glucogenic amino acids
Summary
Glucogenic amino acids are amino acids whose carbon skeletons are degraded to pyruvate or TCA cycle intermediates (α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate), which can be converted to glucose via gluconeogenesis. Purely ketogenic amino acids are only leucine and lysine; isoleucine, phenylalanine, tryptophan, and tyrosine are both glucogenic and ketogenic. All the others are purely glucogenic.
Detail
DEFINITION: Glucogenic amino acids yield pyruvate or a TCA cycle intermediate on catabolism. These can be converted to oxaloacetate and then to phosphoenolpyruvate (via PEPCK) for net glucose synthesis. Ketogenic amino acids yield acetyl-CoA or acetoacetate. These cannot give net glucose in humans, because the two carbons of acetyl-CoA are lost as CO2 in the TCA cycle.
CLASSIFICATION (high-yield): - Purely ketogenic: Leucine, Lysine. Mnemonic: "Leucine and Lysine are Lone ketogenic." - Both glucogenic and ketogenic: Isoleucine, Phenylalanine, Tryptophan, Tyrosine. Mnemonic: "PITT" (Phe, Ile, Thr, Trp). Threonine is also sometimes listed as both. Tyrosine is included as the product of Phe. Mnemonic: "Ill Pho Tyr Trp" or "PITT." - Purely glucogenic: all the remaining amino acids.
ENTRY POINTS INTO METABOLISM: - Pyruvate: Alanine, Serine, Glycine, Cysteine, Threonine, Tryptophan (via alanine pathway) - α-Ketoglutarate: Glutamate, Glutamine, Proline, Arginine, Histidine - Succinyl-CoA: Valine, Isoleucine, Methionine, Threonine (and odd-chain fatty acids). Requires vitamin B12 (methylmalonyl-CoA mutase) and biotin (propionyl-CoA carboxylase). - Fumarate: Phenylalanine, Tyrosine (also aspartate via the urea cycle) - Oxaloacetate: Aspartate, Asparagine - Acetyl-CoA / acetoacetate (ketogenic): Leucine, Lysine, Isoleucine, Phenylalanine, Tyrosine, Tryptophan, Threonine
PHYSIOLOGY AND CLINICAL SIGNIFICANCE: - Fasting/starvation: after glycogen is depleted (about 12-18 hours), muscle protein is broken down. Alanine (glucose-alanine/Cahill cycle) and glutamine are released to the liver (and kidney) for gluconeogenesis. The amino group is disposed of via the urea cycle. - Prolonged starvation: the brain adapts to use ketone bodies, which decreases protein catabolism and spares muscle. - Cortisol and glucagon upregulate gluconeogenic enzymes and promote proteolysis. - Alanine is the major gluconeogenic amino acid carried from muscle to liver. - Transamination (ALT, AST; cofactor B6/PLP) moves amino groups to α-ketoglutarate, forming glutamate, and links amino acid catabolism to gluconeogenesis. - Ketogenic-only amino acids matter in disease. In maple syrup urine disease (branched-chain α-ketoacid dehydrogenase deficiency), Leu, Ile, and Val accumulate (thiamine-responsive in some cases). In phenylketonuria and alkaptonuria, Phe/Tyr catabolism is impaired. Their products are fumarate and acetoacetate. - Because only leucine and lysine are exclusively ketogenic, fatty acids (even-chain) cannot be converted to net glucose, but glycerol and odd-chain propionyl-CoA can. - Ethanol metabolism: increased NADH shifts pyruvate to lactate and oxaloacetate to malate, impairing gluconeogenesis and causing fasting hypoglycemia.
BOARD PEARL: Only leucine and lysine are purely ketogenic. They are the only amino acids that can't contribute to net glucose production. Isoleucine, Phe, Thr, Trp, and Tyr yield both.
Sources
- First Aid for the USMLE Step 1 (Biochemistry: Amino Acid Metabolism, Metabolism)
- Lippincott Illustrated Reviews: Biochemistry (Amino Acid Degradation and Synthesis)
- Harper's Illustrated Biochemistry
- Pathoma/Sketchy Biochemistry (Gluconeogenesis, Fasting vs. Starvation)
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.