Overview of pathways and dynamic interactions of c-type cytochromes in Geobacter sulfurreducens.

Extracellular electron transfer (EET) is a central process in the respiratory metabolism of electrogenic bacteria, enabling cells to exchange electrons with extracellular acceptors and donors. This capability has opened opportunities to exploit these bacteria in diverse practical applications, many of which would benefit from a precise understanding of the electron transfer (ET) events occurring within their respiratory chains. These chains rely on highly populated networks of c-type cytochromes that mediate electron flow across cellular compartments toward terminal electron acceptors. Although genomic, proteomic, biochemical, and structural studies have identified many ET components, defining their physiological redox partners and the directionality of electron flow remains challenging. This difficulty arises from the similar spectroscopic properties of key cytochromes, their transient interactions, and the complex overlap of their redox potential windows. In the present review, we examine how recently developed NMR-based strategies have been used to identify redox complexes established between cytochromes located in the inner membrane, periplasm, outer membrane, and extracellular conductive structures in the bacterium Geobacter sulfurreducens. These strategies exploited the distinct redox-dependent NMR fingerprints of individual heme groups, allowing direct detection of electron exchange, redox equilibria, and transient protein-protein interactions. Collectively, these studies reveal that ET pathways are not organized as single linear chains, but rather as dynamic and highly interconnected redox networks characterized by weak transient redox complexes and extensive promiscuity. This organization provides bacterial electron-buffering capacity, functional flexibility, and adaptability to changing environmental redox conditions.