The murburn precepts for photoreception

Abstract

Murburn concept is a redox mechanistic scheme involving interactive equilibriums of discretized or organized protein/substrate 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 thermodynamically and kinetically viable explanations 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 of 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 several facts: (i) Layers of photoreceptive neural cells precede the rod/cone cells (with respect to the presentation to oncoming light ray/photon), (ii) There exists high oxygen demand in the retina, (iii) Copious DROS are detected in functional retina, (iv) NAD(P)H/reductase is involved in the cycle, and (v) 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). In the new scheme, photo-electric activation leads to charge separation and hyperpolarization in rod/cone cells due to negative charge build-up. This electro-chemical signaling serves as the front-runner to trigger an 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 simple electrical connectivity of the retina-photoreceptors with the brain via the optic nerve, is anatomically correlated with the structure and resolution of the retina, and is kinetically viable.

Murburn model of vision: Precepts and proof of concept

“…Perhaps, another mechanism is at work. The high resolution power of 5 arcsec (smaller than the radial dimensions of the rod cell to rod cell distance of ~50 arcsec) does not agree with the classical view (Manoj & Jacob, 2020). Also, it is difficult to fathom how the brain could avail a depth-perception if only the rod/cone cells were to be involved in the visual transduction process.…”

“…Recent literature supports the murburn proposal that ganglion cells also contribute to vision (Zele et al, 2018). Also, the classical view is highly energy intensive and fails to account for the structural aspects of the tapetum lucidum found in nocturnal carnivores (Manoj & Jacob, 2020).…”

“…Also, the classical view is highly energy intensive and fails to account for the structural aspects of the tapetum lucidum found in nocturnal carnivores (Manoj & Jacob, 2020). Most importantly, rods and cones should now be seen as the basic powering source for the visual process, quite similar to the LEDs projecting light from the bottom level of a modern day TV-screen.…”

“…Continuing along our earlier advocated lines (Manoj & Jacob, 2020), we hypothesized that the physiology of the phototransduction process could be simpler if its initiation were afforded by murburn phosphorylation (a DROS-mediated process at the rhodopsin-transducin complex), with GDP serving as a signaling molecule to phosphodiesterase. Concomitantly, hyperpolarization could be afforded with the ECS at rhodopsin, which would reflect in a change in TMP, and its subsequent effects.…”

Explaining trans-phase potential differences with membrane theory, association-induction hypothesis and murburn concept

“…We have solved the K > Na > Mg > Ca order of cation distribution in cells, based on the predictability of murburn concept [48]. The murburn model affords a fresh perspective to redox metabolism and electrophysiology, also enabling a new explanation for the physiology of vision [48][49][50]. Extrapolating those findings to the current experiments could suggest that since our works were done in oxic environment, the diradical could spontaneously sponsor effective charge separation (in conjunction with the matrix material) to give anionic species in milieu and this could potentially account for the TMP trends seen.…”