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📚 Study Guide: Amino Acid Degradation & Synthesis

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1. Overview

• Catabolism begins with removal of the α-amino group → carbon skeletons enter metabolism as one of 7 key intermediates:
  • Pyruvate, Oxaloacetate, α-Ketoglutarate, Fumarate, Succinyl-CoA, Acetyl-CoA, Acetoacetate
• End products are used for energy (TCA cycle) or biosynthesis (glucose, lipids)
• Nonessential AAs: synthesized from metabolic intermediates or other AAs
• Essential AAs: must come from diet

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2. Glucogenic vs. Ketogenic AAs

High-yield: Leucine & Lysine are the ONLY exclusively ketogenic AAs (cannot form glucose)

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3. Specific Catabolic Pathways

→ Oxaloacetate

→ α-Ketoglutarate

→ Pyruvate

⚠️ Glycine → Oxalate pathway: excess oxalate causes kidney stones in Primary Oxaluria Type 1

→ Fumarate (+ Acetoacetate) - Glucogenic AND Ketogenic

• Phenylalanine → Tyrosine (via phenylalanine hydroxylase + BH₄) → Fumarate & Acetoacetate
• Disorders: PKU, Alkaptonuria, Albinism

→ Succinyl-CoA via Methionine

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4. Homocysteine & Vascular Disease ⚠️

• ↑ Homocysteine → endothelial damage, inflammation, thrombosis risk
• Causes of ↑ Homocysteine: deficiency in folate, B₆, B₁₂
• Associated with neural tube defects
• Classic Homocystinuria (CBS deficiency) → premature vascular events

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5. AAs → Succinyl-CoA, Acetyl-CoA, Acetoacetyl-CoA

→ Succinyl-CoA (Glucogenic)

→ Acetyl-CoA / Acetoacetyl-CoA (Ketogenic)

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6. Branched-Chain Amino Acids (BCAAs) 🔺

• Valine → Succinyl-CoA (glucogenic)
• Leucine → Acetoacetate + Acetyl-CoA (ketogenic)
• Isoleucine → Acetyl-CoA + Succinyl-CoA (both)

⚠️ MSUD (Maple Syrup Urine Disease): deficiency in branched-chain α-keto acid dehydrogenase Isovaleryl-CoA dehydrogenase deficiency → "sweaty feet" odor

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7. Folic Acid & One-Carbon Metabolism

• One-carbon units: formyl, methenyl, methylene, methyl - transferred via THF (Tetrahydrofolate)
• THF = active form of folic acid, synthesized by dihydrofolate reductase (uses 2 NADPH)
• THF transfers 1C units at N5, N10, or both
• Key in DNA synthesis (purines, TMP)
• Folate deficiency → Megaloblastic anemia
• Biotin = 1C carrier for CO₂ (not part of the one-carbon pool)

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8. Biosynthesis of Nonessential Amino Acids

Via Transamination from α-Keto Acids

Via Amidation (ATP-dependent)

Special Pathways

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9. Inborn Errors of Amino Acid Metabolism

Phenylketonuria (PKU) ⭐ Most Clinically Relevant

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Maple Syrup Urine Disease (MSUD)

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Albinism

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Homocystinuria

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Alkaptonuria

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10. Quick-Reference Summary Tables

Vitamin Cofactors in AA Metabolism

Disease Summary - "Which AA, Which Enzyme"

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11. High-Yield Exam Mnemonics

• "Leu & Lys = Pure Keto" - only two exclusively ketogenic AAs
• "BCAAs in Muscle" - Valine, Leucine, Isoleucine catabolized in muscle, not liver
• "SAM I AM" - SAM = S-Adenosylmethionine = major methyl donor
• "Folate + B12 = Recycle Homocysteine" - deficiency → hyperhomocysteinemia
• "Musty Mouse = PKU" - urine/body odor of phenylketones
• "Maple Syrup = MSUD" - sweet urine odor from BCAA accumulation
• "Alkaptonuria Triad" - Dark urine + Ochronosis + Arthritis

• Navy banner headers for each section
• Color-coded tables for every pathway, disease, and cofactor
• Red alert boxes for high-yield warnings (e.g., exclusively ketogenic AAs, clinical pearls)
• Teal info boxes for key concepts
• Full disease profiles for PKU, MSUD, Homocystinuria, Alkaptonuria, and Albinism
• Quick-reference summary tables and mnemonics at the end

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🧬 AMINO ACID DEGRADATION & SYNTHESIS — Complete Detailed Notes

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SECTION 1: Overview of Amino Acid Catabolism

What Happens First: Removal of the α-Amino Group

1. Transamination - The amino group is transferred to an α-keto acid (usually α-ketoglutarate), converting it to glutamate. The original amino acid becomes its corresponding α-keto acid. This reaction needs Vitamin B6 (PLP) as a cofactor and is catalyzed by aminotransferases (transaminases).

   Amino Acid + α-Ketoglutarate → α-Keto Acid + Glutamate

2. Oxidative Deamination - Glutamate is then oxidized by glutamate dehydrogenase (GDH) in the liver mitochondria, releasing the amino group as free NH₄⁺ (ammonium) and regenerating α-ketoglutarate.

   Glutamate + NAD⁺ → α-Ketoglutarate + NH₄⁺ + NADH

What Happens to the Carbon Skeleton?

• Energy generation via the TCA cycle and oxidative phosphorylation
• Gluconeogenesis (making new glucose)
• Lipid synthesis
• Ketone body synthesis

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SECTION 2: Glucogenic vs. Ketogenic Amino Acids

Glucogenic Amino Acids

• Their carbon skeletons produce pyruvate, OAA, α-KG, fumarate, or succinyl-CoA
• All of these are gluconeogenic precursors - they can be converted to glucose
• Examples: Alanine, Glycine, Serine, Threonine, Valine, Methionine, Aspartate, Asparagine, Glutamate, Glutamine, Proline, Arginine, Histidine

Ketogenic Amino Acids

• Their carbon skeletons produce acetyl-CoA or acetoacetate
• These cannot be used for gluconeogenesis (acetyl-CoA cannot be converted to OAA in mammals because the pyruvate dehydrogenase reaction is irreversible)
• They instead form ketone bodies

Exclusively Ketogenic (THE MOST IMPORTANT FACT HERE):

Leucine and Lysine - these are the ONLY two amino acids that are PURELY ketogenic. Their carbon skeletons cannot be used to make glucose under ANY circumstances.

Both Glucogenic AND Ketogenic:

• Isoleucine → both acetyl-CoA AND succinyl-CoA
• Tryptophan → both alanine (glucogenic) AND acetoacetyl-CoA (ketogenic)
• Phenylalanine → both fumarate (glucogenic) AND acetoacetate (ketogenic)
• Tyrosine → same as phenylalanine

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SECTION 3: Specific Catabolic Pathways in Detail

AAs That Form Oxaloacetate

• Asparagine is first deamidated by asparaginase → Aspartate + NH₄⁺
• Aspartate is then transaminated by aspartate aminotransferase (AST) → Oxaloacetate
• Clinical importance: L-asparaginase is used as chemotherapy in ALL (Acute Lymphoblastic Leukemia). Leukemic cells cannot synthesize asparagine (they lack asparagine synthetase), so they depend on plasma asparagine. Giving asparaginase depletes this supply and starves the cancer cells.

