---

1. Inborn Errors of Aromatic Amino Acids - PKU, Alkaptonuria, Albinism: each explained with the missing enzyme, what builds up, symptoms, and diagnostic tests
2. Gout - Purine breakdown pathway, primary vs secondary causes, clinical signs (podagra, tophi, kidney stones), and how allopurinol works as a suicide inhibitor
3. Lipoproteins & Dyslipidemias - How chylomicrons, VLDL, LDL, HDL are assembled and cleared; reverse cholesterol transport; and the LDL receptor defect in Familial Hypercholesterolemia
4. Porphyrias - The heme synthesis pathway with enzyme positions; AIP (no skin signs, abdominal crisis, port-wine urine) vs PCT (skin blistering only, alcohol-related)
5. HMP Shunt - Oxidative phase makes NADPH; non-oxidative phase makes ribose; why G6PD-deficient RBCs lyse under oxidant stress (Heinz bodies, bite cells)
6. Ketone Bodies - Ketogenesis steps in liver, ketolysis in extrahepatic tissues, and why DKA causes high anion gap metabolic acidosis
7. Isoenzymes - LDH (flipped pattern in MI, LDH5 in liver disease), CK (CK-MB for MI), ALP (liver cholestasis vs bone disease)
8. Fluid Mosaic Model - Membrane structure, passive vs active transport, receptor-mediated endocytosis (clathrin-coated pits)
9. Gluconeogenesis - 3 bypass reactions (pyruvate carboxylase/PEPCK, F1,6-bisphosphatase, G6Pase), substrates, reciprocal regulation via F2,6-bisphosphate, Cori cycle
10. Dietary Fibers - Soluble (lowers cholesterol via bile acid binding, blunts glucose spikes) vs insoluble (adds bulk, prevents constipation)
11. Iron Absorption - Heme vs non-heme iron, DMT1/ferroportin/hepcidin axis, mucosal block theory, transferrin transport, ferritin storage
12. Scurvy and Rickets - Vitamin C needed for hydroxylation of proline/lysine (stable collagen); Vitamin D needed for calcium absorption and bone mineralization

---

📚 Biochemistry SN Paper 1 — Deep Elaboration with Diagrams, Mnemonics & Clinical Examples

---

TOPIC 1: Inborn Errors of Aromatic Amino Acids

The Big Picture First

PHENYLALANINE
     │
     │ ← PAH + BH4  ← BLOCKED in PKU
     ▼
  TYROSINE
     │
     ├──→ DOPA → DOPAMINE → Adrenaline
     │
     ├──→ MELANIN  ← BLOCKED in ALBINISM (tyrosinase missing)
     │
     ▼
HOMOGENTISIC ACID (HGA)
     │
     │ ← Homogentisate oxidase  ← BLOCKED in ALKAPTONURIA
     ▼
  Maleylacetoacetate → Fumarate + Acetoacetate

---

🔴 Phenylketonuria (PKU) — Full Elaboration

• Converts: Phenylalanine → Tyrosine
• Requires: BH4 (tetrahydrobiopterin) as cofactor, plus O₂ and NADPH
• Gene: chromosome 12q
• Mutation type: autosomal recessive

Phenylalanine
    │ (PAH blocked)
    ├──→ Phenylpyruvate   (phenylketone - gives "musty/mousy" urine odor)
    ├──→ Phenylacetate    (gives the distinctive body odor)
    └──→ Phenyllactate

• Excess phenylalanine competes with other large neutral amino acids (tryptophan, tyrosine) for the blood-brain barrier transporter
• Less tryptophan → less serotonin → mood/cognitive issues
• Less tyrosine → less dopamine and myelin
• Phenylketones directly impair myelination → intellectual disability if untreated

• P - Pale complexion, pale hair (less melanin; tyrosine diverted away from melanin)
• A - Autism-like behavior, intellectual disability if untreated
• L - Lemon-yellow urine / mousy odor
• S - Seizures (eczema too)

• Some PKU is NOT due to PAH mutation but to deficient BH4 synthesis/recycling
• BH4 is also needed for synthesis of dopamine, serotonin, NO
• These patients respond poorly to diet alone
• Treatment needs neurotransmitter precursors (L-DOPA, 5-hydroxytryptophan) in addition

• Phenylalanine-restricted diet (avoid high-protein foods: meat, fish, dairy, eggs, nuts, aspartame)
• Must start within 3 weeks of birth to prevent intellectual damage
• Continue for life (stop causes adult neuropsych symptoms)
• Sapropterin (synthetic BH4): for BH4-responsive PKU (~30% of patients)
• Pegvaliase (enzyme substitution): breaks down Phe in gut for severe cases

---

🟤 Alkaptonuria — Full Elaboration

• Gene: HGD (chromosome 3q)
• Autosomal recessive
• Rare: 1 in 250,000

• HGA accumulates silently
• Only sign: dark urine (parents notice diaper stains turn black when exposed to air)

• HGA oxidizes and polymerizes into a dark pigment that binds to collagen fibers
• Cartilage turns blue-black (ochronosis = "ochre-osis" from the Greek word for dark color)
• Visible in: sclerae (eyes look blue-grey), ear cartilage, tendons

• Hardened, brittle cartilage cracks and fragments → joint destruction
• Especially spine (disc calcification) and large joints (hips, knees)
• Unlike typical osteoarthritis: spine severely affected, peripheral joints mildly

---

⚪ Albinism — Full Elaboration

1. OCA Type 1 (most severe): Tyrosinase gene mutation → no melanin at all → white hair, pink skin, red eyes
2. OCA Type 2: OCA2 gene mutation (chloride channel in melanosomes) → some melanin, cream/yellow hair, blue eyes; most common globally
3. Ocular albinism: Only eyes affected (X-linked)
4. Hermansky-Pudlak syndrome: Albinism + platelet dysfunction + pulmonary fibrosis (melanosomes and other lysosome-related organelles affected)

