Aspartic acid energy production

A summary of the sources of ATP produced from one molecule of glucose is provided in Table 10.2. ATP production from fatty acids, the other important energy source, is discussed in Chapter 12. Several aspects of this summary require further discussion. Recall that two molecules of NADH are produced during glycolysis. When oxygen is available, the oxidation of this NADH by the ETC is preferable (in terms of energy production) to lactate formation. The inner mitochondrial membrane, however, is impermeable to NADH. Animal cells have evolved several shuttle mechanisms to transfer electrons from cytoplasmic NADH to the mitochrondrial ETC. The most prominent examples are the glycerol phosphate shuttle and the malate-aspartate shuttle. 

The enzymatic hydrolysis of these important co-amides of glutamic and aspartic acids is widespread in organisms. These enzymes have been reviewed by Zittle, and little more can be added here. Except in the renal production of urinary ammonia the hydrolytic cleavage of glutamine is probably an unimportant reaction, since uncoupled hydrolysis of these amides would involve a considerable loss of free energy as heat. 

This experiment told us some new things about arginine and urea synthesis (a) that half of the nitrogen of the urea molecule originated specifically in aspartic acid, (b) that fumarate was a product of the reaction and (c) that phosphate bond energy, specifically in the form of ATP, was required for the new carbon to nitrogen bond formed. 

Biotin also serves as a coenzyme for deamination (removal of -NH2) reactions that are necessary for the production of energy from certain amino acids (at least aspartic acid, serine, and threonine) for amino acids to be used as a source of energy, they must first be deaminated—the amino group must be split off. 

Usually only L-amino acids are utilized for producing proteins (Werpy and Petersen, 2004). Thus L-aspartic acid takes part along with other amino acids in protein production via the urea cycle, which helps in detoxifying ammonia (Werpy and Petersen, 2004). As with other amino acids. Asp can also be used to produce glucose for energy, and it can be converted into oxaloacetic acid, which also has a role in energy production (Fig. 15.4). 

FIGURE 15.4 Aspartic acid biosynthesis during a citric acid cycle and its role in energy production. Modified firm... 

After its transport back into the mitochondrial matrix, fumarate is hydrated to form malate, a component of the citric acid cycle. The oxaloacetate product of the citric acid cycle can be used in energy generation, or it can be converted to glucose or aspartate. The relationship between the urea cycle and the citric acid cycle, often referred to as the Krebs bicycle, is outlined in Figure 15.2 ... 

The synthesis of fumarate is a link between the urea cycle and the citric acid cycle. Fumarate is, of course, an intermediate of the citric acid cycle, and it can be converted to oxaloacetate. A transamination reaction can convert oxa-loacetate to aspartate, providing another link between the two cycles (Figure 23.19). In fact, both pathways were discovered by the same person. Flans Krebs. Four high-energy phosphate bonds are required because of the production of pyrophosphate in the conversion of aspartate to argininosuccinate. 

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