Here, the authors present the 2.9 angstrom crystal structure of the HiPIP-bound LH1-RC complex from the thermophilic purple sulfur bacterium Thermochromatium tepidum and discuss mechanistic implications for the electron transfer pathway.

Automated summary

The study achieved co-crystallization of a bacterial photosynthetic electron transfer complex, offering insights into the molecular basis of interprotein electron tunneling and providing a detailed model for long-range trans-protein electron tunneling pathways in biological systems. The high potential iron-sulfur proteins play a crucial role as direct electron donors in LH1-RC complexes in photosynthetic beta- and gamma-Proteobacteria, with implications for the electron transfer pathway discussed based on the crystal structure of the HiPIP-bound LH1-RC complex.