• No retinal pigment epithelium pigment → you see the blood vessels through the iris and retina

• Retinal pigment is needed for normal development of visual pathways
• Fovea (area of sharp vision) is underdeveloped (foveal hypoplasia)
• Optic fibers cross abnormally at chiasm → nystagmus (involuntary eye movements), reduced visual acuity

• Mainly clinical (characteristic appearance)
• Hair bulb tyrosinase assay: Pluck 10 hairs including bulb; incubate in L-tyrosine solution for 4 hours:
  • Darkening = tyrosinase-positive (OCA2)
  • No darkening = tyrosinase-negative (OCA1)
• Electron microscopy of melanosomes: Staging of melanin production
• Genetic testing: TYR, OCA2, TYRP1, SLC45A2 genes

• Sunscreen SPF 50+ every day
• UV-protective glasses
• Regular ophthalmology follow-up
• Annual skin cancer screening
• No cure for the enzyme defect

---

TOPIC 2: Gout

The Big Picture

DNA/RNA (Adenine, Guanine)
         │ breakdown of nucleotides
         ▼
    AMP / GMP
         │
    Nucleosidases/kinases
         │
    Adenosine / Guanosine
         │
    Hypoxanthine / Guanine
         │
    XANTHINE OXIDASE ← Allopurinol blocks HERE
         │
      XANTHINE
         │
    XANTHINE OXIDASE (again)
         │
      URIC ACID → (normally excreted in urine)

---

Purine Salvage Pathway (why this matters for gout)

• Hypoxanthine + PRPP → IMP (by HGPRT enzyme)
• Guanine + PRPP → GMP (by HGPRT enzyme)

• Hypoxanthine and guanine CANNOT be salvaged
• All goes to xanthine oxidase → massive uric acid production
• Clinical: Gout + intellectual disability + self-mutilation (biting fingers and lips - very characteristic)

---

Primary vs Secondary Hyperuricemia — Story Format

---

Clinical Features — The Gout Timeline

STAGE 1: Asymptomatic hyperuricemia
→ Uric acid elevated in blood but no symptoms
→ Crystals slowly depositing in joints

STAGE 2: Acute Gouty Arthritis
→ Sudden severe joint pain (often 1st MTP joint = big toe = "PODAGRA")
→ Triggered by: alcohol, dehydration, big meal, trauma, surgery, illness
→ Joint: red, hot, swollen, exquisitely tender (can't tolerate bedsheet touching it)
→ Resolves spontaneously in 1-2 weeks even without treatment

STAGE 3: Intercritical Gout
→ No symptoms between attacks
→ Crystals still accumulating

STAGE 4: Chronic Tophaceous Gout
→ Tophi (chalky white deposits) at ear pinna, Achilles tendon, finger knuckles, elbow bursa
→ Chronic joint destruction
→ Kidney stones (radiolucent on X-ray; uric acid stones)
→ Gouty nephropathy

• Needle-shaped crystals
• Negatively birefringent under polarized light (yellow when parallel to slow wave = "negative" birefringence)
• Contrast with CPPD (pseudogout): rhomboid crystals, positively birefringent (blue when parallel)

• Gout = Negative birefringence, Needle shape
• Pseudogout = Positive birefringence, rhomboid shape

---

Allopurinol Mechanism — Detailed

1. Allopurinol (structural analog of hypoxanthine) enters the active site of xanthine oxidase
2. Xanthine oxidase starts to oxidize allopurinol (as it would hypoxanthine)
3. Allopurinol → oxypurinol (alloxanthine) - this metabolite stays tightly bound in the active site
4. Oxypurinol irreversibly inhibits xanthine oxidase (suicide inhibitor / mechanism-based inhibitor)
5. Both xanthine and hypoxanthine now accumulate instead of uric acid (they are MORE soluble, easily excreted)
6. Uric acid levels fall 60-70%

• Do NOT start during acute attack (mobilizes crystals → worsens attack)
• Causes flare-ups when first started (6-12 months): always co-prescribe colchicine or NSAIDs as prophylaxis
• Dose adjust in renal failure (oxypurinol is renally excreted)
• Dangerous drug interaction: Azathioprine/6-mercaptopurine - these are also metabolized by xanthine oxidase; if you block XO with allopurinol, these drugs accumulate to toxic levels → bone marrow suppression. Must reduce dose by 75%.
• Febuxostat = alternative non-purine XO inhibitor (doesn't need dose adjustment for mild-moderate CKD)

---

TOPIC 3: Lipoproteins & Dyslipidemias

The Postal Service Analogy

• Chylomicrons: Huge refrigerated trucks from the intestine, carrying dietary fat
• VLDL: Medium trucks from the liver, carrying liver-made fat
• LDL: Small vans after VLDL offloads its cargo; now mostly carrying cholesterol to cells
• HDL: Tiny street sweepers collecting cholesterol from gutters (artery walls) and taking it back to the liver

---

Chylomicron Assembly — Step by Step (in intestinal cells)

LUMEN OF SMALL INTESTINE
Dietary TGs → digested by lipase + bile salts → Fatty acids + Monoglycerides

                    ↓ absorbed into ENTEROCYTE

INSIDE ENTEROCYTE (rough ER):
Fatty acids + Glycerol → Triglycerides re-synthesized
                         + Cholesterol esters
                         + Phospholipids
                         + ApoB-48 synthesized (key protein for chylomicron assembly)
                         + ApoA-I, A-II, A-IV added
                         ↓
                     Chylomicron assembled in Golgi
                         ↓
                     Secreted into LYMPHATICS (lacteal)
                         ↓
                     Thoracic duct → LEFT SUBCLAVIAN VEIN → Blood

• Chylomicrons are too large to enter capillaries
• Enter lacteal lymphatics first, then drain into blood at thoracic duct
• (This is different from short-chain fatty acids, which go directly to portal blood)

