Transfer of Free Radicals between Proteins and Membrane Lipids: Implications for aquatic biology

The thermodynamic potential of many reaction intermediates of redox enzymes allows the transfer of their electrons to oxygen as an alternative substrate. Membranes act as a physical barrier against the diffusion of oxygen and its reactive species derivatives (ROS); the intensity of the effect is, however, highly specific. For O2, the barrier effect is so low that it can be considered nonexistent. If at all, O2 movement is controlled at the lipid–water interface and this control is enhanced at high temperature and, in more rigid membranes, due to accumulation of cholesterol (Moller et al. 2005). For practical purposes, biological membranes do not represent any barrier for oxygen. Transfer of O•−2 is energetically unfavorable, since it is a charged species and needs to lose its hydration shell to cross membranes. However, cells have specific channels to transport biologically required ions (O•−2 included) across membranes (Lynch and Fridovich 1978). H2O2 is a small noncharged hydrophilic molecule which is transported across membranes through specific aquaporins (Bienert et al. 2007). HO• is the most reactive of the partially reduced oxygen species and reacts with phospholipids in the lipid bilayer, first with the polar heads which are inert to other ROS and, afterwards,with the acyl chains. Thus, membranes pose a physical barrier for the diffusion of HO• through reaction with the lipid or protein components of the bilayer. Organisms may alter the composition of their membranes in a controlled fashion to reduce permeability as observed in Saccharomyces cerevisiae (Folmer et al. 2008).