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

The two calorespirometric ratios, the ratio of metabolic heat rate to the rate of CO2 production and the ratio of metabolic heat rate to the rate of O2 uptake (Rq/RCO2 and Rq/RO2, respectively), provide different information about the activities of metabolic pathways. In a steady state system, Rq/RCO2 depends only on the oxidation state of the substrate and Rq/RO2 equals Thornton's constant or the oxycaloric equivalent. In a growing or developing system, the measured ratio Rq/RO2 differs from the oxycaloric equivalent only if reactions that do not consume oxygen and have nonzero ΔH are present. Relative rates of aerobic and anaerobic (with ΔH ≠ 0) reactions can thus be calculated from the measured Rq/RO2, but the substrate carbon conversion efficiency cannot. The difference between the Rq/RCO2 ratio predicted from Thornton's rule and the actual measured ratio contains information on the rates of anaerobic reactions with ΔH ≈ 0. The latter ratio thus allows partitioning of the CO2 production rate between oxidative catabolism and anabolic reactions with ΔH ≈ 0. This partitioning allows calculation of substrate carbon conversion efficiency and rates of growth or development.