Chylomicron Clearance — What Happens in Blood

Chylomicron in blood
     │
     │← Gets ApoC-II and ApoE donated by HDL
     ▼
ApoC-II activates LIPOPROTEIN LIPASE (LPL)
(LPL sits on capillary walls of muscle and adipose tissue)
     │
     │LPL breaks down TGs → Fatty acids released
     │Fatty acids taken up by:
     │  - Muscle (oxidation for energy)
     │  - Adipose (storage)
     ▼
CHYLOMICRON REMNANT
(shrunken; lost most TGs; still has cholesterol + ApoE; returns ApoC-II to HDL)
     │
     │ApoE recognized by LRP receptor on LIVER
     ▼
LIVER takes up remnant → degraded in lysosomes

• LPL absent → chylomicrons NEVER cleared → massive hypertriglyceridemia
• Milky plasma ("lactescent serum")
• Eruptive xanthomas (tiny yellow papules on buttocks, elbows)
• Pancreatitis (triglycerides >1000 mg/dL → pancreatic inflammation)
• NO increased risk of heart disease (chylomicrons are too large to enter artery walls)

---

VLDL, IDL, LDL — The Conversion Chain

LIVER makes:
VLDL (Very Low Density Lipoprotein)
 - ApoB-100, ApoC-II, ApoE
 - ~55% triglycerides
     │
     │ LPL removes TGs (in blood)
     ↓
IDL (Intermediate Density Lipoprotein)
 - ApoB-100, ApoE
 - ~30% TG, more cholesterol
     │
     ├─50%→ Liver (ApoE recognized by LDL receptor)
     │
     └─50%→ Hepatic lipase removes more TGs
     ↓
LDL (Low Density Lipoprotein)
 - ApoB-100 ONLY
 - ~50% cholesterol (cholesterol-rich!)
 - Delivers cholesterol to peripheral cells via LDL receptor

1. LDL binds LDL receptor (via ApoB-100)
2. Clathrin-coated pit forms → endocytosis
3. Endosome → lysosome; LDL degraded, cholesterol released into cell
4. Receptor recycled to surface
5. Intracellular cholesterol:
   • Suppresses HMG-CoA reductase (less new cholesterol made)
   • Suppresses LDL receptor synthesis (less LDL taken up)
   • Activates ACAT (stores excess as cholesterol esters)

---

HDL and Reverse Cholesterol Transport — The Full Story

LIVER/INTESTINE secrete:
Nascent HDL (disc-shaped; contains ApoA-I, phospholipids)
     │
     │ Encounters peripheral cell with excess cholesterol
     │
     │ ABCA1 transporter on cell surface
     │ flips cholesterol from inner to outer leaflet → donated to HDL
     ▼
Pre-beta HDL (starts filling with cholesterol)
     │
     │ LCAT enzyme (on HDL surface, activated by ApoA-I)
     │ Lecithin + cholesterol → Lysolecithin + Cholesterol ESTER
     │ Cholesterol ester moves to HDL core (can't flip back)
     ▼
Mature spherical HDL3 → grows to HDL2
     │
     ├──→ SR-B1 receptor on LIVER: selective cholesterol ester uptake
     │    (HDL gives cholesterol to liver, particle stays in blood)
     │
     └──→ CETP (Cholesteryl Ester Transfer Protein):
          Transfers cholesterol esters from HDL → LDL/VLDL
          in exchange for TGs
          LDL/VLDL then taken up by liver via LDL receptor

• Directly removes cholesterol from artery walls
• Anti-inflammatory, antioxidant properties
• Statins slightly raise HDL, but dramatically raising HDL artificially (e.g., CETP inhibitors) has NOT shown cardiovascular benefit in trials - the quality of HDL matters, not just quantity

---

Familial Hypercholesterolemia (FH) — Molecular Basis

• Tendon xanthomas (Achilles tendon, extensor tendons of fingers) - pathognomonic
• Xanthelasma (yellow plaques around eyelids) - not specific to FH
• Corneal arcus (white ring around cornea) - before age 45, suggests FH
• Premature CAD (coronary artery disease before age 55 in men, 65 in women)

---

TOPIC 4: Porphyrias

The Heme Synthesis Pathway — with Enzyme Names and Locations

MITOCHONDRIA:
Glycine + Succinyl-CoA
     ↓ ALA SYNTHASE (rate-limiting; induced by drugs, fasting; needs pyridoxal phosphate)
   δ-ALA (aminolevulinic acid)
     ↓ moves to CYTOPLASM

CYTOPLASM:
δ-ALA
     ↓ ALA DEHYDRATASE (inhibited by LEAD)
   Porphobilinogen (PBG)
     ↓ PBG DEAMINASE ← [AIP - deficient here]
   Hydroxymethylbilane
     ↓ Uroporphyrinogen III cosynthase
   Uroporphyrinogen III
     ↓ UROPORPHYRINOGEN DECARBOXYLASE ← [PCT - deficient here]
   Coproporphyrinogen III
     ↓ moves back to MITOCHONDRIA

MITOCHONDRIA:
Coproporphyrinogen III
     ↓ Coproporphyrinogen oxidase
   Protoporphyrinogen IX
     ↓ Protoporphyrinogen oxidase
   Protoporphyrin IX
     ↓ FERROCHELATASE (adds Fe²⁺; inhibited by LEAD)
   HEME

---

Acute Intermittent Porphyria (AIP) — Full Clinical Picture

• These triggers induce ALA synthase (the first enzyme) via activating the gene
• More ALA synthase → more substrate pushed into the pathway
• But the pathway is blocked at PBG deaminase → ALA and PBG flood the system
• ALA is directly neurotoxic → explains ALL the neurological features

• Pain (abdominal - most common, 95%)
• Psychiatric symptoms
• Peripheral neuropathy (motor)
• Port-wine colored urine
• Precipitants (drugs, fasting, OCPs, infections)

