Biologic oxidation of foodstuffs
Liberation offood energy in the body is not a process of instant combustion as in the bomb calorimeter. It is a slow, gradual redistribution of energy of the original molecules into intermediates oflower energy value with concomitant release of both heat and usable energy (as ATP) from the oxidative and other energy-yielding reactions of the metabolic pathways.
Biologic oxidations include all reactions in which electrons are removed from an atom or ion that is part of the substrate; in some oxidations, the electron loss is accompanied by an addition of oxygen to or removal of hydrogen from the substance being oxidized. Oxidation of one substance (loss of electrons) always results in the reduction of another substance (gain of electrons, which may be accompanied by loss of oxygen or gain of hydrogen); oxygen is commonly referred to as an oxidizing agent or electron acceptor. Although oxygen may serve as an electron (hydrogen) acceptor in biologic reactions, with the formation of water (H20) or hydrogen peroxide (H202), most of the intermediary steps in the oxidation of foodstuffs initially involve other electron or hydrogen acceptors. The major ones are the coenzymes, nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD). Both are required for the initial breakdown of glucose, amino acids, and fatty acids as well as the final oxidative cycle of energy production, the citric acid cycle, where the carbons of these nutrients become oxidized to CO2, The resulting reduced coenzymes, often referred to as reducing equivalents (NADH + H+ and F ADH2), are either used in the synthesis of new compounds (in energy requiring reductive steps) or reoxidized by the enzymes of the respiratory chain. Oxygen serves as the final hydrogen acceptor in this stepwise electron transfer, which is accompanied by capture of energy as A TP in the so-called oxidative or respiratory chain phosphorylation. This oxidative phosphorylation at the respiratory chain level is the major means of ATP production in the body.
A sizable fraction of total ATP formation takes place directly at the substrate level, linked to the cleavage of a high-energy bond, as seen in the conversion of 1,3-diphosphoglyceric acid to 3-phosphoglyceric acid in the glycolytic breakdown of glucose.The quantitative aspects of ATP production from oxidative metabolism are discussed in the following sections.
