Bioenergetics as a biological electrical circuit

Biological processes, including motility and enzymatic catalysis, rely on the ability of organisms to chemically transform various forms of energy into adenosine triphosphate (ATP). This molecule acts as the 'energy storage currency' in cells. Over the course of evolution, a myriad of mechanisms and protein molecules have emerged, acting individually or in tandem with others to provide the best possibility for an organism to fully exploit its living environment. As relatively high-resolution structures are available, we have focused herein on the structure-function relationship succinate:quinone oxidoreductases (SQOR). These proteins form a family of complexes unique within the electron transport chain (ETC) that provides the bulk of ATP for cellular activities. As most enzymes involved in the ETC contain one or more redox cofactors, we provide a discussion on their chemical properties in the context of SQOR. These complexes also act as a model protein to understand how biological life is capable of synthesising a limited set of cofactors and then incorporate them into proteins in a chemically-defined manner to perform catalytic reactions that are otherwise energetically unfavourable.