Cellular Assays for Ferredoxins: A Strategy for Understanding Electron Flow through Protein Carriers That Link Metabolic Pathways

The
ferredoxin (Fd) protein family is a structurally diverse group
of iron–sulfur proteins that function as electron carriers,
linking biochemical pathways important for energy transduction, nutrient
assimilation, and primary metabolism. While considerable biochemical
information about individual Fd protein electron carriers and their
reactions has been acquired, we cannot yet anticipate the proportion
of electrons shuttled between different Fd-partner proteins within
cells using biochemical parameters that govern electron flow, such
as holo-Fd concentration, midpoint potential (driving force), molecular
interactions (affinity and
kinetics), conformational changes (allostery), and off-pathway electron
leakage (chemical oxidation). Herein, we describe functional and structural
gaps in our Fd knowledge within the context of a sequence similarity
network and phylogenetic tree, and we propose a strategy for improving
our understanding of Fd sequence–function relationships. We
suggest comparing the functions of divergent Fds within cells whose
growth, or other measurable output, requires electron transfer between
defined electron donor and acceptor proteins. By comparing Fd-mediated
electron transfer with biochemical parameters that govern electron
flow, we posit that models that anticipate energy flow across Fd interactomes
can be built. This approach is expected to transform our ability to
anticipate Fd control over electron flow in cellular settings, an
obstacle to the construction of synthetic electron transfer pathways
and rational optimization of existing energy-conserving pathways.