ketogenic amino acids

DEFINITION: Ketogenic amino acids are degraded to acetyl-CoA or acetoacetyl-CoA/acetoacetate. Because mammals cannot convert acetyl-CoA into a net gain of oxaloacetate (the pyruvate dehydrogenase reaction is irreversible, and the 2 carbons of acetyl-CoA are lost as CO2 in the TCA cycle), these amino acids cannot contribute to net glucose synthesis. They can instead be converted to ketone bodies (acetoacetate, beta-hydroxybutyrate) in hepatic mitochondria, or to fatty acids.

CLASSIFICATION: - Purely ketogenic: Leucine and Lysine. - Both ketogenic and glucogenic (5): Isoleucine, Phenylalanine, Tyrosine, Threonine, Tryptophan. Mnemonic: "PITT" (Phe, Ile, Thr, Trp) plus Tyr. Isoleucine yields acetyl-CoA and propionyl-CoA (-> succinyl-CoA); Phe/Tyr yield fumarate (glucogenic) and acetoacetate (ketogenic); Thr yields pyruvate/succinyl-CoA and acetyl-CoA via some pathways; Trp yields alanine/pyruvate and acetyl-CoA. - All the others are purely glucogenic (yield pyruvate or TCA intermediates such as alpha-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate).

KEY PATHWAYS: - Leucine: transamination to alpha-ketoisocaprohexanoate, then oxidative decarboxylation by branched-chain alpha-ketoacid dehydrogenase (BCKDH) -> ... -> acetoacetate + acetyl-CoA (via HMG-CoA). - Lysine: degraded via saccharopine -> alpha-ketoadipate -> glutaryl-CoA -> crotonyl-CoA -> acetoacetyl-CoA -> acetyl-CoA. Lysine does not undergo transamination at its alpha-amino group like most amino acids.

CLINICAL CORRELATIONS: - Maple syrup urine disease (MSUD): deficiency of BCKDH (requires thiamine/B1, lipoate, CoA, FAD, NAD+) -> accumulation of branched-chain amino acids (Leu, Ile, Val) and their ketoacids. Presents with poor feeding, vomiting, lethargy, encephalopathy, sweet-smelling (maple syrup) urine and neurologic damage. Leucine is the main neurotoxic agent. Treatment: restrict BCAAs in diet, thiamine supplementation in some. - Phenylketonuria (PKU): phenylalanine hydroxylase or BH4 deficiency -> Phe accumulation; tyrosine becomes essential. Musty/mousy odor, intellectual disability, fair skin, eczema. Treatment: low Phe diet, supplement tyrosine; sapropterin for BH4-responsive; avoid aspartame. - Alkaptonuria (homogentisate oxidase deficiency in Tyr degradation): dark urine on standing, ochronosis, arthritis. - Glutaryl-CoA dehydrogenase deficiency (glutaric acidemia type 1): lysine/hydroxylysine/tryptophan catabolism defect; macrocephaly, striatal injury during catabolic stress. - Fasting/starvation and diabetic ketoacidosis: increased proteolysis supplies glucogenic amino acids (alanine, glutamine) for gluconeogenesis; ketogenic amino acids contribute to ketone production but not glucose. Fatty acid beta-oxidation of even-chain fatty acids similarly yields only acetyl-CoA and cannot produce net glucose, the same logic as the ketogenic amino acids. - Odd-chain fatty acids and Ile/Val/Met/Thr produce propionyl-CoA -> methylmalonyl-CoA -> succinyl-CoA, which is glucogenic (needs biotin and B12).

HIGH-YIELD TESTABLE POINTS: 1. Only Leu and Lys are exclusively ketogenic. 2. Ketogenic products: acetyl-CoA / acetoacetate; cannot generate net glucose. 3. Leucine is the classic ketogenic BCAA; BCKDH defect = MSUD. 4. Essential amino acids include Leu, Lys, Ile, Phe, Thr, Trp (and Val, Met, His, Arg conditionally). Both purely ketogenic amino acids are essential, so they must come from the diet. 5. Ketogenic diets and ketone-body physiology: ketone bodies are made in liver mitochondria via HMG-CoA synthase (rate-limiting); the liver cannot use them because it lacks thiophorase (succinyl-CoA:acetoacetate CoA transferase).