Amino Acids that form Acetyl CoA or Acetoacetyl CoA

Tryptophan, leucine, isoleucine, and lysine form acetyl CoA or acetoacetyl CoA directly, without pyruvate serving as an intermediate. As noted earlier, phenylalanine and tyrosine also give rise to acetoacetate during their catabolism . Therefore, there are a total of six partly or wholly ketogenic amino acids.
1. Tryptophan: This amino acid is both glucogenic and ketogenic, because its catabolism yields alanine and acetoacetyl CoA (Fig. 20.10). [Note: Quinolinate from tryptophan catabolism is used in the synthesis of nicotinamide adenine dinucleotide ([NAD].]

Figure 1: Metabolism of tryptophan by the kynurenine pathway (abbreviated). CoA = coenzyme A; PRPP = phosphoribosyl pyrophosphate; NAD(H) = nicotinamide adenine dinucleotide.
2. Leucine: This amino acid is exclusively ketogenic, because its catabolism yields acetyl CoA and acetoacetate (Fig. 2). The first two reactions in the catabolism of leucine and the other BCAA, isoleucine and valine, are catalyzed by enzymes that use all three BCAA (or their derivatives) as substrates .

Figure 2:  Degradation of leucine, valine, and isoleucine. [Note: β-Methylcrotonyl CoA carboxylase is one of four biotin-requiring carboxylases . The other three are pyruvate carboxylase, acetyl CoA carboxylase, and propionyl CoA carboxylase.] TPP = thiamine pyrophosphate; FAD = flavin adenine dinucleotide; CoA = coenzyme A; NAD = nicotinamide adenine dinucleotide; HMG = hydroxymethylglutarate.
3. Isoleucine: This amino acid is both ketogenic and glucogenic, because its metabolism yields acetyl CoA and propionyl CoA.
4. Lysine: This amino acid is exclusively ketogenic and is unusual in that neither of its amino groups undergoes transamination as the first step in catabolism. Lysine is ultimately converted to acetoacetyl CoA.