A patient presents with elevated branched-chain α-keto acids in urine and a maple syrup odor. This accumulation results from deficiency in which enzyme, and what step does it normally catalyze?
ABranched-chain aminotransferase (BCAT) — the initial transamination that generates branched-chain α-keto acids
BBranched-chain α-keto acid dehydrogenase (BCKDH) — the irreversible oxidative decarboxylation that commits BCAAs to catabolism
CGlutamate dehydrogenase — the step that regenerates α-ketoglutarate from glutamate in the transamination cycle
DPyruvate dehydrogenase — which processes the final ketogenic products of leucine degradation
BCKDH catalyzes the second, irreversible step: oxidative decarboxylation of the branched-chain α-keto acids. Because this step is committed and irreversible, its absence causes α-keto acids (produced by the still-functional BCAT step) to accumulate. If BCAT were deficient instead, the keto acids would never be produced and couldn't accumulate. BCKDH is structurally analogous to the pyruvate dehydrogenase complex and requires the same five cofactors.
Question 2 Multiple Choice
During prolonged fasting, a patient relies heavily on muscle protein breakdown for glucose maintenance. Which BCAA catabolism product directly supports gluconeogenesis?
ALeucine's acetoacetate and acetyl-CoA, which enter gluconeogenesis via the citric acid cycle
BValine's succinyl-CoA, a citric acid cycle intermediate that feeds into gluconeogenesis
CLeucine's acetyl-CoA, which the liver converts directly to glucose
DIsoleucine's acetyl-CoA, which feeds gluconeogenesis through the glyoxylate cycle in mammals
Valine is purely glucogenic — its carbon skeleton yields succinyl-CoA, a citric acid cycle intermediate that can feed gluconeogenesis. Leucine is purely ketogenic: its products (acetoacetate and acetyl-CoA) cannot contribute to net glucose synthesis in mammals. Isoleucine is both — it yields succinyl-CoA (glucogenic) and acetyl-CoA (ketogenic). Options C and D describe reactions that do not occur in mammalian metabolism.
Question 3 True / False
Leucine is the only purely ketogenic common amino acid, meaning its carbon skeleton cannot be used for net glucose synthesis.
TTrue
FFalse
Answer: True
Leucine's catabolism ultimately yields acetoacetate and acetyl-CoA — both ketogenic products. Because mammalian metabolism cannot achieve net conversion of acetyl-CoA to oxaloacetate (the glyoxylate cycle is absent), these products cannot contribute to gluconeogenesis. This distinguishes leucine from valine (purely glucogenic, yields succinyl-CoA) and isoleucine (both, yields succinyl-CoA and acetyl-CoA). During fasting, leucine carbons go to ketone body production, not glucose.
Question 4 True / False
Branched-chain amino acids are primarily degraded in the liver, like most other amino acids, because the liver expresses the highest activity of BCAA catabolic enzymes.
TTrue
FFalse
Answer: False
BCAAs are unusual among amino acids precisely because they are catabolized primarily in skeletal muscle, not the liver. The liver has low BCAT activity for BCAAs but high activity for most other amino acid degradation pathways. Muscle expresses high BCAT and substantial BCKDH activity, making it the dominant site of BCAA catabolism. This is why plasma BCAA levels rise rapidly after a protein-containing meal — the liver does not clear them as it does other amino acids.
Question 5 Short Answer
Why is leucine specifically marketed as an effective BCAA supplement for muscle building, beyond its role as a building block for protein synthesis?
Think about your answer, then reveal below.
Model answer: Leucine is a potent activator of the mTOR (mechanistic target of rapamycin) signaling pathway, which directly stimulates muscle protein synthesis at the ribosomal level. This is a signaling function independent of leucine's caloric or structural role. When leucine enters muscle cells after a meal or supplementation, it signals amino acid abundance and activates mTORC1, triggering translation of muscle-structural proteins. No other common amino acid has this mTOR-activating potency, which is why leucine content of protein supplements is specifically highlighted.
The mTOR connection elevates leucine from a mere protein building block to a metabolic signaling molecule. This dual role — fuel and signal — explains the disproportionate emphasis on leucine in sports nutrition relative to valine or isoleucine.