• Barbiturates (phenobarbitone)
• Phenytoin
• Sulfonamides, rifampicin
• Oral contraceptive pills (estrogen/progesterone)
• Alcohol
• Safe drugs: Aspirin, paracetamol, penicillin, propranolol, chlorpromazine, bromides

• Urine: Elevated PBG and ALA (Watson-Schwartz test: Ehrlich's reagent turns pink/red with PBG)
• Urine turns dark red on standing (PBG → porphyrins by oxidation)
• Normal skin appearance (no photosensitivity - PBG and ALA are NOT photoactive)
• Hyponatremia (SIADH from CNS involvement)

1. Remove trigger (stop offending drugs)
2. High glucose (carbohydrate loading): IV dextrose 10-20% → suppresses ALA synthase (glucose inhibits FOXO1 transcription factor which drives ALA synthase gene)
3. IV Hemin (hematin): Provides heme directly → feedback suppresses ALA synthase (most effective)
4. Givosiran (siRNA therapy, 2019): Silences ALA synthase mRNA → prevents future attacks
5. Symptomatic: morphine for pain, propranolol for tachycardia/hypertension

---

Porphyria Cutanea Tarda (PCT) — Full Clinical Picture

• Uroporphyrins accumulate (they are the substrate of the missing enzyme)
• Uroporphyrins are photoactive - they absorb light at ~400 nm (Soret band)
• Light energy → uroporphyrins become excited → release reactive oxygen species
• ROS damage skin cells → blistering, fragility

• Urine: Pink-red fluorescence under Wood's lamp (365 nm UV) - uroporphyrins fluoresce
• Plasma scan: fluorescence peak at 619 nm
• Elevated urinary uroporphyrin (>300 µg/24h)
• Elevated hepatic iron stores

1. Phlebotomy (500 mL blood every 2 weeks): Removes iron (iron stimulates production of uroporphyrins via activation of ALA synthase)
2. Low-dose hydroxychloroquine (100 mg twice weekly): Chelates uroporphyrins in liver → excreted
3. Avoid triggers: alcohol, estrogens, sun exposure
4. Treat underlying hepatitis C

---

TOPIC 5: HMP Shunt (Pentose Phosphate Pathway)

Why This Pathway Exists

1. NADPH (reducing power, not ATP) for antioxidant defense and biosynthesis
2. Ribose-5-phosphate for making DNA, RNA, and nucleotides HMP shunt is the answer to both needs.

---

Oxidative Phase — Detailed with Why

Glucose-6-phosphate (G6P)
     │
     ↓  G6PD enzyme (Glucose-6-Phosphate Dehydrogenase)
     │  NADP⁺ → NADPH  [FIRST NADPH produced]
     │
6-Phosphogluconolactone
     │
     ↓  Lactonase (spontaneous + enzyme)
     │
6-Phosphogluconate
     │
     ↓  6-Phosphogluconate Dehydrogenase
     │  NADP⁺ → NADPH  [SECOND NADPH produced]
     │  CO₂ released
     │
Ribulose-5-Phosphate
     │
     ↓  (Non-oxidative phase begins here)

---

Non-Oxidative Phase — The "Traffic Interchange"

• Transketolase (needs thiamine/B1 as cofactor - important!)
• Transaldolase

• In thiamine deficiency (Wernicke's encephalopathy): non-oxidative phase impaired
• RBC transketolase activity is used as a lab test for thiamine deficiency
• Adding thiamine in lab - if activity increases >25% = thiamine deficient

---

NADPH - Why It's Life or Death for RBCs

         NADPH
          │
          ↓  Glutathione reductase
    Oxidized Glutathione (GSSG)
          ↑                    ↓ reduced = GSH
          │
    GSH = Reduced Glutathione
          │
          ↓  Glutathione peroxidase
    H₂O₂ + 2 GSH → 2 H₂O + GSSG

• H₂O₂ (hydrogen peroxide) and other free radicals are made constantly in RBCs (from hemoglobin oxygenation)
• GSH (reduced glutathione) neutralizes them
• NADPH regenerates GSH from its oxidized form (GSSG)
• Without NADPH → GSH depleted → oxidative damage unchecked → RBC membrane and hemoglobin are oxidized → hemolysis

• F - Fatty acid synthesis (fatty acid synthase needs NADPH)
• A - Antioxidant defense (glutathione reductase)
• S - Steroid synthesis (in adrenal cortex)
• C - Cytochrome P450 reactions (drug metabolism)
• O - Oxidative burst in phagocytes (NADPH oxidase kills bacteria)

---

G6PD Deficiency — Full Elaboration

• Most common enzyme deficiency globally (~400 million people)
• X-linked recessive (gene on Xq28)
• Males affected; females usually carriers but can be affected (if homozygous)
• Most common in malaria-endemic regions (G6PD deficiency provides some protection against malaria - infected RBCs are preferentially destroyed)

Oxidant stress (drug/infection/fava beans)
     ↓
H₂O₂ builds up in RBC (G6PD absent → no NADPH → no reduced glutathione)
     ↓
H₂O₂ oxidizes Hemoglobin → METHEMOGLOBIN → precipitates as HEINZ BODIES
     ↓
Heinz body RBCs → become rigid and fragile
     ↓
Spleen macrophages "bite" out Heinz bodies → BITE CELLS (keratocytes)
     ↓
Remaining cell is unstable → destroyed → HEMOLYSIS
     ↓
Hemoglobinuria (dark urine), jaundice, anemia

• Primaquine, dapsone (antimalarials and antileprotics - classic exam)
• Fava beans (favism) - contain divicine and isouramil (oxidants)
• Infections (most common real-world trigger)
• Naphthalene (mothballs)
• Vitamin K (high dose)
• Note: NOT aspirin at low doses (contrary to old teaching)

