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📚 Pharm D 1st Year — Biochemistry Complete Notes

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LONG ANSWER QUESTIONS (15 Marks Each)

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LONG ANSWER 1 — Urea Cycle: Detailed Note with Reactions & Metabolic Disorders

🔑 Introduction

Key fact: Normal blood urea nitrogen (BUN) = 7–20 mg/dL; plasma ammonia (NH₄⁺) = 30–60 µmol/L

🔬 Source of Nitrogen for Urea

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⚙️ Reactions of the Urea Cycle

FLOWCHART — Urea Cycle

MITOCHONDRIA
───────────────────────────────────────────────────────
NH₄⁺ + HCO₃⁻ + 2ATP
        ↓ [CPS-I — activated by N-acetylglutamate (NAG)]
 ① CARBAMOYL PHOSPHATE (high-energy compound)
        ↓ + Ornithine
        ↓ [Ornithine Transcarbamylase (OTC)]
 ② CITRULLINE + Pᵢ
        ↓ (transported to cytosol via antiporter)
───────────────────────────────────────────────────────
CYTOSOL
───────────────────────────────────────────────────────
 Citrulline + Aspartate + ATP
        ↓ [Argininosuccinate Synthetase] → AMP + PPᵢ
 ③ ARGININOSUCCINATE
        ↓ [Argininosuccinate Lyase]
 ④ ARGININE + Fumarate
        ↓ [Arginase-I (liver only)] + H₂O
 ⑤ UREA + ORNITHINE ←────────────────────────────────┐
                                                      │
         Ornithine transported back to mitochondria ──┘

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📋 Step-by-Step Reactions (Tabular)

Overall Stoichiometry:

NH₄⁺ + HCO₃⁻ + Aspartate + 3 ATP + H₂O
        → Urea + Fumarate + 2 ADP + AMP + 2 Pᵢ + PPᵢ

4 high-energy phosphate bonds are consumed per mole of urea. The cycle is irreversible (large negative ΔG).

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🧪 Regulation of the Urea Cycle

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⚠️ Major Metabolic Disorders of the Urea Cycle

FLOWCHART — Urea Cycle Disorders

Step 1 → CPS-I deficiency     → ↑ NH₄⁺ (no carbamoyl phosphate formed)
Step 2 → OTC deficiency        → ↑ NH₄⁺ + ↑ Orotic acid in urine (most common; X-linked)
Step 3 → AS deficiency         → Citrullinemia (↑ citrulline in blood/urine)
Step 4 → AL deficiency         → Argininosuccinic acidemia (↑ argininosuccinate)
Step 5 → Arginase deficiency   → Argininemia (↑ arginine in blood)

AR = Autosomal Recessive

Common Clinical Features of ALL Urea Cycle Disorders:

• Neonatal vomiting, lethargy
• Hyperammonemia → Cerebral edema → Coma → Death
• Protein aversion (older children)
• Treatment: Low-protein diet + Sodium benzoate/phenylbutyrate (alternative nitrogen excretion pathways)

Other causes of Hyperammonemia:

• Acquired (Liver disease): Viral hepatitis, alcoholic cirrhosis → shunting of portal blood → urea cycle bypassed
• Liver failure → ammonia cannot be converted to urea → encephalopathy

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LONG ANSWER 2 — Transamination Reaction in Catabolism of Amino Acids

🔑 Introduction

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⚙️ General Reaction

       Amino Acid₁      +    α-Keto acid₂
            ↕ [Transaminase / Aminotransferase + PLP]
       α-Keto acid₁     +    Amino Acid₂

Amino acid + α-Ketoglutarate  ⇌  α-Keto acid + Glutamate
                [Transaminase + PLP]

Aspartate + α-Ketoglutarate  ⇌  Oxaloacetate + Glutamate

Alanine + α-Ketoglutarate  ⇌  Pyruvate + Glutamate

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🔬 Key Features

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🔄 Mechanism (Step-by-Step)

Phase 1 — Amino group transfer TO enzyme (PLP → PMP):
    Amino Acid + Enzyme-PLP → α-Keto Acid + Enzyme-PMP

Phase 2 — Amino group transfer FROM enzyme (PMP → PLP):
    Enzyme-PMP + α-Ketoglutarate → Glutamate + Enzyme-PLP

PLP = Pyridoxal Phosphate (Schiff base with enzyme lysine) PMP = Pyridoxamine Phosphate

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🔗 Role of Transamination in Amino Acid Catabolism (FLOWCHART)

Dietary Amino Acids (20 types)
        ↓ Transamination (collects α-amino groups)
        ↓ All nitrogen funneled → GLUTAMATE
        ↓
   Glutamate Dehydrogenase (GDH)
        ↓ Oxidative Deamination
   α-Ketoglutarate + NH₄⁺
        ↓
   NH₄⁺ enters → UREA CYCLE → Urea (excreted)
   α-Keto acids → TCA cycle / gluconeogenesis / ketogenesis

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🔗 Link with Other Pathways

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🏥 Clinical Importance of Transamination

• Basis of diagnostic importance of transaminases (see Long Answer 4)
• Reversible reactions allow biosynthesis of non-essential amino acids
• Central to the glucose–alanine cycle (muscle → liver nitrogen transport)

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LONG ANSWER 3 — Bile Pigments: Types, Metabolism & Associated Diseases

🔑 What Are Bile Pigments?

