In the catabolism of amino acids, the carbon skeletons follow pathways leading to the formation of glucose and/or ketones. Amino acids may be glucogenic, ketogenic or both (Chapter 1). The process of ketogenesis from leucine is shown in Fig. 4.13. It will be noted that leucine is exclusively ketogenic. Lysine is also ketogenic (Fig. 4.14); however, it should be noted that lysine itself does not participate in transamination. On the other hand, two separate transamination reactions are involved in the catabolism of cysteine leading to the glucogenic precursor, pyruvate (Fig. 4.15). The pathways of gluconeogenesis and ketogenesis from amino acids have been elucidated and are presented in Fig. 4.16. The key intermediate in the synthesis of glucose is phosphoenolpyruvate.
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