Breaking biological symmetry in membrane proteins: The asymmetrical orientation of PsaC on the pseudo-C2 symmetric Photosystem I core

Abstract

The elucidation of assembly pathways of multi-subunit membrane proteins is of growing interest in structural biology. In this study, we provide an analysis of the assembly of the asymmetrically oriented PsaC subunit on the pseudo C(2)-symmetric Photosystem I core. Based on a comparison of the differences in the NMR solution structure of unbound PsaC with that of the X-ray crystal structure of bound PsaC, and on a detailed analysis of the PsaC binding site surrounding the F(X) iron-sulfur cluster, two models can be envisioned for what are likely the last steps in the assembly of Photosystem I. Here, we dissect both models and attempt to address heretofore unrecognized issues by proposing a mechanism that includes a thermodynamic perspective. Experimental strategies to verify the models are proposed. In closing, the evolutionary aspects of the assembly process will be considered, with special reference to the structural arrangement of the PsaC binding surface.

Bidirectional Electron Transfer in the Reaction Centre of Photosystem I

“…In this case, as the two RC subunits are identical, it is expected that there are either no differences, or only very limited differences in the properties of the two electron transfer chains possibly induced by the dimerization of the RC subunits or by the binding of the additional subunit that harbors the terminal acceptors, F A and F B (e.g. Jagannathan and Golbeck 2009). Henceforth, both ET chains are expected to be active in green‐sulfur bacteria, even though experimental support for this process is still lacking.…”

Back‐reactions, short‐circuits, leaks and other energy wasteful reactions in biological electron transfer: Redox tuning to survive life in O2

“…One feature of PSI that may reflect redox tuning to limit O 2 reduction is the fact that the more stable, highest potential acceptor is not the F B , the exposed terminal acceptor that interacts with ferredoxin, but rather the F A centre which is buried inside the protein (see [27]). The more sequestered location of the reduced F A could slow its reaction with O 2 .…”

A Robust Genetic System for Producing Heterodimeric Native and Mutant Cytochrome bc1

“…The three-dimensional structure of ccNiR is a homodimer in which the distance separating the hemes located at the edge of the dimer interface is short (~12 Å), 44,45 whereas that of PSI is a pseudohomodimer containing similar but not identical (PsaA and PsaB) monomers. 46,47 …”