To understand what this research paper means, one has to really understand ATP Synthase (Complex V) and its function in context with this paper.
To date, we know ATP Synthase activity regulates (1) the state of flux of OXPHOS, (2) execution of cell death (3) mitochondrial signalling by ROS. The protein IF1 (ATPase inhibitory factor 1) referred to in the paper, is an inhibitor of ATP Synthase in normal physiological conditions - when dephosphorylated it binds to ATP Synthase and inhibits ATP synthesis. In the same manner, it also inhibits ATP hydrolysis, when ATP Synthase runs in reverse. ATP Synthase is a reversible turbine, each revolution yields 3 ATP but in reverse consumes 3 ATP. Essentially, IF1 acts just like a brake and could be the cause of downregulated ATP Synthase.
Inhibition of ATP Synthase by IF1 reduces ATP output by OXPHOS and rewires energy metabolism to enhanced glycolysis, and this paper suggests that non-mitochondrial oxygen consumption was enhanced. What I find strange here is this research finds no impairment of glycolysis, yet previous findings of a PDH deficiency, low pyruvate and normal lactate argue against this. Unless of course pyruvate is being converted to Acetyl-CoA and taken up by hypermetabolic mitochondria more rapidly than is being produced. A hyperactivated Complex I, as reported, might support this possibility, and perhaps glycolysis isn’t impaired after all. A study of the pyruvate kinase enzyme, the final rate-limiting step in glycolysis is needed to answer that question.
The question why ATP Synthase is insufficient remains unanswered, we can only speculate. ATP Synthase is pro-apoptotic whereas inhibition by IF1 is anti-apoptotic by inducing mitohormesis, cell survival and adaption to stress. The finding of proton leak might suggest inhibition of ATP Synthase by IF1 to increase mitochondrial membrane potential, by increasing the proton pool. This could all be a hypoxia response or Warburg effect. However, the paper proposes spare capacity of early complexes suggests this Complex V inhibition is unwarranted.
There are many other unexplored possibilities though. The cAMP / Protein Kinase A (PKA) signalling pathway is relevant in PKA’s ability to phosphorylate IF1 preventing it binding to and inhibiting ATP Synthase. PKA’s functions include regulation of glycogen, sugar and lipid metabolism, all thought to be disturbed in ME/CFS. Has cAMP activator Forskolin improved anyone or made them worse? A β-adrenergic agonist has been shown to phosphorylate IF1, Beta-Blockers do the opposite, and this may be relevant in the context of ME/CFS.
Similarly, we know SIRS metabolically resembles ME/CFS, and in septic shock ATP Synthase function is suppressed and IF1 depleted which favours ATP hydrolysis. Without IF1 to prevent ATP hydrolysis, early complexes would need to be hyperactive, as reported, to maintain the proton gradient. The SIRS/Sepsis model adopts the Warburg effect, whereby both glycolysis and OXPHOS are impaired, which may still be relevant for ME/CFS.
This paper is certainly significant progress to validating the crisis of ME/CFS patients, but also shows how much more research we desperately need. I'd love to know definitively if glycolysis is impaired or not, I'm not at all confident yet there's no problem there. Afterall, if glycolysis is impaired, would Complex V downregulate in order to to induce increased glycolysis as a normal response?