1. Bilirubin (yellow-orange) — major bile pigment
2. Biliverdin (green) — intermediate
3. Urobilinogen (colorless) — intestinal reduction product
4. Urobilin / Stercobilin (yellow/brown) — oxidized urobilinogen

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⚙️ Metabolism of Bilirubin (Step-by-Step Flowchart)

┌─────────────────────────────────────────────────────────┐
│  RBC breakdown (in spleen/liver/bone marrow)            │
│  Hemoglobin → Heme + Globin                             │
│  Heme + O₂ → [Heme Oxygenase] → Biliverdin + CO + Fe²⁺ │
│  Biliverdin → [Biliverdin Reductase] → BILIRUBIN        │
│            (Unconjugated / Indirect / Free)              │
└──────────────────┬──────────────────────────────────────┘
                   │ Transported bound to ALBUMIN in blood
                   ▼
┌─────────────────────────────────────────────────────────┐
│  LIVER (Hepatocytes)                                    │
│  Bilirubin → [UDP-Glucuronyl Transferase]               │
│           → Bilirubin Diglucuronide                     │
│            (Conjugated / Direct / Water-soluble)         │
└──────────────────┬──────────────────────────────────────┘
                   │ Excreted into bile → intestine
                   ▼
┌─────────────────────────────────────────────────────────┐
│  INTESTINE                                              │
│  Bilirubin glucuronide → [Intestinal bacteria]          │
│           → Urobilinogen (colorless)                    │
│                ├─ 20% reabsorbed → portal blood → liver │
│                │   (small amount → kidney → URINE       │
│                │    as Urobilin = yellow color of urine) │
│                └─ 80% oxidized → Stercobilin            │
│                   (brown color of feces)                 │
└─────────────────────────────────────────────────────────┘

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📋 Types of Bilirubin

Total normal serum bilirubin: 0.3–1.0 mg/dL Jaundice appears when bilirubin >1.5–2 mg/dL (visually detectable ~3× normal)

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🏥 Diseases Associated with Bile Pigment Metabolism

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LONG ANSWER 4 — Transaminases: Definition & Diagnostic Importance

🔑 Definition

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🔬 Clinically Important Transaminases

AST: Aspartate + α-Ketoglutarate  ⇌  Oxaloacetate + Glutamate
ALT: Alanine   + α-Ketoglutarate  ⇌  Pyruvate     + Glutamate

Blood half-life: AST = 17 h; ALT = 47 h; Mitochondrial AST = 87 h

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🏥 Diagnostic Importance of Transaminases

FLOWCHART — Disease Diagnosis Using AST & ALT

                  Serum Transaminases Elevated
                         /              \
               ALT > AST              AST >> ALT (ratio >2:1)
                  |                          |
         Acute Liver Disease         Alcoholic Hepatitis
       (Viral Hepatitis, Drugs)    (DeRitis Ratio = AST/ALT ≥3:1)

Key Points for Exam:

1. ALT is LIVER-SPECIFIC — predominantly found in liver cytoplasm; most sensitive marker of hepatocellular damage
2. AST is LESS SPECIFIC — found in liver, heart, skeletal muscle, kidneys, RBCs
3. DeRitis Ratio (AST/ALT):
   • >2 → Alcoholic liver disease
   • <1 → Viral hepatitis / non-alcoholic fatty liver
4. Vitamin B₆ deficiency can give falsely LOW transaminase levels (PLP is required for enzyme activity — especially important in alcoholics)
5. Both enzymes are used to monitor liver function, assess severity of hepatitis, guide treatment decisions, and detect drug toxicity

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LONG ANSWER 5 — Porphyrins: Types and Clinical Importance of Porphyria

🔑 What Are Porphyrins?

