Amino acid degradation glucose-alanine cycle

The glucose-alanine cycle between the liver and the musculature is particularly significant. In muscle tissue, ammonia is generated during the degradation of amino acids (particularly the branched-chain amino acids). The transfer of ammonia to pyruvate yields alanine, which is then transported through the bloodstream to... 

See also Amino Acid Degradation, Transamination in Amino Acid Metabolism, Utilization of Ammonia, Glucose Alanine Cycle... 

Fig. 38.10. The glucose/alanine cycle. Within the muscle, amino acid degradation leads to the transfer of nitrogens to a-ketoglutarate and pym-vate. The alanine formed travels to the liver, where the carbons of alanine are used for gluconeogenesis and the alanine nitrogen is used for urea biosynthesis. This could occur during exercise, when the muscle uses blood-borne glucose (see Chapter 47).

Pyruvic acid is important as an intermediate in sugar metabolism. This acid is formed as the final product of the glycolytic pathway from 3-phosphoglycerate via phosphoenolpyruvate. Pyruvic acid is also formed through the oxidative pentose phosphate cycle. The degradation of glucose in the cycle yields the C3 product glyceraldehyde 3-phosphate, which can be oxidized to pyruvate. Pyruvic acid is the principal precursor for the biosynthesis of amino acids such as alanine, as well as leucine and valine. 

Acetyl CoA serves as a common point of convergence for the major pathways of fuel oxidation. It is generated directly from the (3-oxidation of fatty acids and degradation of the ketone bodies (3-hydroxybutyrate and acetoacetate (Fig. 20.14). It is also formed from acetate, which can arise from the diet or from ethanol oxidation. Glucose and other carbohydrates enter glycolysis, a pathway common to all cells, and are oxidized to pyruvate. The amino acids alanine and serine are also converted to pyruvate. Pyruvate is oxidized to acetyl CoA by the pyruvate dehydrogenase complex. A number of amino acids, such as leucine and isoleucine are also oxidized to acetyl CoA. Thus, the final oxidation of acetyl CoA to CO2 in the TCA cycle is the last step in all the major pathways of fuel oxidation. 

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