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AAs That Form α-Ketoglutarate

• Glutaminase (in kidney, intestine, liver): Glutamine → Glutamate + NH₄⁺
• Glutamate dehydrogenase (GDH): Glutamate → α-KG + NH₄⁺ (or via transamination)
• This is the primary way the body handles ammonia transport from peripheral tissues to the liver

• Proline is oxidized by proline oxidase → glutamate-5-semialdehyde → glutamate → α-KG

• Arginine (from urea cycle or diet) → Ornithine → Glutamate-5-semialdehyde → Glutamate → α-KG

• This is a longer pathway: Histidine → Urocanate → 4-imidazolone-5-propionate → N-formiminoglutamate (FIGlu)
• FIGlu transfers its formimino group to THF (tetrahydrofolate) → Glutamate
• This step is folate-dependent
• The FIGlu Test: When folate is deficient, THF is low, so FIGlu cannot be processed and accumulates in urine. Measuring urine FIGlu after a histidine load is a functional test for folate deficiency.

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AAs That Form Pyruvate

• The simplest transamination: Alanine + α-KG → Pyruvate + Glutamate (via ALT/GPT)
• Alanine is the major gluconeogenic amino acid released from muscle during fasting - the "glucose-alanine cycle": muscle makes alanine from pyruvate + amino groups; the liver takes alanine, makes glucose from pyruvate, and processes the amino group in the urea cycle

1. Serine + THF → Glycine + N⁵,N¹⁰-methylene-THF (serine hydroxymethyltransferase, Vit B6)
2. Serine → Pyruvate directly via serine dehydratase (also needs B6; removes -OH and -NH₂ together)

• Glycine can be converted back to serine (adding a 1C unit from N⁵,N¹⁰-methylene-THF)
• Glycine can also be degraded by the glycine cleavage system → CO₂ + NH₃ + N⁵,N¹⁰-methylene-THF
• Glyoxylate pathway: Glycine → Glyoxylate → Oxalate
  • Normally, glyoxylate is transaminated back to glycine by alanine-glyoxylate aminotransferase (requires Vit B6) in peroxisomes
  • If this enzyme is deficient (genetic) → Primary Hyperoxaluria Type 1: oxalate accumulates, precipitates as calcium oxalate in the kidney → recurrent kidney stones, can lead to renal failure

• Cystine (the disulfide-linked dimer) is reduced to cysteine
• Cysteine is degraded by desulfuration: Cysteine → Pyruvate + H₂S + NH₄⁺ (via cystathionase)
• The sulfur is oxidized to sulfate, which combines with 3'-phosphoadenosine-5'-phosphate to form PAPS (3'-phosphoadenosine-5'-phosphosulfate) - the activated sulfur donor used in sulfation reactions (e.g., heparan sulfate, drug conjugation)

• Route 1: Threonine → Pyruvate + Acetaldehyde (via threonine aldolase)
• Route 2: Threonine → α-Ketobutyrate → Propionyl-CoA → Succinyl-CoA (via propionyl-CoA carboxylase + methylmalonyl-CoA mutase; needs biotin and B12)

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AAs That Form Fumarate (Phenylalanine & Tyrosine)

Phenylalanine
    ↓  (phenylalanine hydroxylase + BH₄)
Tyrosine
    ↓  (tyrosine aminotransferase)
4-Hydroxyphenylpyruvate
    ↓  (4-hydroxyphenylpyruvate dioxygenase)
Homogentisate
    ↓  (homogentisate oxidase)  ← DEFICIENT in ALKAPTONURIA
Maleylacetoacetate
    ↓
Fumarylacetoacetate
    ↓
Fumarate  +  Acetoacetate

• Phenylalanine hydroxylase requires BH₄ (tetrahydrobiopterin) as a coenzyme and O₂
• BH₄ gets oxidized to BH₂ and must be regenerated by dihydropteridine reductase (DHPR)
• Fumarate → glucogenic; Acetoacetate → ketogenic → so Phe and Tyr are BOTH glucogenic and ketogenic

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Methionine → Succinyl-CoA (The SAM Pathway — Very Detailed)

1. Methionine + ATP → SAM (S-Adenosylmethionine)
   • Catalyzed by methionine adenosyltransferase
   • SAM is the universal methyl donor in the body — it donates its methyl group to DNA, proteins, lipids, neurotransmitters, creatine, phosphatidylcholine, etc.
   • After donating a methyl group, SAM becomes SAH (S-adenosylhomocysteine)
2. SAM → SAH → Homocysteine
   • SAH is hydrolyzed → Homocysteine + Adenosine (by SAH hydrolase)
   • Homocysteine is at a metabolic crossroads
3. Homocysteine has two fates:
   a) Remethylation back to Methionine (to conserve methionine)
   • Requires: N⁵-methyl-THF (donates the methyl group) and Vitamin B12 (as methylcobalamin, the cofactor for methionine synthase)
   • This is why B12 deficiency causes "methyl-THF trap" - THF gets stuck as methyl-THF and can't be used for other 1C reactions → folate deficiency symptoms even with adequate folate intake
   b) Transsulfuration → Cysteine (if methionine is in excess)
   • Homocysteine + Serine → Cystathionine (via cystathionine β-synthase, CBS, needs Vitamin B6)
   • Cystathionine → Cysteine + α-Ketobutyrate (via cystathionase, also needs B6)
   • α-Ketobutyrate → Propionyl-CoA → Succinyl-CoA (needs biotin + B12)
   This is why Cysteine is considered conditionally essential — it can only be made if methionine (an essential AA) is available.

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SECTION 4: Homocysteine & Vascular Disease

• Directly damages endothelial cells (oxidative stress, toxic to vascular lining)
• Induces inflammation in vessel walls
• Promotes thrombosis - activates platelets and coagulation factors
• Inhibits anticoagulant pathways (reduces nitric oxide, thrombomodulin)

1. Folate deficiency - can't make N⁵-methyl-THF → can't remethylate homocysteine
2. Vitamin B12 deficiency - methionine synthase can't work even with folate
3. Vitamin B6 deficiency - CBS and cystathionase don't work → transsulfuration blocked
4. CBS enzyme deficiency (Classic Homocystinuria - genetic)

• ↑ risk of atherosclerosis, MI, stroke
• Associated with neural tube defects in the fetus (reason folic acid supplementation is recommended in pregnancy)
• In Classic Homocystinuria: severe vascular events even in young people, ectopia lentis, skeletal abnormalities, osteoporosis, intellectual disability

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SECTION 5: BCAAs → Succinyl-CoA, Acetyl-CoA

→ Succinyl-CoA (Glucogenic)

1. Transamination → respective α-keto acids (α-ketoisovalerate for Val, α-methylbutyryl-CoA pathway for Ile)
2. Oxidative decarboxylation by BCAA dehydrogenase
3. Further degradation → Propionyl-CoA
4. Propionyl-CoA carboxylase + biotin → Methylmalonyl-CoA
5. Methylmalonyl-CoA mutase + B12 → Succinyl-CoA

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SECTION 6: Branched-Chain Amino Acids (BCAAs) — Full Detail

Why Are They Special?