• During attack: Low Hb, elevated reticulocytes, elevated LDH and bilirubin (hemolysis markers)
• Blood film: Heinz bodies (supravital stain with crystal violet), bite cells, polychromasia
• G6PD enzyme assay: Measure after attack resolves! (During attack, old RBCs destroyed → only young reticulocytes remain which have normal/higher G6PD → falsely normal result)

---

TOPIC 6: Ketone Bodies

Why Ketones Exist — The Big Picture

---

Ketogenesis — Step by Step

FAT CELLS:
Triglycerides → (lipase) → Fatty acids + Glycerol
     ↓
Fatty acids enter LIVER MITOCHONDRIA
     ↓
BETA-OXIDATION → Acetyl-CoA builds up

BUT WHY CAN'T ACETYL-CoA ENTER TCA CYCLE?
→ TCA cycle needs Oxaloacetate (OAA)
→ During fasting/starvation, OAA is used up for gluconeogenesis
→ Less OAA → Acetyl-CoA piles up → diverted to ketone body synthesis

KETOGENESIS in liver mitochondria:

Acetyl-CoA + Acetyl-CoA
     ↓  Thiolase (acetyl-CoA acetyltransferase)
Acetoacetyl-CoA
     ↓  HMG-CoA SYNTHASE ← RATE-LIMITING STEP of ketogenesis
     + Acetyl-CoA
HMG-CoA (3-hydroxy-3-methylglutaryl-CoA)
     ↓  HMG-CoA LYASE ← specific to mitochondria
     (Note: cytoplasmic HMG-CoA reductase makes cholesterol - different pathway)
ACETOACETATE + Acetyl-CoA
     │
     ├──→ (spontaneous decarboxylation) → ACETONE (lost in breath - fruity smell)
     │
     └──→ (Beta-hydroxybutyrate dehydrogenase + NADH)
          BETA-HYDROXYBUTYRATE (3-hydroxybutyrate)

1. Acetoacetate - the primary, measured in clinical labs
2. Beta-hydroxybutyrate - most abundant in DKA (predominates because of high NADH:NAD+ ratio)
3. Acetone - volatile, exhaled (fruity breath), NOT metabolized

---

Ketolysis in Extrahepatic Tissues

MUSCLE/BRAIN/HEART:

Beta-hydroxybutyrate enters cell
     ↓  Beta-hydroxybutyrate dehydrogenase
Acetoacetate
     ↓  SUCCINYL-CoA TRANSFERASE (THIOPHORASE)
     + Succinyl-CoA from TCA cycle
Acetoacetyl-CoA + Succinate
     ↓  Thiolase
2 x Acetyl-CoA
     ↓
Enters TCA cycle → ATP production

• Present in heart, brain, muscle, kidney
• ABSENT in liver → liver cannot use its own ketones (prevents futile cycling)

• First 24 hours: glucose from liver glycogen
• 24-72 hours: glucose from gluconeogenesis; some ketones
• After 1-2 weeks: brain adapts to use ketone bodies for 60-70% of its energy
• This reduces glucose requirement dramatically → protein (muscle) spared

---

Diabetic Ketoacidosis (DKA) — Why Acidosis Occurs

• Inhibiting hormone-sensitive lipase in adipose → less fatty acid release
• Inhibiting HMG-CoA synthase in liver
• Promoting glucose uptake by cells

No insulin → Glucagon predominates → "Starvation mode" in full feeding

Fat cells: ↑ Lipolysis → massive fatty acid release
Liver: Fatty acids → massive ketone production (no insulin to suppress it)
AND no glucose uptake by cells

Result: Blood glucose HIGH + Blood ketones HIGH + Blood pH LOW

• Acetoacetate and beta-hydroxybutyrate are acids (donate H⁺)
• H⁺ + HCO₃⁻ → H₂O + CO₂ (bicarbonate consumed)
• HCO₃⁻ falls
• But Cl⁻ does NOT rise proportionally
• So gap = Na⁺ - (Cl⁻ + low HCO₃⁻) → GAP WIDENS
• Anion gap elevated = Keto-acid anions replace bicarbonate

1. IV 0.9% NaCl (rehydration first - patients are severely dehydrated)
2. IV Insulin (stops ketogenesis; lowers blood glucose)
3. Potassium (give proactively - insulin shifts K⁺ into cells; hypokalemia is dangerous)
4. Treat the precipitant (infection, missed insulin, etc.)

---

TOPIC 7: Isoenzymes

The Concept — Why Isoenzymes Are Diagnostically Useful

---

LDH — Detailed

• H subunit (heart type): favors pyruvate → lactate direction (AEROBIC tissues)
• M subunit (muscle type): favors lactate → pyruvate direction (in recovering anaerobic tissues)

LDH1 = H₄    Heart, RBCs, kidney cortex, testes
LDH2 = H₃M   Heart, RBCs (dominant in serum normally)
LDH3 = H₂M₂  Lung, platelets, lymphoid tissue
LDH4 = HM₃   Liver, kidney
LDH5 = M₄    Liver, skeletal muscle (runs fastest toward anode in electrophoresis)

---

CK — Detailed

ACUTE MI timeline:
0h: Normal
3-4h: CK-MB starts rising
12-24h: CK-MB PEAKS
36-48h: CK-MB returns to normal
(Total CK peaks slightly later than CK-MB)

• Total CK also rises in muscle injuries (running, injections, rhabdomyolysis)
• CK-MB >5% of total CK or absolute CK-MB >25 IU/L = cardiac damage

• CK-MM: Myositis, rhabdomyolysis, muscular dystrophy, statin myopathy, IM injections, seizures
• CK-MB: Myocarditis, cardiac surgery, electrical cardioversion
• CK-BB: Rarely detected in serum (doesn't cross BBB well); brain injury mainly diagnosed by other markers

---

ALP — Detailed

1. Get GGT (gamma-glutamyl transpeptidase):
   • If GGT elevated = liver/biliary origin
   • If GGT normal = bone origin
2. Or run ALP isoenzyme electrophoresis

