Role of thylakoid membranes in oxygenic photosynthesis: A comparative perspective using murburn concept
Murburn concept is a new redox metabolic paradigm which advocates that several redox enzymes generate/stabilize diffusible reactive (oxygen) species (DRS or DROS) to carry out useful electron/moiety transfer reactions at biological membrane interfaces (Manoj 2020a). Herein, we show that the components and principles of redox reactions within chloroplasts/cyanobacteria share several similarities with soluble and simple extracellular or peroxisomal heme-enzymes that carry out electron/group transfer. We explore the comparison in detail with membrane-embedded and complex systems that catalyze: (i) microsomal xenobiotic metabolism and (ii) mitochondrial oxidative phosphorylation. We point out that the murburn interpretations of catalytic phenomena are consistent through the various reaction systems cited above. Further, we argue that evolutionary constraints and the physiological restrictions of neutral pH ranges discount proton-gradient based explanations for bioenergetic phosphorylations in chloroplasts. Therefore, we propose that the highly packed thylakoid membranes with minute aqueous volumes serve to enhance the lifetimes of oxygen-centered radicals and intermediates. The murburn perspective could also potentially explain protein supercomplexes in chloroplasts, and generation of ATP in mitochondria by photo-activation. Our proposal also highlights the evolutionary significance of lipid membranes and utility of oxygen in diverse life processes.