• Unlike most amino acids (which are primarily catabolized in the liver), BCAAs are catabolized mostly in skeletal muscle, heart, and adipose tissue
• This is because the liver has very low branched-chain aminotransferase activity
• Muscle uses BCAAs as a direct fuel source and for protein synthesis

Three-Step Catabolism:

• Enzyme: Branched-chain aminotransferase
• Cofactor: Vitamin B6 (PLP)
• Products: branched-chain α-keto acids (α-ketoisocaproate from Leu, α-ketoisovalerate from Val, α-methylbutyryl from Ile)

• Enzyme: Branched-chain α-keto acid dehydrogenase (BCKAD)
• This is a multi-enzyme complex (similar to pyruvate dehydrogenase and α-KG dehydrogenase)
• Cofactors: TPP (Thiamine/B1), Lipoic Acid, FAD (B2), NAD⁺ (B3), CoA (B5) — remember "TL-FNC"
• Products: branched-chain acyl-CoAs + CO₂ + NADH
• Deficiency of BCKAD → Maple Syrup Urine Disease (MSUD)

• Forms α,β-unsaturated acyl-CoA intermediates (similar to fatty acid β-oxidation)
• Isovaleryl-CoA dehydrogenase acts on leucine's product
  • Deficiency → accumulation of isovaleric acid → characteristic "sweaty feet" or "cheesy" odor

End Products:

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SECTION 7: Folic Acid & One-Carbon Metabolism

What Are "One-Carbon Units"?

• Methyl (-CH₃)
• Methylene (=CH₂)
• Methenyl (=CH-)
• Formyl (-CHO)
• Formimino (-CH=NH)

THF: The One-Carbon Carrier

• Dietary folate (folic acid) → Dihydrofolate (DHF) → Tetrahydrofolate (THF)
• Each reduction step uses NADPH
• Enzyme: Dihydrofolate Reductase (DHFR)
• Methotrexate inhibits DHFR → blocks folate metabolism → cancer chemotherapy

• N⁵-methyl-THF → donates methyl group to homocysteine (makes methionine)
• N⁵,N¹⁰-methylene-THF → donates methylene to dUMP (makes dTMP) and to glycine (makes serine)
• N⁵,N¹⁰-methenyl-THF, N¹⁰-formyl-THF → involved in purine synthesis

• Without THF, you can't make dTMP from dUMP (thymidylate synthase reaction stalls)
• Without dTMP → no DNA synthesis → cells can't divide
• Red blood cell precursors keep growing (RNA/protein synthesis continues) but can't divide → large, immature cells (megaloblasts)
• Results in megaloblastic anemia (also seen in B12 deficiency via the same mechanism)

• Biotin also carries a one-carbon unit, BUT only for CO₂ carboxylation reactions (e.g., pyruvate carboxylase, propionyl-CoA carboxylase)
• Biotin is NOT part of the THF-mediated one-carbon pool

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SECTION 8: Biosynthesis of Nonessential Amino Acids

From α-Keto Acids by Transamination

By Amidation (adding an -NH₂ to the side chain)

• Enzyme: Glutamine synthetase (in brain, liver, muscle)
• Reaction: Glutamate + NH₄⁺ + ATP → Glutamine + ADP + Pi
• Why important: Glutamine is the major non-toxic transport form of ammonia in the blood. It carries excess NH₄⁺ from peripheral tissues to the liver and kidney.

• Enzyme: Asparagine synthetase
• Uses glutamine (not free NH₃) as the amino group donor
• Asparagine = transport and storage form of ammonia in some contexts

Special Biosynthesis Pathways

• Made from glutamate via several steps involving reduction and cyclization
• Glutamate → Glutamate-5-semialdehyde → Δ¹-Pyrroline-5-carboxylate → Proline
• This pathway is reversible (proline can be catabolized back to glutamate)

1. From 3-phosphoglycerate (glycolytic intermediate): 3-PG → 3-phosphohydroxypyruvate → 3-phosphoserine → Serine
2. From Glycine: Glycine + N⁵,N¹⁰-methylene-THF → Serine (catalyzed by serine hydroxymethyltransferase, needs B6)

• The reverse of serine synthesis: Serine → Glycine + N⁵,N¹⁰-methylene-THF
• Glycine is also made from glyoxylate via transamination

• This is the transsulfuration pathway described above
• Homocysteine + Serine → Cystathionine → Cysteine + α-Ketobutyrate
• Cysteine is "conditionally essential" — it requires methionine (essential AA) as sulfur donor
• So if methionine is low in diet, cysteine becomes essential

• Phenylalanine + O₂ + BH₄ → Tyrosine + BH₂ + H₂O
• Enzyme: Phenylalanine hydroxylase
• Tyrosine is also conditionally essential — requires phenylalanine
• If phenylalanine hydroxylase is absent → PKU; tyrosine must then be supplemented in diet

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SECTION 9: Inborn Errors of Amino Acid Metabolism

Understanding the General Concept

1. The substrate of the blocked reaction accumulates (can be toxic)
2. The product is deficient (may itself be essential)
3. The substrate may overflow into alternative pathways, producing toxic metabolites
4. The CNS is particularly vulnerable because many of these metabolites are neurotoxic

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PHENYLKETONURIA (PKU) — The Prototype

• Phenylalanine (Phe) cannot be converted to Tyrosine
• Phe accumulates in blood and tissues
• Phe overflows into alternative pathways:
  • Phe → Phenylpyruvate (via transamination) — phenylketone
  • Phenylpyruvate → Phenyllactate (reduction)
  • Phenylpyruvate → Phenylacetate (oxidative decarboxylation) — causes the odor
• Tyrosine becomes deficient (must be supplemented)

• Phe competes with other large neutral amino acids (Tyr, Trp, Leu, etc.) for transport across the blood-brain barrier via the same transporter (LAT1)
• High Phe blocks entry of these other AAs into the brain
• Tryptophan deficiency → ↓ serotonin synthesis
• Tyrosine deficiency → ↓ dopamine, norepinephrine, melanin synthesis
• Phenylpyruvate directly inhibits pyruvate dehydrogenase → impairs energy metabolism in neurons

