Abstract
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.
Highlights
Photosynthetic electron transfers occur through multiple components ranging from small soluble proteins to large integral membrane protein complexes
We present here the crystallographic structure of an high-potential iron-sulfur proteins (HiPIP)-bound reaction center (RC) in which the RC contains the tetraheme Cyt subunit and is associated with its core light-harvesting complex (LH1-RC, ~390 kDa)
Both the HiPIP and LH1-RC were isolated from the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum (γ-Proteobacteria)[11] and their individual structures have been determined to high resolutions[12,13]
Summary
Photosynthetic electron transfers occur through multiple components ranging from small soluble proteins to large integral membrane protein complexes. 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. We present here the crystallographic structure of an HiPIP-bound RC in which the RC contains the tetraheme Cyt subunit and is associated with its core light-harvesting complex (LH1-RC, ~390 kDa). Both the HiPIP and LH1-RC were isolated from the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum (γ-Proteobacteria)[11] and their individual structures have been determined to high resolutions[12,13]. Our structural results verify predictions of protein–protein interactions between the Tch. tepidum tetraheme Cyt subunit and HiPIP and go well beyond this to provide important new insight into the mechanism of electron transfer that occurs in this key step of photosynthetic energy generation
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