Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Metabolism of Carbohydrates and Lipids
Oxidative Phosphorylation and Mitochondrial Transport Systems
The Role of the Respiratory Chain in Energy Capture

ADP serves as the molecule that captures a portion of the Free energy released during catabolic processes in the form of high-energy phosphates. The resulting ATP subsequently supplies this free energy to drive energy-requiring processes. Therefore, ATP can be described as the cellular energy "currency" (see Fig. 11.8).

Glycolysis yields two high-energy phosphate groups, with an energy equivalent of approximately 61 kJ∙mol-1 of glucose (see Table 18.1). Since the complete combustion of glucose releases about 2780 kJ, the fraction of energy captured via phosphorylation during glycolysis is quite small. The Citric Acid Cycle reactions, which complete The process of total glucose oxidation, include another phosphorylation stage; this accompanies The conversion of succinyl-CoA to succinate, yielding two additional high-energy phosphates per mole of glucose. All of the phosphorylation reactions discussed above occur at the substrate level. Estimates of energy-capture efficiency in intact Cell/35.html">Mitochondria show that during substrate oxidation mediated by NAD-dependent dehydrogenases and the Respiratory Chain, 3 moles of inorganic phosphate are incorporated into ADP, producing 3 moles of ATP per 1/2 mole of oxygen consumed. The P : O ratio equals 3 (Fig. 13.6). At the same time, The oxidation of a substrate via flavoprotein dehydrogenase yields only 2 moles of ATP, meaning P : O = 2. These reactions are referred to as Oxidative Phosphorylation at the respiratory chain level. As a result of dehydrogenation processes during Glucose Catabolism through glycolysis and The Citric Acid cycle—culminating in oxidative phosphorylation, alongside substrate-level phosphorylation—approximately 42% of the Free energy of glucose combustion is captured in the form of high-energy phosphates. Clearly, ATP formation occurs primarily through the functioning of the respiratory chain.



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