• Intellectual disability (if untreated) — the most devastating consequence; begins accumulating damage from birth
• Seizures (EEG abnormalities)
• Microcephaly — brain doesn't grow normally
• Failure to thrive in infancy
• Musty or mousey odor — from phenylacetate (excreted in urine and sweat)
• Hypopigmentation — fair skin, blonde hair, blue eyes even in naturally dark children; because tyrosine is needed for melanin synthesis, and it is deficient in PKU
• Eczema in some patients

• Universal Newborn Screening (Guthrie test, now tandem mass spectrometry): heel-prick blood spot at 24-48 hours after birth (must have had some protein feeding)
• Confirmed by quantitative plasma phenylalanine measurement
• Prenatal diagnosis possible by PAH gene analysis (100+ mutations known)

• Must start within the first 7-10 days of life to prevent intellectual disability
• Low-phenylalanine diet: special medical formula that provides all other amino acids without Phe + careful intake of natural foods (vegetables low in protein)
• Tyrosine supplementation (since Phe can't make it)
• Avoid aspartame (the artificial sweetener is phenylalanine + aspartate → dangerous for PKU patients)
• Lifelong management recommended; stopping the diet in childhood leads to IQ decline
• Sapropterin (BH₄) — available for BH₄-responsive forms; reduces plasma Phe

• A pregnant woman with PKU who doesn't control her diet exposes the fetus to high Phe (fetal blood Phe is even higher than maternal)
• Phe is teratogenic: causes fetal microcephaly, cardiac defects, low birth weight, intellectual disability — even if the baby doesn't have PKU itself
• Strict dietary control must begin before conception and continue throughout pregnancy

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MAPLE SYRUP URINE DISEASE (MSUD)

• The branched-chain keto acids, particularly leucine and α-ketoisocaproate, are directly neurotoxic
• They inhibit transport of other amino acids into the brain
• They impair the pyruvate dehydrogenase complex and TCA cycle
• Cause cerebral edema and demyelination

• Maple syrup smell in urine, sweat, and cerumen (earwax) — from the keto acids
• Usually presents in the first few days of life: poor feeding, lethargy, vomiting
• Progresses rapidly to: metabolic acidosis, seizures, altered consciousness, brain herniation
• If untreated → death in infancy or severe neurological disability

• Classic (most common): Severe, neonatal onset, < 2% enzyme activity
• Intermediate: Onset after infancy, milder
• Intermittent: Normal between episodes; triggered by illness/fasting
• Thiamine-responsive: Thiamine (B1, a cofactor for BCKAD) supplementation partially restores enzyme activity in some mutations

• Acute crisis: Dialysis or glucose infusion to remove toxic metabolites and suppress catabolism
• Long-term: Lifelong diet restricting BCAA intake, with special formula
• Leucine is most closely monitored (most toxic)
• Thiamine supplementation in responsive cases
• Liver transplant has been used in some cases (BCKAD is expressed mainly in liver)

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ALBINISM

• Hypopigmentation of skin, hair, and eyes — from absence of melanin
• Photophobia — eyes are very sensitive to light (melanin normally absorbs light in the retinal pigment epithelium)
• Reduced visual acuity — abnormal development of the fovea and optic nerve pathways (melanin is needed for proper visual development)
• Nystagmus — involuntary eye movements
• Greatly increased risk of skin cancer — especially squamous cell carcinoma; melanin protects against UV radiation-induced DNA damage

• Oculocutaneous Albinism (OCA) — affects skin, hair, eyes; autosomal recessive; several subtypes
  • OCA Type 1 (Tyrosinase-negative): most severe; complete absence of pigment; due to mutations in TYR gene
  • OCA Type 2: mutations in P gene; some residual pigment; most common worldwide
• Ocular Albinism — affects only the eyes; X-linked; skin pigmentation normal
• Rare syndromes: Hermansky-Pudlak (albinism + platelet storage pool disease + pulmonary fibrosis); Chediak-Higashi (albinism + immunodeficiency + neurological problems)

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HOMOCYSTINURIA

• ↑ Homocysteine and ↑ Methionine in blood and urine (both accumulate because the transsulfuration pathway is blocked, and the remethylation pathway runs backward trying to clear homocysteine)
• ↓ Cysteine (can't be made because the pathway is blocked)

1. Eyes: Ectopia lentis — downward dislocation of the lens (vs. upward in Marfan syndrome); myopia; glaucoma; retinal detachment
2. Skeleton: Marfan-like habitus — tall, thin, long limbs, arachnodactyly; osteoporosis (especially vertebral); scoliosis; sternal deformities
3. Vascular: Thromboembolism (the most life-threatening feature) — venous and arterial; strokes, pulmonary embolism, MI; even in young children; homocysteine damages endothelium and promotes clotting
4. CNS: Intellectual disability (variable), psychiatric disorders, seizures

• B6-responsive (about 50% of cases): Milder disease; PLP (active B6) is a cofactor for CBS, and these patients have residual enzyme activity that is boosted by high-dose B6
• B6-nonresponsive: Severe disease; no residual activity

• Low methionine diet (restricts the substrate)
• High-dose Vitamin B6 (for responsive cases) - largest therapeutic dose of a B vitamin used in medicine
• Betaine - an alternative methyl donor that can remethylate homocysteine via betaine-homocysteine methyltransferase (BHMT) — bypasses the need for folate/B12
• Folate and B12 supplementation — to maximize remethylation

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ALKAPTONURIA

1. Dark urine (Homogentisic aciduria):
   • HGA is excreted in urine; urine is normal-colored when fresh but darkens on standing (oxidation to alkapton)
   • Classic sign: diaper staining in infancy — parents notice dark stains on diapers
   • This is often the ONLY finding for the first 3-4 decades of life
2. Ochronosis:
   • HGA and its polymers bind to and accumulate in collagen-containing connective tissues
   • Causes a blue-black or grayish-black pigmentation visible in:
     • Sclerae of the eyes (first visible sign in adults)
     • Ear cartilage
     • Nasal cartilage
     • Tendons
     • Skin over bony prominences
3. Arthritis:
   • HGA deposits in joint cartilage → degeneration → severe arthropathy
   • Affects large joints primarily: spine, hips, knees
   • Can become severely debilitating; mimics osteoarthritis but at earlier age
   • Spinal involvement → calcification, fused vertebrae (bamboo spine appearance on X-ray, similar to ankylosing spondylitis)

• Patient is essentially asymptomatic for 30-40 years (the dark urine is an abnormality, not a symptom)
• Ochronosis and arthritis develop in middle age (40s-50s)
• Alkaptonuria is actually one of the first inborn errors of metabolism described (by Archibald Garrod in 1902)

• Nitisinone (NTBC, an inhibitor of 4-hydroxyphenylpyruvate dioxygenase) — blocks HGA production upstream; reduces HGA accumulation; used in clinical trials
• Low phenylalanine & tyrosine diet — reduces substrate
• Management of arthritis (analgesics, joint replacement)
• No treatment corrects the disease fully

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SECTION 10: Key Cofactor Summary

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SECTION 11: High-Yield Exam Points