• Obstructive (cholestasis): ALP ↑↑↑ (>> AST/ALT) - classic pattern: biliary obstruction, drugs, PBC, PSC
• Hepatocellular (hepatitis): AST/ALT ↑↑↑ >> ALP

• Middle-aged woman, itching, elevated ALP 3x normal, GGT elevated, normal bilirubin: Primary biliary cholangitis (PBC)
• Elderly man, bone pain, elevated ALP 10x normal, GGT normal, calcium normal: Paget's disease
• Child, bowed legs, elevated ALP, low Ca and PO₄, elevated PTH: Rickets

---

TOPIC 8: Fluid Mosaic Model

Why "Fluid" and Why "Mosaic"

---

Membrane Structure — Layer by Layer

EXTRACELLULAR SPACE
═══════════════════════════════════════════════════════
 🔴🔴🔴🔴🔴🔴  GLYCOCALYX (carbohydrate chains of
                           glycoproteins and glycolipids)
──────────────────────────────────────────────────────
 ○○○○○○○○○○○○  OUTER LEAFLET phospholipid heads
 ||||||||||||  Fatty acid tails (hydrophobic core)
─────────[CHOLESTEROL everywhere]─────────────────────
 ||||||||||||  Fatty acid tails
 ○○○○○○○○○○○○  INNER LEAFLET phospholipid heads
──────────────────────────────────────────────────────
  |INTEGRAL|   Transmembrane proteins
CYTOPLASM
═══════════════════════════════════════════════════════

• Phosphatidylcholine, phosphatidylethanolamine: Most abundant phospholipids
• Sphingomyelin: Enriched in outer leaflet; stiffens membrane
• Cholesterol: Between phospholipids; the "temperature buffer":
  • At LOW temperatures: prevents solidification (increases fluidity)
  • At HIGH temperatures: prevents excessive fluidity (stabilizes)
  • The "Goldilocks" molecule of the membrane
• Lipid rafts: Cholesterol-rich microdomains that cluster certain proteins (signal transduction platforms)

• Outer leaflet: Phosphatidylcholine, sphingomyelin (neutral)
• Inner leaflet: Phosphatidylserine (negatively charged), phosphatidylethanolamine
• This asymmetry is maintained by flipases (active transport)
• Loss of asymmetry = signal for apoptosis (phosphatidylserine flips to outside → "eat me" signal for macrophages)

---

Transport Mechanisms — Complete Comparison

• Pumps 3 Na⁺ OUT, 2 K⁺ IN (per ATP)
• Maintains: high K⁺ inside cells, high Na⁺ outside
• This gradient drives secondary active transport, maintains resting membrane potential, controls cell volume
• Inhibited by cardiac glycosides (digoxin, ouabain) → used in heart failure (raises intracellular Na⁺ → less Na/Ca exchange → more intracellular Ca²⁺ → stronger cardiac contraction)

---

Receptor-Mediated Endocytosis — The Clathrin Story

LDL (or transferrin, insulin, viruses, etc.)
     │
     │ binds specific RECEPTOR on cell surface
     ▼
Receptor-ligand complex accumulates in
CLATHRIN-COATED PIT
(clathrin is a scaffold protein that curves the membrane)
     │
     │ dynamin GTPase pinches off the pit
     ▼
CLATHRIN-COATED VESICLE forms
     │
     │ clathrin coat removed
     ▼
EARLY ENDOSOME
(pH drops to ~6.5 → ligand-receptor separated)
     │
     ├──→ Receptor: recycling endosome → back to cell surface
     │
     └──→ Late endosome → LYSOSOME
          (pH ~5.0; lysosomal enzymes activated)
          LDL degraded; cholesterol released for cell use

• No LDL receptor → no coated pit formation → LDL stays in blood → atherosclerosis

• Caveolae: Flask-shaped invaginations rich in caveolin protein and cholesterol; important for signaling (eNOS activation)
• Macropinocytosis: Bulk uptake of fluid
• Phagocytosis: Uptake of large particles (bacteria) by macrophages/neutrophils

---

TOPIC 9: Gluconeogenesis

When Does Gluconeogenesis Run?

GLUCOSE SOURCES BY TIME SINCE LAST MEAL:
0-4 hours:   Liver GLYCOGENOLYSIS (breaking down stored glycogen)
4-8 hours:   Liver glycogen + start of gluconeogenesis
8-24 hours:  Gluconeogenesis predominates (liver glycogen depleted)
>24 hours:   Gluconeogenesis (liver + kidney); ketones start substituting

---

The 3 Bypass Reactions — Complete Explanation

PYRUVATE (in cytoplasm)
     │
     │ transported into MITOCHONDRIA
     ▼
PYRUVATE + CO₂ + ATP
     │  PYRUVATE CARBOXYLASE (needs: BIOTIN, activated by ACETYL-CoA)
     ▼
OXALOACETATE (OAA) [in mitochondria]
     │
     │ OAA cannot cross inner mitochondrial membrane directly
     │ → converted to MALATE (by malate dehydrogenase)
     │ → MALATE exits mitochondria
     │ → converted back to OAA in cytoplasm
     ▼
OAA (in cytoplasm)
     │  PEPCK (Phosphoenolpyruvate Carboxykinase)
     │  (needs GTP; also needs Zinc)
     ▼
PEP (phosphoenolpyruvate) + CO₂
     │
     │ (glycolysis runs in reverse from here)

• High acetyl-CoA signals: "too much fat is being burned, make glucose to refuel"
• This is the key signal that fasting/fatty acid oxidation activates gluconeogenesis

• Enzyme: Fructose-1,6-bisphosphatase (FBPase-1)
• Simply removes one phosphate group (hydrolyzes it, releases Pi)
• Inhibited by: AMP (low energy = don't make glucose) and Fructose-2,6-bisphosphate (F2,6-BP)
• Activated by: Citrate

