Energy Partitioning: Hormonal, Organelle-Level, and Redox Organization of Mitochondrial Energy Allocation Between ATP Production and Thermal Dissipation

Living organisms must continuously regulate not only how much energy is produced, but also how available substrates are directed into different metabolic processes, where resulting intermediates are used, and how catabolic energy is distributed between ATP production and thermal dissipation. In this review, energy partitioning refers to the regulated distribution of catabolic energy between ATP production and heat production, whereas metabolic partitioning refers more broadly to the allocation of substrates toward storage, biosynthesis, oxidation, and inter-tissue transfer. The proposed model is based on mitochondrial heterogeneity. Different mitochondrial subpopulations may coexist within the same cell, including peridroplet mitochondria associated with lipid synthesis and storage, P5CS-enriched mitochondria associated with proline and ornithine biosynthesis, mitochondria with high oxidative ATP-producing capacity, thermogenic mitochondrial states, and potentially additional functional subpopulations that remain to be defined. A central proposal is that these subpopulations may participate not only in parallel metabolic tasks but also in sequential metabolic division of labor. Metabolites generated by one mitochondrial subpopulation may enter the cellular metabolite pool and become substrates for another, such that the mitochondrion in which a metabolic pathway begins need not be the same mitochondrion in which ATP is ultimately produced. Because tissues contain different proportions of mitochondrial functional subpopulations, local metabolic production and consumption are not expected to be completely matched. Metabolites not consumed locally enter the circulation and become substrates for other tissues, extending intracellular mitochondrial division of labor to inter-tissue metabolic organization. A further central hypothesis is that NADH-Complex I-dominant electron entry and flavin-CoQ-linked electron entry may preferentially support different energetic outcomes. NADH-Complex I flux may favor ATP production, whereas FAD/FADH2-linked and other flavin-CoQ pathways may contribute more strongly to thermal dissipation in appropriate thermogenic mitochondrial states. The framework therefore views mitochondria not merely as energy-producing organelles, but as components of a dynamic metabolic organization extending from mitochondrial subpopulations within a cell to substrate exchange between tissues.