Amino Acid Degradation and Synthesis

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Chapter: Biochemistry : Amino Acid Degradation and Synthesis

The catabolism of the amino acids involves the removal of α-amino groups, followed by the degradation of the resulting carbon skeletons.


Amino Acid Degradation and Synthesis

OVERVIEW

The catabolism of the amino acids involves the removal of α-amino groups, followed by the degradation of the resulting carbon skeletons. These pathways converge to form seven intermediate products: oxaloacetate, pyruvate, α-ketoglutarate, fumarate, succinyl coenzyme A (CoA), acetyl CoA, and acetoacetate. These products directly enter the pathways of intermediary metabolism, resulting either in the synthesis of glucose or lipid or in the production of energy through their oxidation to CO2 by the tricarboxylic acid (TCA) cycle. Figure 20.1 provides an overview of these pathways, with a more detailed summary presented in Figure 20.14. Nonessential amino acids (Figure 20.2) can be synthesized in sufficient amounts from the intermediates of metabolism or, as in the case of cysteine and tyrosine, from essential amino acids. In contrast, the essential amino acids cannot be synthesized (or produced in sufficient amounts) by the body and, therefore, must be obtained from the diet in order for normal protein synthesis to occur. Genetic defects in the pathways of amino acid metabolism can cause serious disease.


Figure 20.1 Amino acid metabolism shown as a part of the essential pathways of energy metabolism. (See Figure 8.2 , for a more detailed view of these processes.) CoA = coenzyme A.


Figure 20.2 Classification of amino acids. [Note: Some amino acids can become conditionally essential. For example, supplementation with glutamine and arginine has been shown to improve outcomes in patients with trauma, postoperative infections, and immunosuppression.]


Figure 20.14 Summary of the metabolism of amino acids in humans. Genetically determined enzyme deficiencies are summarized in white boxes. Nitrogen-containing compounds derived from amino acids are shown in small, yellow boxes. Classification of amino acids is color coded: Red = glucogenic; brown = glucogenic and ketogenic; green ketogenic. Compounds in BLUE ALL CAPS are the seven metabolites to which all amino acid metabolism converges. CoA = coenzyme A; NAD(H) = nicotinamide adenine dinucleotide.

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