The murburn precepts for photoreception

Abstract

Murburn concept is a redox mechanistic scheme involving interactive equilibriums of discretized or organized proteins/substrates molecules, unbound ions and radicals (or reactive species); which may afford selective/specific electron transfers, particularly at phospholipid interfaces. Earlier, we have applied murburn concept to provide parsimonious explanations (grounded in thermodynamics and kinetics) for various physiological/bioenergetic routines like xenobiotic metabolism, unusual dose responses, aerobic respiration, thermogenesis, homeostasis, trans-membrane potential, oxygenic photosynthesis, etc. While proposing the murburn model for photophosphorylation, we had projected that the murburn mechanism could also be relevant for photoreception physiology. Herein, we expand on this aspect and present the basic scheme and evidence in support for the murburn model of photoreception, with retinal/opsin as the salient photon-impingement response-transducing element. In alignment with our earlier murburn schemes, we propose that diffusible reactive oxygen species (DROS, as exemplified by superoxide, which is currently deemed a toxic product of all-trans retinal and NADPH oxidase interactions) is produced in rod/cone cells upon photoactivation and it plays a crucial role in the visual cycle. This is supported by the fact that layers of photoreceptive neural cells precede the rod/cone cells (with respect to the presentation to oncoming light ray/photon), there exists high oxygen demand in retina, copious ROS are detected in functional retina, NAD(P)H/reductase is involved in the cycle, events occur at sub-micrometer dimensioned phospholipid disks stacked to stabilize DROS and minimize free protons (quite like the thylakoids that harbor carotenoids in chloroplasts and cyanobacteria), etc. In the new scheme, photo-electric activation leads to charge separation and hyperpolarization. This electro-chemical signaling serves is the front-runner to a trigger for action potential relay along a neuron and the superoxide mediated phosphorylation of GDP bound to transducin serves as the initiator of signal transduction cascade. The newly proposed scheme allows a facile electrical connectivity of the retina-photoreceptors with the brain via the optic nerve, and is anatomically correlated with the structure and resolution or retina, and is kinetically viable.