Glucose, aerobic oxidation formation

TABLE 16-1 Stoichiometry of Coenzyme Reduction and ATP Formation in the Aerobic Oxidation of Glucose via Glycolysis, the Pyruvate Dehydrogenase Complex Reaction, the Citric Acid Cycle, and Oxidative Phosphorylation... 

The analytical data show that gold catalysis and enzymatic catalysis allow fast and selective aerobic oxidation of glucose according to the same stoichiometry characterized by the formation of hydrogen peroxide as the by-product (Eq. (21.1)) [8]. However, it is not surprising that completely different catalytic systems adopt different reaction mechanisms as shown by the kinetic studies on commercial enzymatic preparations containing /wcose oxidase and catalase [13]. The results of the research support a Michaelis-Menten type mechanism where the kinetic... 

Glucose is metabolized to pyruvate by the pathway of glycolysis, which can occur anaerobically (in the absence of oxygen), when the end product is lactate. Aerobic tissues metabolize pyruvate to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO2 and HjO, linked to the formation of ATP in the process of oxidative phosphorylation (Figure 16-2). Glucose is the major fuel of most tissues. 

Figure 22.17 Summary of mechanisms to maintain the ATP/ADP concentration ratio in hypoxic myocardium. A decrease in the ATP/ADP concentration ratio increases the concentrations of AMP and phosphate, which stimulate conversion of glycogen/ glucose to lactic acid and hence ATP generation from glycolysis. The changes also increase the activity of AMP deaminase, which increases the formation and hence the concentration of adenosine. The latter has two major effects, (i) It relaxes smooth muscle in the arterioles, which results in vasodilation that provides more oxygen for aerobic ATP generation (oxidative phosphorylation). (ii) It results in decreased work by the heart (i.e. decrease in contractile activity), (mechanisms given in the text) which decreases ATP utilisation.

Under aerobic conditions, the glycolytic pathway becomes the initial phase of glucose catabolism (fig. 13.2). The other three components of respiratory metabolism are the tricarboxylic acid (TCA) cycle, which is responsible for further oxidation of pyruvate, the electron-transport chain, which is required for the reoxidation of coenzyme molecules at the expense of molecular oxygen, and the oxidative phosphorylation of ADP to ATP, which is driven by a proton gradient generated in the process of electron transport. Overall, this leads to the potential formation of approximately 30 molecules of ATP per molecule of glucose in the typical eukaryotic cell. 

The formation of acetyl CoA from carbohydrates is less direct than from fat. Recall that carbohydrates, most notably glucose, are processed by glycolysis into pyruvate (Chapter 16). Under anaerobic conditions, the pyruvate is converted into lactic acid or ethanol, depending on the organism. Under aerobic conditions, the pyruvate is transported into mitochondria in exchange for OH by the pyruvate carrier, an antiporter (Section 13.4). In the mitochondrial matrix, pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex to form acetyl CoA. 

Under aerobic conditions, pyruvate can be oxidatively decarboxylated via the pyruvate dehydrogenase multienzyme complex to yield acetyl-CoA, which can then be completely oxidised via the citric acid cycle (Fig. 2). In eubacteria growing anaerobically, pyruvate is metabolised fermentatively, thus serving as an electron sink for reducing equivalents generated in its formation from glucose. The diverse array of possible fermentative reactions from pyruvate is reviewed in [5]. 

While this book is not the appropriate place for a detailed discussion of the ADP-ATP cycle, perhaps a bit more mechanistic detail is useful. The aerobic metabolism of glucose to produce ATP from ADP can be-considered to consist of three parts the fermentation of glucose to form pyruvate (and a small amount of ATP from ADP), the conversion of the pyruvate to carbon dioxide in the citric acid cycle in which NAD" " and FAD (the oxidized form of flavin adenine dinucleotide) are converted to NADH and FADH2, and their oxidation in the respiratory chain, resulting in the formation of a larger quantity of ATP from ADP. If oxygen is not available, so the reaction is anaerobic, only the first step, formation of pyruvate, occurs with a small amount of ATP production. 

---