1. Leucine & Lysine = only purely ketogenic AAs
2. Glucose-Alanine Cycle: muscle → alanine → liver → glucose (alanine = major gluconeogenic AA)
3. SAM = major methyl donor for all methylation reactions in the body
4. FIGlu test = functional test for folate deficiency (histidine load → FIGlu accumulates in urine if folate low)
5. Asparaginase = leukemia drug that exploits asparagine dependency of leukemic cells
6. BCAAs are catabolized in muscle, not liver (opposite of most AAs)
7. BCKAD cofactors = TPP, lipoic acid, FAD, NAD⁺, CoA (same as PDH complex)
8. Methyl-THF trap: B12 deficiency traps folate as methyl-THF → functional folate deficiency even with adequate dietary folate
9. Cysteine & Tyrosine are conditionally essential (depend on Met and Phe respectively)
10. PKU: Phe hydroxylase deficient → musty odor, hypopigmentation, intellectual disability; avoid aspartame
11. Maternal PKU = high Phe is teratogenic even in non-PKU fetus
12. MSUD: BCKAD deficient → maple syrup odor, neonatal onset, lethal if untreated; thiamine-responsive forms exist
13. Homocystinuria: CBS deficient → ectopia lentis (downward), thrombosis, skeletal changes; B6-responsive in 50%
14. Alkaptonuria: homogentisate oxidase deficient → dark urine + ochronosis + arthritis; asymptomatic until ~40 years
15. Albinism: tyrosinase deficient → no melanin → hypopigmentation + ↑ skin cancer risk
16. Primary Oxaluria Type 1: alanine-glyoxylate aminotransferase deficient → glycine → oxalate → kidney stones
17. Folate deficiency → megaloblastic anemia (via inability to make dTMP for DNA synthesis)
18. Biotin carries CO₂ only (not part of the one-carbon/THF pool)

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⏱️ 30-MINUTE MASTER PLAN

Amino Acid Degradation & Synthesis

Strategy: Each block has a core concept + a memory anchor. Read → close your eyes → recall → move on. Don't re-read. Forward momentum only.

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⏱️ MINS 0–3 | THE BIG PICTURE (Foundation)

"P-O-A-F-S-A-A" → Pyruvate, OAA, Alpha-KG, Fumarate, Succinyl-CoA, Acetyl-CoA, Acetoacetate

• First 5 (Pyruvate → Succinyl-CoA) → GLUCOGENIC (can make glucose)
• Last 2 (Acetyl-CoA, Acetoacetate) → KETOGENIC (make ketone bodies, NOT glucose)

1. Transamination (needs B6) → gives glutamate
2. Oxidative deamination (GDH) → releases NH₄⁺ → urea cycle

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⏱️ MINS 3–6 | GLUCOGENIC vs KETOGENIC

• Ala → Pyruvate
• Asp, Asn → OAA
• Glu, Gln, Pro, Arg, His → α-KG
• Phe, Tyr → Fumarate + Acetoacetate
• Val, Ile, Met, Thr → Succinyl-CoA
• Leu → Acetyl-CoA + Acetoacetate
• Lys → Acetoacetyl-CoA

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⏱️ MINS 6–10 | THE 5 KEY PATHWAYS

🔵 PATH 1 — Asparagine → OAA

• Asparaginase cuts asparagine → aspartate
• Clinic: Leukemia drug — leukemic cells can't make their own asparagine, so asparaginase starves them

🔵 PATH 2 — Histidine → α-KG (via FIGlu)

• His → FIGlu → (needs folate/THF) → Glutamate → α-KG
• FIGlu in urine = folate deficiency (FIGlu piles up when there's no THF to accept it)

🔵 PATH 3 — Glycine danger zone

• Glycine → Glyoxylate → Oxalate → kidney stones
• Disease: Primary Oxaluria Type 1 (alanine-glyoxylate aminotransferase deficient)
• 🧠 "Glycine gone wrong = kidney stones"

🔵 PATH 4 — The SAM/Methionine Superhighway ★★★

Methionine
    ↓ (+ATP)
   SAM  ← THE methyl donor for everything
    ↓ (donates -CH₃)
   SAH
    ↓
Homocysteine ←——————————————————┐
    ↓ (needs Folate + B12)       │ REMETHYLATION
    → back to Methionine ————————┘

    ↓ (needs B6)  TRANSSULFURATION
Cystathionine
    ↓ (needs B6)
Cysteine + α-Ketobutyrate
    ↓
Propionyl-CoA → (Biotin + B12) → Succinyl-CoA

• SAM = universal methyl donor
• ↑ Homocysteine: damages endothelium → thrombosis → caused by low folate, B6, or B12
• Cysteine is conditionally essential (needs methionine to make it)

🔵 PATH 5 — Phe/Tyr → Fumarate + Acetoacetate

Phe → (phenylalanine hydroxylase + BH₄) → Tyr → … → Fumarate + Acetoacetate
                                                      ↓
                                                  Homogentisate ← Alkaptonuria blocks HERE

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⏱️ MINS 10–14 | BCAAs — THE MUSCLE AAs

• Val → Succinyl-CoA (glucogenic)
• Leu → Acetoacetate + Acetyl-CoA (ketogenic)
• Ile → Both (Acetyl-CoA + Succinyl-CoA)

• BCKAD deficient → MSUD (maple syrup urine disease)
• Isovaleryl-CoA dehydrogenase deficient → sweaty feet odor

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⏱️ MINS 14–17 | FOLATE & ONE-CARBON METABOLISM

THF (active folate) carries single-carbon fragments at N5 and/or N10, critical for DNA synthesis and homocysteine recycling.

• Folate → Dihydrofolate (DHF) → Tetrahydrofolate (THF) via dihydrofolate reductase (DHFR) (needs 2 NADPH)
• Methotrexate blocks DHFR → cancer treatment
• N⁵-methyl-THF → remethylates homocysteine (needs B12)
• N⁵,N¹⁰-methylene-THF → makes dTMP from dUMP (thymidylate synthase)
• No THF → no dTMP → no DNA → megaloblastic anemia

• Biotin carries CO₂ only — it is NOT part of the THF one-carbon pool
• (Biotin is used by: propionyl-CoA carboxylase, pyruvate carboxylase)

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⏱️ MINS 17–20 | NONESSENTIAL AA SYNTHESIS

• Pyruvate + amino group → Alanine
• OAA + amino group → Aspartate
• α-KG + amino group → Glutamate

• Glutamate + NH₃ → Glutamine (glutamine synthetase) — ammonia transport in blood
• Aspartate + NH₃ → Asparagine (asparagine synthetase, uses glutamine as donor)

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⏱️ MINS 20–28 | THE 5 DISEASES — One Comparison Table

1. Maternal PKU — high maternal Phe is teratogenic even in a non-PKU baby → microcephaly, cardiac defects → strict diet BEFORE conception
2. Homocystinuria lens goes DOWN (vs. Marfan's goes UP) — "Homocystinuria = H = down like a hammock"
3. Alkaptonuria was the first inborn error described (Garrod, 1902) — diapers stain dark in infancy, but patient feels fine until age 40

