a-Amino acids, in addition to their role as protein mono- meric units, are energy metabolites and precursors ofmany biologically important nitrogen containing compounds. no tably heme, physiologically active amines ghitathione nucleotides, and nucleotide coenzymes. Amino acids are classified into two groups: essential and nonessential. Mammals synthesize the nonessential amino acids from metabolic precursors but must obtain the essential amino acids from their diet. Excess dietary amino acids are neither stored for future use nor excreted. Rather, they are con- verted to common metabolic intermediates such pyru- vate, oxaloacetate, and ketoglutarate. Consequently, amino acids are also precursors of glucose, faty acids, and ketone bodies are therefore metabolic fiels. In this chapter, we consider the pathways of amino acid breakdown, synthesis, and utilization. We begin by exam- ining the three common stages of amino acid breakdown: Deamination (amino group removal), whereby amino groups are converted either ammonia or to the amino group of aspartate. 2. Incorporation of ammonia and aspartate nitrogen atoms into urea excretion.