Amino Acid Catabolism & Inborn Errors Tournament
Two onboarding diagrams orient you in amino-acid metabolism. Then eight MCAT-DoK quiz rounds: PKU + BH4 logic, MSUD's BCKD complex (PDC's third cousin), the Phe -> Tyr -> homogentisate -> fumarate path that hides alkaptonuria, ketogenic vs. glucogenic amino acid carbons, and the B6 / B12 cofactor backbone of inborn-error pharmacology.
Where the Metabolism of amino acids & derivatives fits in Whole-cell metabolism overview
Amino acid catabolism funnels 20 different carbon skeletons into seven TCA-cycle entry points (or acetyl-CoA / acetoacetate for the ketogenic ones). The classic inborn errors of metabolism - PKU, alkaptonuria, MSUD, homocystinuria - are point failures along these pathways. Click the highlighted Metabolism of amino acids box to enter the tournament.
Click the highlighted Metabolism of amino acids & derivatives box to continue.
What this tournament tests
Each task maps to a distinct MCAT cognitive demand. The first two orient you in the broader topology; the next eight test the high-yield mechanism, regulation, sequence and quantitative reasoning that consistently appear on test day.
The Bigger Picture
Anchor amino-acid catabolism in whole-cell metabolism on the live Reactome map.
Whole-Pathway Overview
Pan and zoom the curated WikiPathways Phe-Tyr pathway figure before you start answering.
Fill in the Blank
Recall PAH (BH4-dependent) as the deficient enzyme in classic PKU.
Disruptor
Diagnose MSUD from maple-syrup urine + branched-chain α-keto acid accumulation.
Sequence Ordering
Trace Phe -> Tyr -> p-OH-phenylpyruvate -> homogentisate -> fumarate + acetoacetate.
Match the Pairs
Pair each inborn error (PKU, alkaptonuria, MSUD, homocystinuria, MMA) with its deficient enzyme.
Numeric Input
Count the two purely ketogenic amino acids (lysine + leucine).
Select All That Apply
Identify TRUE facts about transamination, B6, glutamine vs. alanine N-carriers, and glucogenic potential.
Odd One Out
Distinguish vitamin-cofactor-dependent enzymes (B6, B12, folate) from glucose-6-phosphatase.
Homocystinuria Disruptor
Predict why classic CBS deficiency raises BOTH homocysteine AND methionine, with marfanoid + thrombosis features.
Public leaderboard
Your score posts to a global, persistent leaderboard scored by points first, time as tiebreaker.
Amino acid catabolism in 60 seconds
Most amino acids start with transamination (PLP / B6 cofactor) to yield an α-keto-acid + glutamate. Glutamate hands off the nitrogen to glutamine or alanine for safe plasma transport to the liver, where the urea cycle finishes the job.
Carbon skeletons converge on seven TCA entry points: pyruvate, acetyl-CoA, acetoacetate, α-KG, succinyl-CoA, fumarate, OAA. Lysine + Leucine are purely ketogenic; Phe, Tyr, Trp, Ile, Thr are mixed; everything else is purely glucogenic.
The classic inborn errors are point failures in this network: PKU = PAH (or BH4); alkaptonuria = homogentisate oxidase; MSUD = BCKD (PDC's third cousin); homocystinuria = CBS (B6); methylmalonic acidemia = methylmalonyl-CoA mutase (B12).
Two cofactors anchor most of these enzymes: B6 (PLP) for transaminases + CBS, and B12 (cobalamin) for methylmalonyl-CoA mutase + methionine synthase. B6-responsive homocystinuria responds to high-dose pyridoxine; some MMA cases respond to B12.
FAQ
Why does PKU cause intellectual disability?
Phenylalanine accumulates and competes with tyrosine + tryptophan for the LAT1 transporter at the blood-brain barrier, depleting brain dopamine and serotonin. Phe metabolites (phenylpyruvate, phenyllactate) also disrupt myelination and neurotransmitter synthesis.
Why does alkaptonuria turn urine black?
Homogentisate accumulates and oxidizes spontaneously to a black quinone-like polymer on standing. The same pigment deposits in cartilage and connective tissue (ochronosis), causing premature osteoarthritis later in life.
How do you tell classic homocystinuria from a methionine synthase or MTHFR defect?
Methionine LEVEL is the discriminator. Classic CBS deficiency: homocysteine HIGH, methionine HIGH. Methionine synthase / MTHFR defect: homocysteine HIGH, methionine LOW (because the remethylation route is broken).
Do I need an account to play?
No. The tournament is fully public. You get a randomized handle and your score posts to the public leaderboard at the bottom of this page.
Keep going
Where the nitrogen ends up - the liver-only path from NH3 + aspartate to urea.
Glucogenic amino acids feed gluconeogenesis via OAA, pyruvate, α-KG, succinyl-CoA, fumarate.
Overview diagram: Reactome Pathway R-HSA-1430728, licensed CC BY 4.0.