Title The Embden-Meyerhof cycle and the Rapoport-Luebering cycle: A flow non-equilibrium perspective

Blood lactic acid concentration (lactic acidosis) is an important clinical parameter that has generally been excluded from modern metabolic modelling. Recently, a nonequilibrium-based framework of glycolysis and fermentation has been proposed, incorporating in vivo dynamics such as the generation of a lactate concentration gradient, unidirectional lactic acid import or export, simultaneous ATP consumption and regeneration, and biosynthetic reactions. This manuscript synthesizes current knowledge on the biochemical mechanisms governing transmembrane lactic acid transport and extends the understanding of fermentation. Fermentation is characterized as an incomplete glycolytic degradation of 3-phosphoglyceric acid, defined by lactic acid export and the concurrent synthesis of 3-phosphoglycerate. The manuscript further proposes that oscillation of the Embden-Meyerhof cycle between fermentative and glycolytic states drive 3-phosphoglycerate biosynthetic cycles. Within this framework, 2,3-diphosphoglyceric acid is introduced as a synthesis product of the 3-phosphoglycerate driven diphosphoglyceric acid cycle (Rapoport-Luebering cycle) and 2,3-diphosphoglycerate (2,3-DPG) is proposed as a biomarker for pathway activity. The Embden-Meyerhof cycle thereby establishes a mechanistical link between clinical and metabolic parameters including 2,3-DPG, lactic acid, pyruvate, glucose, and nonequilibrium NADH/NAD⁺ flow dynamics. The cycles are presented in context to lactic acidosis, malaria, cancer, sickle cell disease, oxidative stress, hyperglycaemia, and exercise-induced hyperinsulinism, proposing that malaria and cancer are biochemically linked to dysregulated activity of the 3-phosphoglycerate biosynthesis cycles.