Mapping the scope of metabolic flexibility by respiratory flux control: Bioenergetic cluster analysis of mitochondrial fitness

Abstract

Cell respiration studied in coupling-control states of living cells provides diagnostic information on cellular bioenergetics and mitochondrial function. Routine respiration R is under cell physiological control of aerobic ATP demand within limits set by mitochondrial performance. By contrast, mitochondrial coupling control is assessed by relating leak respiration L – when aerobic ATP turnover is arrested – to electron transfer capacity E when oxidative phosphorylation is decoupled from electron transfer. E is not a surrogate for the capacity of oxidative phosphorylation (OXPHOS, P) which requires OXPHOS analysis with permeabilized cells.

Normalization of O₂ consumption by cell count and cell size provides information on cell-specific respiration, whereas normalization to a mitochondrial marker expresses respiration independent of cell-metric variables. Cell respiration normalized to E or R, determined within the coupling-control protocol, yields the respiratory flux control indices:

• coupling efficiency, j_E−L = (E−L)/E
• E−R excess ratio, j_E−R = (E−R)/E
• R-ATP coupling ratio, j_RLE = (R−L)/E
• R-ATP control efficiency, j_R−L = (R−L)/R

Bioenergetic cluster analysis (BCA) provides fundamental quality control by identifying apparent artifacts. More importantly, BCA reveals mechanistic relationships among flux control indices for improved evaluation of mitochondrial fitness. Trajectories and breakpoints in BCA define the theoretical scope of metabolic flexibility. The direction of a respiratory change alone does not necessarily distinguish physiological adaption from emerging dysfunction. Precision OXPHOS analysis delivers the next level.

Cite: Gnaiger E (2026) Mapping the scope of metabolic flexibility by respiratory flux control: Bioenergetic cluster analysis of mitochondrial fitness. BEC preprints 2026.5. https://doi.org/10.26124/becprep.2026-0005