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⏱️ MINS 28–30 | RAPID-FIRE REVIEW

1. Name the 7 carbon skeleton entry points → P-O-A-F-S-A-A
2. Which 2 AAs are ONLY ketogenic? → Leu & Lys
3. What's the universal methyl donor? → SAM
4. What does FIGlu in urine indicate? → Folate deficiency
5. What cofactors does BCKAD need? → TPP, Lipoic, FAD, NAD⁺, CoA
6. Where are BCAAs catabolized? → Muscle (not liver)
7. Which disease: BCKAD deficient + maple syrup odor? → MSUD
8. Which disease: Phe hydroxylase deficient + musty odor? → PKU
9. Which disease: CBS deficient + thrombosis + ectopia lentis DOWN? → Homocystinuria
10. Which disease: Homogentisate oxidase deficient + dark urine + arthritis? → Alkaptonuria
11. Which disease: Tyrosinase deficient + skin cancer risk? → Albinism
12. High homocysteine is caused by low ___, ___, or ___? → Folate, B6, B12
13. What enzyme makes THF from DHF? → Dihydrofolate reductase (DHFR)
14. What drug blocks DHFR? → Methotrexate
15. Folate deficiency → what type of anemia? → Megaloblastic
16. Biotin carries what? → CO₂ (not 1C fragments)
17. Cysteine is conditionally essential — needs what AA? → Methionine
18. Tyrosine is conditionally essential — needs what AA? → Phenylalanine

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🗺️ THE ULTIMATE ONE-PAGE CHEAT SHEET

AA CATABOLISM
─────────────────────────────────────────────────────────
AA → (remove -NH₂ via B6 transamination) → carbon skeleton

GLUCOGENIC intermediates:          KETOGENIC intermediates:
Pyruvate ← Ala, Ser, Gly, Cys     Acetyl-CoA ← Leu, Lys, Ile
OAA ← Asp, Asn                    Acetoacetate ← Leu, Lys, Phe, Tyr, Ile, Trp
α-KG ← Glu, Gln, Pro, Arg, His
Fumarate ← Phe, Tyr (also keto)
Succinyl-CoA ← Val, Ile, Met, Thr

ONLY KETOGENIC = LEU & LYS

THE SAM CYCLE (methylation + sulfur):
Met → SAM → SAH → Homocysteine
              ↓(B12+Folate)→ back to Met
              ↓(B6)→ Cys → Succinyl-CoA

HIGH HOMOCYSTEINE = low Folate/B6/B12 = vascular damage

BCAAs (Muscle only): Val→Succinyl, Leu→Ketones, Ile→Both
BCKAD cofactors: TPP·Lipoic·FAD·NAD⁺·CoA  |  Defect→MSUD

FOLATE: THF carries 1C units | Deficiency→Megaloblastic anemia
BIOTIN: carries CO₂ only

DISEASES:
PKU: Phe↑, musty urine, hypopig., avoid aspartame, treat early
MSUD: BCAA↑, maple syrup, neonatal, thiamine helps some
Homocystinuria: Hcy↑, thrombosis, lens DOWN, B6 helps 50%
Alkaptonuria: dark urine, ochronosis, arthritis @ age 40
Albinism: no melanin, photophobia, ↑ skin cancer

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🧬 MASTER THE 5 KEY PATHWAYS

Learn them once, remember them forever

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🗺️ ORIENTATION FIRST — What are the "5 pathways"?

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PATHWAY 1 — Asparagine & Aspartate → Oxaloacetate

The Story

Asparagine
    │
    │  asparaginase (removes -NH₂ from side chain amide)
    ▼
Aspartate + NH₄⁺
    │
    │  aspartate aminotransferase (AST/GOT) + B6
    │  (transfers α-amino group to α-KG → glutamate)
    ▼
Oxaloacetate (OAA)
    │
    ▼
  TCA Cycle / Gluconeogenesis

Two Reactions, Two Enzymes

The Clinical Bomb 💣 — Asparaginase in Leukemia

• Normal cells have asparagine synthetase — they can make their own asparagine from aspartate + glutamine
• Leukemic cells (especially in ALL — Acute Lymphoblastic Leukemia) lose asparagine synthetase through transformation — they become completely dependent on plasma asparagine
• Give L-asparaginase IV → depletes ALL plasma asparagine → leukemic cells starve and die
• Normal cells survive because they can make their own

🧠 Hook: "Leukemia cells are lazy — they can't make asparagine, so we poison their food supply"

Self-Test

• What enzyme converts Asn → Asp? → Asparaginase
• What enzyme converts Asp → OAA? → AST (B6 needed)
• Why is asparaginase used in leukemia? → Leukemic cells can't synthesize asparagine themselves

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PATHWAY 2 — Histidine, Glutamine, Proline, Arginine → α-Ketoglutarate

The Story

Histidine ──────────────────────────────┐
                                         │ (via FIGlu — needs FOLATE)
Proline ─────────────────────────────┐  │
                                      │  ▼
Arginine ────────────────────────────┼──► GLUTAMATE
                                      │
Glutamine ───────────────────────────┘
    │ (glutaminase, releases NH₄⁺)
    ▼
Glutamate
    │
    │ Option A: Glutamate dehydrogenase (GDH) — releases NH₄⁺, needs NAD⁺
    │ Option B: Transamination — transfers -NH₂ to α-keto acid
    ▼
α-Ketoglutarate (α-KG)
    │
    ▼
  TCA Cycle

Each AA in Detail

Glutamine → Glutamate → α-KG

• Glutaminase (in kidney, intestine, liver): removes side chain amide → Glutamate + NH₄⁺
• This is how the body transports excess ammonia from peripheral tissues to liver — safely packaged as glutamine
• In kidney: NH₄⁺ is excreted directly into urine → helps regulate acid-base balance

Proline → Glutamate → α-KG

• Proline is a cyclic amino acid
• Proline oxidase opens the ring → glutamate-5-semialdehyde → glutamate (spontaneous cyclization in reverse)
• This pathway is reversible — glutamate can also make proline (biosynthesis)

Arginine → Ornithine → Glutamate → α-KG

• Arginine comes from the urea cycle or diet
• Arginase (urea cycle enzyme) → Ornithine + Urea
• Ornithine → glutamate-5-semialdehyde → glutamate → α-KG

Histidine → FIGlu → Glutamate → α-KG (MOST TESTED)

Histidine
    │  histidase
    ▼
Urocanate
    │
    ▼
4-Imidazolone-5-propionate
    │
    ▼
N-Formiminoglutamate (FIGlu) ← ACCUMULATES if folate deficient
    │
    │  formimino transferase (needs THF as acceptor)
    ▼                              ↓
Glutamate              N⁵-formimino-THF
    │
    ▼
α-Ketoglutarate

The FIGlu Test — Clinical Gold 🏆

• If folate (THF) is normal → FIGlu quickly transferred to THF → no FIGlu in urine
• If folate is deficient → no THF available → FIGlu piles up → spills into urine

🧠 Hook: "His-to-FIGlu-to-Glu = a folate detector. No folate = FIGlu floods the urine."