• Enzyme: Glucose-6-phosphatase
• Located in smooth ER of liver and kidney (NOT in muscle or brain!)
• This is why only liver and kidney can release glucose into the bloodstream
• Glucose-6-P is made in all cells, but can only be dephosphorylated and exported by liver/kidney
• Deficiency = Von Gierke's disease (Glycogen Storage Disease Type I): cannot release glucose from liver → severe fasting hypoglycemia + massive glycogen + fat accumulation in liver

---

Reciprocal Regulation with Glycolysis — The Master Switch

Insulin (fed state) → activates PFK-2 enzyme → makes F2,6-BP
F2,6-BP: 
  → ACTIVATES PFK-1 → GLYCOLYSIS ON
  → INHIBITS FBPase-1 → GLUCONEOGENESIS OFF

Glucagon (fasting) → activates protein kinase A → activates FBPase-2 → destroys F2,6-BP
F2,6-BP falls:
  → GLYCOLYSIS OFF
  → GLUCONEOGENESIS ON

---

The Cori Cycle — Full Story with Numbers

                    CORI CYCLE

MUSCLE/RBCs (anaerobic conditions):
Glucose (6C) → 2 Pyruvate (3C each) + 2 ATP
                ↓ lactate dehydrogenase (in RBCs or ischemic muscle)
             2 Lactate released into blood
                             ↓
                        (bloodstream)
                             ↓
LIVER receives 2 Lactate
2 Lactate → 2 Pyruvate (lactate dehydrogenase)
2 Pyruvate → 1 Glucose (gluconeogenesis costs 6 ATP)

Net ATP cost to liver: 6 ATP
Net ATP gained by muscle: 2 ATP
Net overall: COSTS 4 ATP — an energy-expensive cycle!

But it serves vital functions:
→ Keeps muscle working (lactate removed, doesn't build up)
→ Maintains blood glucose for brain
→ Distributes metabolic "debt" to liver

• In muscle: Pyruvate + amino group (from amino acid catabolism) → ALANINE (via alanine aminotransferase)
• Alanine exported to liver
• In liver: Alanine → Pyruvate + NH₃ (via ALT again)
• NH₃ → urea cycle; Pyruvate → gluconeogenesis → glucose back to muscle
• This safely transports nitrogen from muscle to liver for disposal

---

TOPIC 10: Dietary Fibers

What Dietary Fiber Actually Is

---

Soluble vs Insoluble — Detailed Comparison

• It absorbs water → swells → forms a thick, viscous gel
• This gel coats the intestinal mucosa
• Slows down everything: gastric emptying, enzyme access to food, nutrient absorption

• Add bulk to stool
• Absorb water mechanically (like a sponge) → softer, larger stool
• Speed up transit through colon (shorter transit time)
• Reduce time that carcinogens spend in contact with colon mucosa

---

Mechanism in Diabetes — The Full Story

Normal meal (no fiber):
Starch → rapidly broken by amylase → glucose → fast absorption → GLUCOSE SPIKE

Meal with soluble fiber:
     ↓
Fiber forms viscous gel around food particles
     ↓
Amylase can't access starch easily (physical barrier)
     ↓ SLOWER starch breakdown
Less glucose available at a time for absorption
     ↓
Glucose transporters (SGLT1) are not overwhelmed
     ↓
GRADUAL rise in blood glucose (lower glycemic index)
     ↓
Less demand on beta cells
     ↓
Less postprandial insulin spike
     ↓
Better glycemic control

• Improve insulin sensitivity in liver and muscle
• Signal GLP-1 and PYY release → enhance insulin response and satiety
• Nourish colonocytes (colon epithelial cells)

---

Mechanism in Hypercholesterolemia — The Bile Acid Story

NORMAL BILE ACID CYCLE:
Liver → makes bile acids from cholesterol
     ↓
Bile acids secreted into bile → stored in gallbladder
     ↓
Released into small intestine (for fat emulsification)
     ↓
95% REABSORBED in terminal ileum → portal blood → back to liver
(Only 5% lost in stool)
Liver uses this recycled bile acid pool → does NOT need to make much new bile acid
→ Blood cholesterol stable

WITH SOLUBLE FIBER:
Soluble fiber (especially psyllium, oat beta-glucan) in gut
     ↓
BINDS BILE ACIDS (like a sponge)
     ↓
Bile acid-fiber complex NOT absorbed → excreted in stool
     ↓
Less bile acid returned to liver
     ↓
Liver must make NEW bile acids from cholesterol
     ↓
More cholesterol pulled from blood to make bile acids
     ↓
Blood LDL cholesterol FALLS (5-10%)

This is the same mechanism as cholestyramine / colestipol (bile acid sequestrant drugs)!

---

TOPIC 11: Iron Absorption

Why Iron Absorption Is Unique

• Menstruation
• Skin cell shedding
• Intestinal cell shedding (enterocyte turnover)

---

Heme vs Non-Heme Iron — Why Heme is Absorbed Better

Heme (iron inside porphyrin ring) in food
     ↓
HCP1 (Heme Carrier Protein 1) / PCFT transports intact heme INTO enterocyte
     ↓
Heme oxygenase cleaves porphyrin ring inside cell → releases Fe²⁺
     ↓
Fe²⁺ joins intracellular iron pool

• Absorption: 15-35%; NOT affected by dietary factors (phytates, vitamin C, etc.)
• This is why vegetarians are more prone to iron deficiency

Fe³⁺ (ferric - in plants, insoluble)
     ↓
STOMACH HCl acidifies → some Fe³⁺ → Fe²⁺ (partial)
     ↓
DCYTB (Duodenal CytochRome B, a reductase on brush border)
Fe³⁺ → Fe²⁺ (ferrous - soluble, absorbable)
The electron donor for DCYTB is VITAMIN C ← this is why Vit C enhances non-heme iron absorption
     ↓
DMT1 (Divalent Metal Transporter 1) transports Fe²⁺ into enterocyte

