What is the Role of Lipid Membrane-embedded Quinones in Mitochondria and Chloroplasts? Chemiosmotic Q-cycle versus Murbu
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REVIEW PAPER
What is the Role of Lipid Membrane-embedded Quinones in Mitochondria and Chloroplasts? Chemiosmotic Q-cycle versus Murburn Reaction Perspective Kelath Murali Manoj1 Daniel Andrew Gideon1 Abhinav Parashar ●
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Accepted: 16 September 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Quinones are found in the lipid membranes of prokaryotes like E. coli and cyanobacteria, and are also abundant in eukaryotic mitochondria and chloroplasts. They are intricately involved in the reaction mechanism of redox phosphorylations. In the Mitchellian chemiosmotic school of thought, membrane-lodged quinones are perceived as highly mobile conveyors of two-electron equivalents from the first leg of Electron Transport Chain (ETC) to the ‘second pit-stop’ of Cytochrome bc1 or b6f complex (CBC), where they undergo a regenerative ‘Q-cycle’. In Manoj’s murburn mechanism, the membrane-lodged quinones are perceived as relatively slow-moving one- or two- electron donors/acceptors, enabling charge separation and the CBC resets a one-electron paradigm via ‘turbo logic’. Herein, we compare various purviews of the two mechanistic schools with respect to: constraints in mobility, protons’ availability, binding of quinones with proteins, structural features of the protein complexes, energetics of reaction, overall reaction logic, etc. From various perspectives, the murburn mechanism appeals as a viable alternative explanation well-rooted in thermodynamics/kinetics and one which lends adequate structure-function correlations for the roles of quinones, lipid membrane and associated proteins. Keywords murburn concept chemiosmosis cytochrome b6 f ubiquinone plastoquinone Q-cycle ●
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Introduction Ubiquinone (CoQ) and plastoquinone (PQ)/phylloquinone (K) are examples of lipid-membrane-bound biological quinones found in mitochondrial and chloroplastid/cyanobacterial membranes that are involved in energy metabolism. The ‘Chemiosmotically driven Rotary ATP Synthesis’ (CRAS) paradigm for energy metabolism was mooted by Mitchell–Boyer, which was developed on Keilin’s concept of Electron Transport Chain (ETC) [1, 2]. The ETC-CRAS mechanism proposed that quinones ferry
* Kelath Murali Manoj [email protected] * Abhinav Parashar [email protected] 1
Satyamjayatu: The Science & Ethics Foundation, Kulappully, Shoranur-2 (PO), Palakkad, Kerala 679122, India
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Department of Biotechnology, Vignan’s Foundation for Science, Technology & Research, Vadlamudi, Guntur 522213, India
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two-electron equivalents from the ‘primary pit-stop’ of Complex I/II or Photosystem II to the ‘secondary pit-stop’ of Cyt. bc1 (Complex III) or Cyt. b6f complexes (which we collectively term as CBC), in the routines of oxidative phosphorylation (OxPhos) in mitochondria and photophosphorylation (PhotoPhos) in chloroplasts, respectively. An alternative explanation based in murburn concept was recently proposed for OxPhos and PhotoPhos [3–19]. Within this context, we provide a criti
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