Interpreting mitochondrial respiration of cancer tissue is more complex than a single readout usually implies. In many experimental settings, oxygen consumption is used as a practical proxy for oxidative phosphorylation, and oxidative phosphorylation is then interpreted as mitochondrial ATP production. This operational shortcut is understandable, because electron transport, proton pumping, membrane potential, and ATP synthesis are linked in the classical view of OXPHOS. However, oxygen consumption is not synonymous with phosphorylation. It reports electron flow to molecular oxygen at the end of the electron transport chain; it does not, by itself, demonstrate that the resulting protonmotive force is being converted into ATP by the F₀F₁-ATP synthase.
This distinction is the conceptual starting point of the review Mitochondrial respiration supports cancer growth independent of OXPHOS. Much of the literature on tumor mitochondria has been shaped by a deceptively simple question: do cancer mitochondria perform OXPHOS? The question is experimentally tractable, and historically important. It has motivated measurements of basal and maximal oxygen consumption, ATP-linked respiration, spare respiratory capacity, respiratory chain integrity, mitochondrial membrane potential, and the balance between glycolytic and mitochondrial ATP production. However, the question also narrows the biological interpretation of respiration, because oxygen-dependent electron transport may remain essential even when ATP synthesis is not the principal reason.
The present review does not examine whether cancer mitochondria generate ATP through OXPHOS. Instead, it surveys mitochondrial pathways in which oxygen, acting as the terminal electron acceptor of the electron transport chain, supports tumor progression through functions that are not reducible to ADP phosphorylation. In this view, respiration is not treated only as an energy-producing process. It is also examined as a mechanism that sustains distinct biochemical currencies, mitochondrial states, and signaling outputs. These include oxidized ubiquinone, matrix NAD⁺, membrane potential, NADPH-generating capacity, biosynthetic precursor supply, cofactor assembly, and reactive oxygen species-dependent signaling.
The separation between respiration and phosphorylation is especially relevant in metabolically plastic cancer cells. Electron transport may consume oxygen to maintain an oxidized coenzyme Q pool, regenerate NAD⁺, sustain membrane potential, support redox shuttles, or drive protonmotive-force-dependent reactions. Under some bioenergetic conditions, the ATP synthase contributes little to net ATP production, and in extreme cases it operates in reverse, hydrolyzing ATP to maintain mitochondrial polarization. Thus, oxygen consumption requires careful interpretation: it indicates that electrons reach oxygen, but it does not identify ATP as the relevant biological product.
A clear example is de novo pyrimidine synthesis through dihydroorotate dehydrogenase. DHODH oxidizes dihydroorotate to orotate and transfers electrons to ubiquinone. For this reaction to proceed, ubiquinol must be reoxidized by the downstream electron transport chain, ultimately using oxygen as the terminal electron acceptor. The respiratory chain is therefore required to maintain the oxidized ubiquinone pool that supports nucleotide synthesis. In this setting, the growth-promoting role of respiration is not primarily ATP generation, but the maintenance of a redox currency required for pyrimidine biosynthesis.
Aspartate production provides another important example. Proliferating cancer cells require aspartate for the synthesis of pyrimidines, purines, asparagine, arginine-related metabolites, and additional anabolic products. In many settings, mitochondrial aspartate production depends on matrix NAD⁺ regeneration by the respiratory chain, because enzymes such as mitochondrial malate dehydrogenase require an oxidized nicotinamide nucleotide pool to sustain flux. When electron transport is impaired, growth arrest may therefore arise from insufficient aspartate or redox imbalance rather than from a direct deficit in ATP. This has become one of the clearest examples showing that respiratory dependence may be biosynthetic and redox-based rather than energetic in the narrow sense.
Mitochondrial one-carbon metabolism extends the same principle. Enzymes such as SHMT2 and MTHFD2 participate in folate-dependent reactions that support purine synthesis, thymidylate production, formate export, mitochondrial translation, methylation metabolism, and redox homeostasis. These reactions depend on a matrix environment in which NAD⁺/NADH balance is maintained by respiratory electron flow. Here again, the relevance of oxygen consumption lies not in proving ATP synthesis, but in preserving the redox conditions required for anabolic and regulatory chemistry.
The review also emphasizes respiration-supported reducing power, particularly through nicotinamide nucleotide transhydrogenase. NNT uses the protonmotive force to convert NADH and NADP⁺ into NAD⁺ and NADPH. This reaction depends on mitochondrial energization, but it is not phosphorylation. Its immediate product is mitochondrial NADPH, a reducing currency that supports antioxidant systems, lipid-related metabolism, iron-sulfur cluster biogenesis, and other matrix processes. In this case, the protonmotive force is directed toward redox maintenance rather than ATP production, illustrating why respiration and OXPHOS should not be used interchangeably without qualification.
Other pathways broaden this view of mitochondrial respiration. The glycerol-3-phosphate shuttle connects cytosolic redox balance to the mitochondrial ubiquinone pool and influences glycolytic flux, lipid metabolism, and ferroptosis sensitivity. The tricarboxylic acid cycle relies on respiratory NAD⁺ regeneration to sustain the production of citrate, oxaloacetate, α-ketoglutarate, succinyl-CoA, and other intermediates that feed biosynthesis and signaling. Heme synthesis and iron-sulfur cluster biogenesis require membrane potential, iron handling, redox balance, and respiratory competence. Proline and hydroxyproline oxidation, particularly in collagen-rich tumor microenvironments, support anaplerosis, glycine production, and redox signaling. Sulfide oxidation protects cells from H₂S toxicity while feeding electrons into the respiratory chain. Mitochondrial choline oxidation links respiration to betaine, methyl-group metabolism, and nucleotide-related pathways. Taken together, these examples encourage a broader view of the cancer mitochondrion as a respiratory redox platform.
This distinction also affects the interpretation of respiratory inhibition. When a tumor cell is sensitive to complex I, complex III, or complex IV inhibition, the consequence should not automatically be assigned to ATP depletion. The relevant vulnerability may instead be loss of aspartate production, interruption of DHODH activity, depletion of mitochondrial NADPH, failure of iron-sulfur cluster biogenesis, collapse of redox shuttling, or disruption of ROS-dependent signaling. In such cases, ATP measurements alone may miss the reason why respiratory inhibition suppresses growth.
The central message is therefore not that cancer mitochondria should be classified simply as OXPHOS-positive or OXPHOS-negative. Such categories are often too coarse for the metabolic architecture of tumors. A more informative question is what respiration maintains in a given cancer context. In some cases, phosphorylation may be central. In others, oxygen-dependent electron transport may be required mainly to sustain nucleotide synthesis, redox poise, membrane potential, NADPH generation, cofactor assembly, methyl metabolism, lipid homeostasis, or adaptive signaling.
By separating oxygen consumption from ATP synthesis, the review places mitochondrial respiration in a broader setting. Cancer mitochondria breathe for many reasons. Phosphorylation maybe one of them, but it is not the only one, and in some settings it may not be the most relevant one.