Self-Test

• Which AA has a folate-dependent degradation step? → Histidine
• What accumulates in urine when folate is deficient? → FIGlu
• What enzyme converts Glu → α-KG (releasing NH₄⁺)? → Glutamate dehydrogenase (GDH)
• Why is glutamine important for ammonia transport? → Carries NH₄⁺ safely in blood from periphery to liver

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PATHWAY 3 — Glycine, Serine, Alanine, Cysteine, Threonine → Pyruvate

The Story

Alanine ──────────────────────────────────────────────────────────► PYRUVATE
                                                           (direct transamination — ALT)

Serine ──────────────────────────────────────────────────────────► PYRUVATE
         (serine dehydratase, B6)          OR → Glycine + N⁵,N¹⁰-methylene-THF

Glycine ──────── ↔ Serine ────────────────────────────────────────► PYRUVATE
         │ (glycine cleavage system → CO₂ + NH₃ + methylene-THF)
         └─► Glyoxylate ──► OXALATE ⚠️ (kidney stones — Primary Oxaluria Type 1)

Cystine/Cysteine ─► (desulfuration) ─────────────────────────────► PYRUVATE
                     + H₂S → Sulfate → PAPS (sulfur donor)

Threonine ──────────────────────────────────────────────────────► PYRUVATE (minor)
           └────────────────────────────────────────────────────► α-Ketobutyrate → Succinyl-CoA (major)

Each AA in Detail

Alanine → Pyruvate (The Simplest)

Alanine + α-KG ──(ALT/GPT, B6)──► Pyruvate + Glutamate

• The simplest amino acid transamination
• ALT (alanine aminotransferase) = liver enzyme elevated in liver disease
• Alanine is the #1 gluconeogenic amino acid from muscle
• Glucose-Alanine Cycle:
  • Muscle: glucose → pyruvate + NH₂ from AA breakdown → alanine
  • Blood: alanine travels to liver
  • Liver: alanine → pyruvate → glucose (gluconeogenesis) + NH₄⁺ → urea

🧠 "Alanine = muscle's postal service — it ships nitrogen safely to the liver"

Serine → Pyruvate (Two Routes)

Serine ──(serine dehydratase, B6)──► Pyruvate + NH₄⁺

Serine + THF ──(serine hydroxymethyltransferase, B6)──► Glycine + N⁵,N¹⁰-methylene-THF

• This reaction also works in reverse (glycine → serine) for biosynthesis
• Provides the methylene group for dTMP synthesis (critical for DNA)

Glycine → Multiple Fates (Clinically Important)

Glycine + THF ──► CO₂ + NH₄⁺ + N⁵,N¹⁰-methylene-THF

Glycine
   │  glycine oxidase (or spontaneous)
   ▼
Glyoxylate
   │  NORMALLY: alanine-glyoxylate aminotransferase (B6, in peroxisomes) → back to glycine
   │  IF ENZYME DEFICIENT ↓
   ▼
OXALATE
   │
   ▼
Calcium Oxalate crystals → KIDNEY STONES → renal failure

• Deficient enzyme: alanine-glyoxylate aminotransferase (AGT)
• Glyoxylate can't be recycled → all goes to oxalate
• Severe kidney stones from childhood, recurrent UTIs, renal failure by 20s-30s
• Treatment: Vitamin B6 (some cases), liver transplant (AGT is a liver peroxisomal enzyme)

Cysteine → Pyruvate (+ Sulfur Chemistry)

Cystine (dietary disulfide)
   │  cystine reductase (NADH)
   ▼
Cysteine (SH form)
   │  desulfuration enzymes (B6)
   ├──► Pyruvate
   └──► H₂S → oxidized → Sulfate (SO₄²⁻)
                               │
                               ▼
                         PAPS (3'-Phosphoadenosine-5'-phosphosulfate)
                         = Activated sulfur donor for:
                           - Heparan/chondroitin sulfate (ECM)
                           - Drug conjugation (Phase II metabolism)
                           - Bile salt conjugation

🧠 "Cysteine gives up its sulfur → makes PAPS → sulfates everything in the body"

Threonine → Pyruvate OR Succinyl-CoA (Two Fates)

Threonine
   │
   ├── Path A → Pyruvate + Acetaldehyde (threonine aldolase, minor in humans)
   │
   └── Path B → α-Ketobutyrate (threonine dehydratase, B6)
                     │
                     ▼
               Propionyl-CoA
                     │  (propionyl-CoA carboxylase + BIOTIN)
                     ▼
               Methylmalonyl-CoA
                     │  (methylmalonyl-CoA mutase + B12)
                     ▼
               Succinyl-CoA → TCA cycle ✅

Self-Test

• What is the glucose-alanine cycle? → Muscle sends Ala to liver → liver makes glucose, processes NH₄⁺
• What does glycine → glyoxylate → oxalate cause? → Primary Oxaluria Type 1, kidney stones
• What cofactor converts Ser → Gly (and vice versa)? → B6 (and THF)
• What is PAPS and where does the sulfur come from? → Activated sulfur donor, from cysteine
• Threonine → Succinyl-CoA via what? → Propionyl-CoA (needs Biotin + B12)

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PATHWAY 4 — Methionine → SAM → Succinyl-CoA

The Story

ACT 1 — Methionine becomes SAM (the methyl donor)

Methionine
    │
    │  + ATP (methionine adenosyltransferase)
    ▼
S-Adenosylmethionine (SAM)
    │
    │  Donates -CH₃ to: DNA, histones, neurotransmitters, creatine,
    │  phosphatidylcholine, epinephrine, melatonin, etc.
    │
    ▼
S-Adenosylhomocysteine (SAH)
    │
    │  SAH hydrolase
    ▼
Homocysteine + Adenosine

🧠 "SAM is the methyl ATM of the cell — everything withdraws a -CH₃ from SAM"

ACT 2 — Homocysteine at the crossroads (2 choices)

Homocysteine
    │
    │  methionine synthase
    │  needs: N⁵-methyl-THF (folate donates -CH₃) + Vitamin B12 (cofactor)
    ▼
Methionine ♻️

• B12 is needed for methionine synthase
• Without B12, N⁵-methyl-THF can't donate its methyl group
• All folate gets trapped as N⁵-methyl-THF — the "methyl trap"
• No free THF available for other 1C reactions → same as folate deficiency → megaloblastic anemia

Homocysteine + Serine
    │
    │  Cystathionine β-synthase (CBS) — needs B6
    ▼
Cystathionine
    │
    │  Cystathionase (γ-lyase) — needs B6
    ▼
Cysteine + α-Ketobutyrate

