Use of calorespirometric ratios, heat per CO2 and heat per O2, to quantify metabolic paths and energetics of growing cells☆

Abstract The two calorespirometric ratios, the ratio of metabolic heat rate to the rate of CO 2 production and the ratio of metabolic heat rate to the rate of O 2 uptake ( R q / R C O 2 and R q / R O 2 , respectively), provide different information about the activities of metabolic pathways. In a steady state system, R q / R C O 2 depends only on the oxidation state of the substrate and R q / R O 2 equals Thornton's constant or the oxycaloric equivalent. In a growing or developing system, the measured ratio R q / R O 2 differs from the oxycaloric equivalent only if reactions that do not consume oxygen and have nonzero Δ H are present. Relative rates of aerobic and anaerobic (with Δ H ≠ 0) reactions can thus be calculated from the measured R q / R O 2 , but the substrate carbon conversion efficiency cannot. The difference between the R q / R C O 2 ratio predicted from Thornton's rule and the actual measured ratio contains information on the rates of anaerobic reactions with Δ H ≈ 0. The latter ratio thus allows partitioning of the CO 2 production rate between oxidative catabolism and anabolic reactions with Δ H ≈ 0. This partitioning allows calculation of substrate carbon conversion efficiency and rates of growth or development.