No abstract

The classical paradigm of visual physiology comprises of the following features: (i) rod/cone cells located at the rear end of the retina serve as the primary transducers of incoming photo-information, (ii) cis-trans retinal (C 20 H 28 O) transformations on rhodopsin act as the transduction switch to generate a transmittable signal, (iii) signal amplification occurs via GDP-GTP exchange at transducin, and (iv) the amplified signal is relayed (as an action potential) as a flux-based ripple of Na-K ions along the axons of neurons. Fundamental physical principles, chemical kinetics and awareness of architecture of eye/retina prompt a question of these classical assumptions. In lieu, based on experimental and in silico findings, a simple space-time resolved murburn model for the physiology of photo-transduction in the retina is presented wherein molecular oxygen plays key roles. It is advocated that: (a) photo-induced oxygen to superoxide conversion serves as the key step in signal transduction in the visual cycle, (b) all photo-active cells of the retina serve as photoreceptors and rods/cones serve as the ultimate electron source in the retina (deriving oxygen and nutrients from retinal pigmented epithelium), (c) signal amplification is through superoxide mediated phosphorylation of GDP bound to inactive 2 transducin, thereby activating a GDP-based cascade (a new mechanism for trimeric G-proteins), and (d) signal relay is primarily an electron movement along the neuron, from dendritic source to synaptic sink. In particular, we specify the roles for the beta module of transducin and GDPbased activation of phosphodiesterase-6 in the physiology of visual transduction.

Mitochondrial membrane‐embedded redox proteins are classically perceived as deterministic “electron transport chain” (ETC) arrays cum proton pumps; and oxygen is seen as an “immobile terminal electron acceptor.” This is untenable because: (1) there are little free protons to be pumped out of the matrix; (2) proton pumping would be highly endergonic; (3) ETC‐chemiosmosis‐rotary ATP synthesis proposal is “irreducibly complex”/”non‐evolvable” and does not fit with mitochondrial architecture or structural/distribution data of the concerned proteins/components; (4) a plethora of experimental observations do not conform to the postulates/requisites; for example, there is little evidence for viable proton‐pumps/pH‐gradient in mitochondria, trans‐membrane potential (TMP) is non‐fluctuating/non‐trappable, oxygen is seen to give copious “diffusible reactive (oxygen) species” (DRS/DROS) in milieu, etc. Quite contrarily, the newly proposed murburn model's tenets agree with known principles of energetics/kinetics, and builds on established structural data and reported observations. In this purview, oxygen is needed to make DRS, the principal component of mitochondrial function. Complex V and porins respectively serve as proton‐inlet and turgor‐based water‐exodus portals, thereby achieving organellar homeostasis. Complexes I to IV possess ADP‐binding sites and their redox‐centers react/interact with O2/DRS. At/around these complexes, DRS cross‐react or activate/oxidize ADP/Pi via fast thermogenic one‐electron reaction(s), condensing to form two‐electron stabilized products (H2O2/H2O/ATP). The varied architecture and distribution of components in mitochondria validate DRS as (i) the coupling agent of oxidative reactions and phosphorylations, and (ii) the primary reason for manifestation of TMP in steady‐state. Explorations along the new precepts stand to provide greater insights on mitochondrial function and pathophysiology.

The redox metabolic paradigm of murburn concept advocates that diffusible reactive species (DRS, particularly oxygen-centric radicals) are mainstays of physiology, and not mere pathological manifestations. The murburn purview of cellular function also integrates the essential principles of bioenergetics, thermogenesis, homeostasis, electrophysiology, and coherence. In this context, any enzyme that generates/ modulates/utilizes/sustains DRS functionality is called a murzyme. We have demonstrated that several water-soluble (peroxidases, lactate dehydrogenase, hemogoblin, etc.) and membrane-embedded (Complexes I-V in mitochondria, Photosystems I/II in chloroplasts, rhodopsin/transducin in rod cells, etc.) proteins serve as murzymes. The membrane protein of Na,K-ATPase (NKA, also known as sodium-potassium pump) is the focus of this article, owing to its centrality in neurocardio-musculo electrophysiology. Herein, via a series of critical queries starting from the geometric/spatio-temporal considerations of diffusion/mass transfer of solutes in cells to an update on structural/distributional features of NKA in diverse cellular systems, and from various mechanistic aspects of ion-transport (thermodynamics, osmoregulation, evolutionary dictates, etc.) to assays/explanations of inhibitory principles like cardiotonic steroids (CTS), we first highlight some unresolved problems in the field. Thereafter, we propose and apply a minimalist murburn model of trans-membrane ion-differentiation by NKA to address the physiological inhibitory effects of trans-dermal peptide, lithium ion, volatile anesthetics, confirmed interfacial DRS + proton modulators like nitrophenolics and unsaturated fatty acid, and the diverse classes of molecules like CTS, arginine, oximes, etc. These explanations find a pan-systemic connectivity with the inhibitions/uncouplings of other membrane proteins in cells.

We recently proposed a diffusible reactive (oxygen) species (DRS/DROS) based function for cytochrome b complexes (CBC) and quinones (Q)/quinols (QH2) in the murburn model of bioenergetics. This proposal is in direct conflict with the classical purview of Q‐cycle. Via extensive analyses of the structure‐function correlations of membrane‐quinones/quinols and proteins, we present qualitative and quantitative arguments to infer that the classical model cannot explain the energetics, kinetics, mechanism and probabilistic considerations. Therefore, it is proposed that Q‐cycle is neither necessary nor feasible at CBCs. In contrast, we substantiate that the murburn model explains: (a) crucial structural data of CBCs, (b) why quinones/quinols are utilized in bioenergetic membranes, (c) how trans‐membrane potential is generated owing to effective charge separation at CBCs, (d) mobility data of O2, DRS, Q/QH2, and (e) utility of other reaction/membrane components. Further, the murburn model also accommodates the absence of quinones in anaerobic Archaea, wherein methanophenazines are prevalent. The work mandates that the textbooks and research agendas are refreshed to reflect the new perception. Significance The current article must be seen as a critical and detailed analysis of the role and working mechanism of quinone (Q) /quinols (QH2) in bioenergetic membranes. In the classical model, QH2 are perceived as highly mobile electron‐transport agents that bind and donate electrons to cytochrome b complexes (CBCs), using sophisticated electronic circuitries, in order to recycle Q and pump protons. The classical perception sees radicals (such as Q*‐, O2*‐, etc., also called diffusible reactive species, DRS) as wasteful or toxic (patho) physiological manifestations. It is highlighted herein that QH2 has low mobility and matrix has little protons to pump. New insights from the structural analyses of diverse CBCs and quinols, in conjunction with murburn reaction thermodynamics suggest that the electrons from substrates/quinols are effectively utilized via DRS. This perception fits into a much broader analysis of 1 and 2 electron transfers in overall redox metabolism, as recently brought out by the murburn model, wherein DRS are considered obligatory ingredients of physiology. Thus, the findings mandate a reorientation in the pertinent research field.