ACT 3 — α-Ketobutyrate → Succinyl-CoA

α-Ketobutyrate
    │  (same enzymes as threonine pathway)
    ▼
Propionyl-CoA
    │  propionyl-CoA carboxylase + BIOTIN
    ▼
Methylmalonyl-CoA
    │  methylmalonyl-CoA mutase + B12
    ▼
Succinyl-CoA → TCA cycle ✅

The Full Picture Together

         DIET
          │
     METHIONINE (essential AA)
          │ + ATP
          ▼
         SAM ──── donates -CH₃ to everything ──────────►
          │                                     (DNA methylation, neurotransmitters, etc.)
          ▼
         SAH
          │
          ▼
    HOMOCYSTEINE ◄──────────────────────────────────────────┐
          │                                                  │ REMETHYLATION
          │ B6              B12 + Folate                     │
          ▼─── Transsulfuration ──►  OR  ◄── Remethylation ─┘
          │
          ▼ (+ Serine, B6)
     CYSTATHIONINE
          │ (B6)
          ▼
     CYSTEINE ─────────────────► used for proteins, glutathione, PAPS
     α-KETOBUTYRATE
          │
          ▼
     PROPIONYL-CoA
          │ (Biotin)
          ▼
     METHYLMALONYL-CoA
          │ (B12)
          ▼
     SUCCINYL-CoA → TCA ✅

Clinical Points

Self-Test

• What does SAM do? → Universal methyl donor
• SAM → SAH → ? → Homocysteine
• To remethylate Hcy → Met, you need? → N⁵-methyl-THF + B12
• To transsulfurate Hcy → Cys, you need? → Serine + B6 (CBS enzyme)
• What is the methyl trap? → B12 deficiency traps folate as methyl-THF → functional folate deficiency
• What enzyme is deficient in Classic Homocystinuria? → Cystathionine β-synthase (CBS)
• Propionyl-CoA → Succinyl-CoA needs? → Biotin then B12

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PATHWAY 5 — Phenylalanine & Tyrosine → Fumarate + Acetoacetate

The Story

PHENYLALANINE (essential AA)
          │
          │  Phenylalanine hydroxylase (PAH)
          │  cofactor: BH₄ (tetrahydrobiopterin)
          │  BH₄ is regenerated by: dihydropteridine reductase (DHPR)
          │
          │  ← BLOCK HERE = PKU (PAH deficient)
          │    or BH₄ deficiency / DHPR deficiency = malignant PKU
          ▼
TYROSINE (conditionally essential)
          │
          │  Tyrosine aminotransferase
          ▼
4-Hydroxyphenylpyruvate
          │
          │  4-HPP dioxygenase (needs Vitamin C)
          ▼
Homogentisate
          │
          │  ← BLOCK HERE = ALKAPTONURIA (homogentisate oxidase deficient)
          │    → HGA accumulates → dark urine, ochronosis, arthritis
          ▼
Maleylacetoacetate
          │
          ▼
Fumarylacetoacetate
          │
          │  Fumarylacetoacetase
          ▼
   FUMARATE + ACETOACETATE
       (TCA cycle)  (ketone body)
       GLUCOGENIC     KETOGENIC

The Three "Tyrosine Offshoots" (Don't forget these)

TYROSINE
   ├──► MELANIN (via Tyrosinase)
   │         BLOCK = ALBINISM
   │
   ├──► DOPA → DOPAMINE → NOREPINEPHRINE → EPINEPHRINE (catecholamines)
   │    (via Tyrosine hydroxylase, needs BH₄)
   │
   └──► THYROID HORMONES (T3, T4) via iodination

🧠 "Tyrosine is a triple threat — makes melanin, catecholamines, and thyroid hormones"

The Disease Map on This Pathway

Why PKU Causes Hypopigmentation (even though Phe is HIGH, not low)

• Phe competes with Tyrosine for the same transporter into cells
• Very high Phe → blocks tyrosine uptake into melanocytes
• Less tyrosine available → less melanin → fair skin, hair, eyes
• Same competition happens at blood-brain barrier → less Tyr → less dopamine

🧠 "Phe bullies Tyrosine out of the way — no tyrosine in melanocytes or brain"

BH₄ — The Hidden Cofactor

1. Phenylalanine hydroxylase → Phe → Tyr
2. Tyrosine hydroxylase → Tyr → DOPA (catecholamine synthesis)
3. Tryptophan hydroxylase → Trp → 5-HTP (serotonin synthesis)

• Phe → Tyr is blocked (same as PKU biochemically)
• BUT ALSO: Tyrosine hydroxylase and tryptophan hydroxylase fail
• Results in: ↓ Dopamine, ↓ Serotonin, ↓ Norepinephrine — severe neurological disease
• This is "malignant PKU" or BH₄-deficient hyperphenylalaninemia — low-Phe diet alone doesn't help because the neurotransmitter deficiency persists
• Treatment requires BH₄ + neurotransmitter precursors

Self-Test

• Phe → Tyr needs what enzyme + cofactor? → Phenylalanine hydroxylase + BH₄
• Where is Alkaptonuria blocked? → Homogentisate oxidase — HGA accumulates
• What 3 things does Tyrosine make (besides being degraded)? → Melanin, Catecholamines, Thyroid hormones
• Why does PKU cause hypopigmentation? → High Phe competes with Tyr for uptake into melanocytes → less melanin
• Why is BH₄ deficiency worse than plain PKU? → Also blocks catecholamine + serotonin synthesis → severe neurological disease
• What are the end products of Phe/Tyr degradation? → Fumarate (glucogenic) + Acetoacetate (ketogenic)

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🔁 ACTIVE RECALL: THE PATHWAY DRILL

Round 1 — Trace the pathway

1. Asparagine → ??? → OAA
2. Histidine → ??? → ??? → Glutamate → α-KG
3. Alanine → ??? (enzyme + cofactor?)
4. Methionine → ??? → ??? → Homocysteine → (two routes, name both)
5. Phenylalanine → ??? → ??? → Fumarate + Acetoacetate (name a disease at each block)

Round 2 — Name the enzyme

Round 3 — Clinical connection

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✅ Answers — Round 2

✅ Answers — Round 3

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🗺️ THE MASTER MAP — All 5 Pathways

                    AMINO ACIDS
                        │
    ┌───────┬───────────┬────────────┬─────────────┐
    │       │           │            │             │
   Asn     His        Ala          Met           Phe
   Asp   Gln,Pro     Ser,Gly       Val           Tyr
    │    Arg          Cys,Thr      Ile,Thr
    ▼       │           │            │             │
   OAA    Glu        Pyruvate    Succinyl-CoA   Fumarate
     ↘     │ ↓          ↓                      + Acetoacetate
      ►  α-KG    →  TCA Cycle  ←─────────────────┘
           │
        TCA CYCLE
           │
    Energy + Gluconeogenesis (glucogenic AAs)
                      Ketogenesis (ketogenic AAs)