---

Inside the Enterocyte — The Critical Decision Point

Fe²⁺ enters enterocyte from gut lumen
     │
     ├──→ Stored as FERRITIN (apoferritin + Fe³⁺)
     │    (Fe²⁺ oxidized back to Fe³⁺ by ferroxidase activity of apoferritin)
     │    Stored until enterocyte dies and is shed (3-5 days)
     │    This storage = "MUCOSAL BLOCK" - iron trapped and lost
     │
     └──→ Exported via FERROPORTIN (only known iron export channel)
          │
          │ Fe²⁺ oxidized → Fe³⁺ by HEPHAESTIN (membrane ferroxidase)
          ↓
       Enters BLOOD as Fe³⁺
          ↓
       Binds TRANSFERRIN (plasma carrier)

---

Transferrin and Transferrin Saturation

• MW ~80 kDa; 2 iron-binding sites (each binds 1 Fe³⁺)
• Normal serum transferrin: 200-400 mg/dL
• Normal transferrin saturation: ~30% (not all sites filled = safety reserve)

• On all cells needing iron
• Transferrin-Fe³⁺ complex binds TfR1 → endocytosis → acid environment releases Fe³⁺ → cell uses iron → receptor recycled to surface
• Regulated inversely with ferritin:
  • Low cell iron: ferritin mRNA destroyed (IRP system); TfR1 mRNA stabilized → more receptors → more uptake
  • High cell iron: ferritin mRNA stabilized → make more ferritin (storage); TfR1 mRNA destroyed → fewer receptors

---

TOPIC 12: Scurvy and Rickets

Scurvy — The Molecular Story

• Fe²⁺ in its active site (must be kept reduced/ferrous)
• Alpha-ketoglutarate as co-substrate
• Oxygen
• Vitamin C as the electron donor that regenerates Fe²⁺ after each reaction

COLLAGEN HYDROXYLATION REACTION:

Prolyl/Lysyl residue + O₂ + α-ketoglutarate
     ↓  Prolyl/Lysyl hydroxylase (contains Fe²⁺)
     │  Vitamin C keeps the iron as Fe²⁺ (reduces Fe³⁺ back to Fe²⁺)
Hydroxyprolyl/Hydroxylysyl + CO₂ + Succinate

Without Vitamin C:
→ Fe²⁺ is oxidized to Fe³⁺ (Fe³⁺ = inactive)
→ Enzyme cannot perform hydroxylation
→ Procollagen chains NOT hydroxylated
→ Triple helix UNSTABLE (hydroxyproline is essential for H-bonds in helix)
→ Collagen secreted but breaks down rapidly

• Hydroxyproline: H-bonds stabilize the triple helix (without it, collagen unfolds at body temperature)
• Hydroxylysine: Cross-links between collagen fibrils (via lysyl oxidase, also needs vitamin C as cofactor) → tensile strength

---

Scurvy Clinical Features — The Underlying Mechanism for Each Sign

• Clinical + dietary history
• Serum/plasma vitamin C level (< 11 µmol/L = deficient)
• WBC vitamin C (better reflects tissue stores)
• X-ray in children: Trummerfeld zone (dense zone of provisional calcification), Pelkan spur (lateral bone spur), Frankel line, Wimberger ring sign

---

Rickets — The Vitamin D Cascade

SKIN:
7-dehydrocholesterol + UV-B radiation (280-315 nm)
     ↓
Pre-vitamin D3
     ↓ (thermal isomerization)
Vitamin D3 (cholecalciferol) - dietary vitamin D3 from fish oil/eggs; vitamin D2 from plants

LIVER:
Vitamin D3 + O₂
     ↓  25-Hydroxylase (CYP2R1)
25-hydroxyvitamin D3 (calcidiol) - the STORAGE form (half-life = 3 weeks)
     ↓
KIDNEY (proximal tubule):
25-OH-D3
     ↓  1α-HYDROXYLASE (CYP27B1) ← regulated by PTH (↑), FGF23 (↓), hypophosphatemia (↑)
1,25-dihydroxyvitamin D3 (calcitriol) - the ACTIVE hormone (half-life = 4-6 hours)
     ↓
Acts on target tissues via nuclear Vitamin D Receptor (VDR)

---

What Happens Without Vitamin D — Step by Step

Vitamin D deficiency
     ↓
Less intestinal Ca²⁺ absorption
     ↓
Blood Ca²⁺ falls below normal
     ↓
Parathyroid glands sense low Ca²⁺
     ↓
PTH released (↑↑) = SECONDARY HYPERPARATHYROIDISM
     ↓
PTH effects:
  1. Bone: stimulates osteoclasts → resorbs bone to release Ca²⁺ → BLOOD Ca²⁺ normalizes
            but bone becomes DEMINERALIZED (osteoid not mineralized)
  2. Kidney: reabsorbs Ca²⁺ but increases phosphate EXCRETION
             → blood phosphate FALLS further
  3. Kidney: stimulates 1α-hydroxylase (tries to make more active Vit D, but no substrate if severe)
     ↓
Net result:
Blood Ca²⁺ - LOW to low-normal
Blood PO₄ - LOW (hyperparathyroidism wastes phosphate)
Blood ALP - HIGH (osteoblast activity trying to mineralize)
Blood PTH - HIGH (secondary hyperparathyroidism)
Blood 25-OH-D - LOW (the diagnostic test)

---

Final Exam Strategy for This Paper:

For every topic, train your brain to answer 3 questions instantly:

1. What enzyme is missing? (or what pathway is blocked?)
2. What accumulates? (the substrate before the block)
3. What is depleted? (the product that can't be made)

Then the clinical features follow logically from those 3 answers. You will not need to memorize them separately - you can derive them.