Macromolecular crowding in the context of redox biology: effects on protein oxidation and aggregation

Proteins are the most abundant non-water component of most intracellular and extracellular biological systems including plasma and tissues. These, including structural proteins, metabolic enzymes, transport proteins and transcription factors, amongst others, are continuously exposed to oxidant molecules generated in physiology and pathology. A significant number of these are regulated by oxidation-reduction modifications. Understanding the mechanisms governing redox changes on proteins and the time course of these reactions is essential to achieve a greater understanding of how redox reactions modulate signaling pathways, cellular adaptation during physiology, and tissue responses to different stressors. However, most of the mechanistic and kinetic data concerning protein redox reactions comes from in vitro studies often carried out under conditions that do not necessarily mimic physiological environments. Thus most biological systems are known to be heavily crowded environments with up to 40% of the cytosolic volume occupied by macromolecules, and highly heterogeneous. Furthermore, they often contain many nano- and phase-separated domains – conditions that are difficult to generate in experimental setups. Nowadays it is evident that macromolecular crowding, phase separation and nanodomain formation modulate multiple aspects of protein behavior and reactions, including their diffusion, aggregation, kinetics and interactions. Thus, a central question is whether crowded environments also modulate protein oxidation. This lecture will describe and discuss examples of how macromolecular crowding can affect protein oxidation and aggregation illustrating the importance of appropriate choice of in vitro systems to investigate the many factors that govern oxidative processes, and their consequences.