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News on mitochondria: can they be exclusive aerobic ATP producers?
We have recently reported data showing that the functionality of the aerobic ATP synthesis can even be more effective in subcellular districts other than mitochondria, such as rod outer segment disks and myelin sheath, thanks to the ectopic expression of the mitochondrial oxidative phosphorylation (OXPHOS) proteins. This fact will not affect in any way the importance of mitochondria. In fact these constitute a physiological niche where the assembly of the complex molecular system for the (OXPHOS) takes place.
Our recent observations demonstrate that mitochondrial ability to aerobically synthesize ATP is quite poor. This may seem heretical, but descends from experimental data.
The hypothesis emerges that this can be related to the intrinsic functioning of the system for the exchange of ADP and ATP across the two mitochondrial membranes.
Sensitivity of photoreceptors to degenerative processes
Our data published rod outer segment disks that suggests that the metabolic support to phototransduction in the rod outer segment (OS) may originate directly in the OS, able to conduct OXPHOS. This oxygen-handling activity of the rod OS, which was never suspected before, may be a primary source of reactive oxygen species in the OS.
Reactive oxygen species would generated by the ectopic mitochondrial electron transport chain (ETC) expressed in rod OS disks.
Knowledge of this fact may shed new light on the pathogenesis of those neurodegenerative retinal pathologies that are caused by oxidative stress, such as diabetic retinopathy, age-related macular degeneration, retinopathy of the prematurity, and photoreceptor cell death after retinal detachment.
Reassessment of the shunt pentose phosphate pathway
Recent literature data review suggests that the pentose-phosphate shunt would be pivotal for the catabolism of glucose, more than previously thought.
Serious doubt is cast to the stage catalyzed by aldolase in the first step of glycolysis, which it may suffer from a thermodynamic stop.
It can be envisaged that the pentose-phosphate shunt would be able to directly link hexose monophosphates to triose phosphates thus bypassing the aldolase blockage. Furthermore the pentose-phosphate shunt forms fructose-6-P which, thanks to phosphogluco-isomerase, can in turn resynthesizes glucose-6-P that can be sent again to the shunt reactions, thus implementing a highly versatile recycling that leads ultimately to the conversion of sugars to CO2 with consequent energy extraction.
This on a bioenergetic basis assures the cell a source of energy rapid and easily expendable. Therefore, the pentose-phosphate shunt would play both a catabolic and anabolic role.
