Structural Analyses of Membrane Extrinsic Subunits of Rieske b-Type Cytochrome Complexes in Phototrophs

Photosynthesis oxygenates the atmosphere and is essential to modern food networks. The
mechanisms driving photosynthetic energy harvesting are conducted by a stepwise
configuration of protein complexes, which are integrated into specialised membrane systems
of phototrophic organisms to form electron transport chains (ETCs).
The Rieske b-type cytochrome complexes are a critical component of these photosynthetic
ETCs. They sit at the heart of the electron transport system, coordinating the transfer of
reducing energy from the reaction centres to maintain optimum efficiency, and contributing to
the proton motive force which ultimately converts transient light energy to a stable chemical
form. These complexes have been implicated in a number of regulatory processes, some of
which are mediated by interaction partners.
This project has exploited the growing accessibility of single particle cryogenic electron
microscopy to probe the structural and mechanistic activities of Rieske b-type cytochromes
from both prokaryotic and eukaryotic systems. Novel interactions formed by both the
anoxygenic cytochrome bc1 and the oxygenic b6f are revealed by high-resolution structures,
alongside biochemical and kinetic analyses undertaken to establish the roles played by the
small subunits in regulating the electron transport activities of the core complexes.
The essential redox reactions mediated by cytochrome complexes of this family depend on
dynamic movement of extrinsic domains at high frequencies. Recent developments in data
analytical methods are leveraged to elucidate the identity and coordination of the contributing
subunits, and a proposal is offered as to how this movement may be integrated with other
regulatory components to maximise electron transport efficiency in cyt b6f. A structural and
genetic interrogation of alternative isoforms of one of these extrinsic subunits in cyanobacteria
is also presented.
Finally, the long-running hypothesis that cyt b6f in higher plants forms higher-order arrays is
explored, applying single-particle and biochemical techniques to a putative “supercomplex”.