• Fe²⁺ → Heme (in hemoglobin, myoglobin, cytochromes)
• Mg²⁺ → Chlorophyll (plants)
• Co²⁺ → Vitamin B₁₂

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⚙️ Heme Synthesis (Simplified)

MITOCHONDRIA:
Succinyl CoA + Glycine → [ALA Synthase + PLP] → δ-ALA (aminolevulinic acid)

CYTOSOL:
2× δ-ALA → [ALA Dehydratase] → Porphobilinogen (PBG) — 1 pyrrole ring
4× PBG → [Hydroxymethylbilane Synthase] → Hydroxymethylbilane
                → Uroporphyrinogen III → Coproporphyrinogen III

MITOCHONDRIA:
Coproporphyrinogen III → Protoporphyrin IX
Protoporphyrin IX + Fe²⁺ → [Ferrochelatase] → HEME

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🔬 Types of Porphyria

Classification:

PORPHYRIAS
    │
    ├── HEPATIC (liver is primary site of overproduction)
    │       ├── Acute Intermittent Porphyria (AIP) ← most common acute
    │       ├── Porphyria Cutanea Tarda (PCT) ← most common overall
    │       ├── Hereditary Coproporphyria (HCP)
    │       └── Variegate Porphyria (VP)
    │
    └── ERYTHROPOIETIC (bone marrow/RBCs)
            ├── Congenital Erythropoietic Porphyria (CEP) ← Günther disease
            └── Erythropoietic Protoporphyria (EPP)

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📋 Major Types — Detailed Table

AD = Autosomal Dominant; AR = Autosomal Recessive

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⚠️ Clinical Importance of Porphyria

Two Main Clinical Manifestations:

ACUTE ATTACKS (Neuropsychiatric)          CUTANEOUS (Photosensitivity)
——————————————————————                    ——————————————————————————
Caused by ALA + PBG accumulation          Caused by porphyrin accumulation
(toxic to nervous system)                 (porphyrins absorb light → O₂ radicals)

Symptoms:                                  Symptoms:
• Severe colicky abdominal pain            • Blistering, erosions on sun-exposed skin
• Nausea, vomiting                         • Hyperpigmentation
• Motor neuropathy (weakness/paralysis)    • Fragile skin
• Neuropsychiatric symptoms               • Hypertrichosis
• Tachycardia, hypertension               • Red/pink urine or teeth

Precipitating Factors for Acute Attacks:

• Drugs (barbiturates, sulfonamides, alcohol, estrogens) — induce ALA synthase
• Fasting/starvation
• Hormonal changes (menstruation, pregnancy)
• Infections

Diagnosis:

• ↑ Urinary ALA and PBG (Watson-Schwartz test — positive in AIP)
• Urine turns port-wine red on standing (oxidation of PBG)
• Stool porphyrins
• Fluorescence of porphyrins under UV light (Wood's lamp)

Treatment:

• Acute attack: IV Glucose (suppresses ALA synthase) + IV Hemin (negative feedback on ALA synthase)
• Avoid precipitating drugs
• Sunscreen and sun protection (for cutaneous types)

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SHORT ANSWER QUESTIONS (5 Marks)

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SHORT ANSWER 1 — Transamination Reaction in Catabolism of Amino Acids

Amino Acid + α-Ketoglutarate  ⇌  α-Keto Acid + Glutamate
              [Transaminase / PLP]

• Enzyme: Aminotransferase (Transaminase)
• Coenzyme: PLP (Pyridoxal Phosphate = Vitamin B₆ derivative)
• All amino acids except Lysine and Threonine undergo transamination
• Reactions are reversible — used in both catabolism and synthesis
• Most reactions use α-ketoglutarate as the amino group acceptor → forming glutamate, which then undergoes oxidative deamination (by GDH) to release NH₄⁺ → enters urea cycle
• Most important transaminases clinically: ALT (liver marker) and AST (liver + heart marker)

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SHORT ANSWER 2 — Phenylketonuria (PKU) and Alkaptonuria

A. Phenylketonuria (PKU)

Phenylalanine  →[PAH - BLOCKED]→  Tyrosine (CANNOT form normally)
      ↓
Phenylpyruvate (transamination)
      ↓
Phenyllactate / Phenylacetate
      ↓
Excreted in urine → "mousy/musty" odor

• Intellectual disability (untreated) — phenylpyruvate is toxic to brain
• Fair skin, hair, eyes (reduced melanin — needs Tyr)
• Eczema
• Seizures
• Musty odor of urine and sweat (phenylacetate)

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B. Alkaptonuria

Tyrosine → Homogentisate →[Homogentisate Oxidase — BLOCKED]→ Maleylacetoacetate
                ↓
         Accumulates → auto-oxidizes → dark pigment (ALKAPTON)
                ↓
         ┌───────────────────────────────┐
         │ Urine turns dark (black/brown)│ → "Black urine disease"
         │ on exposure to air            │
         └───────────────────────────────┘
         Deposited in cartilage → OCHRONOSIS (bluish-black pigmentation)
         → Arthritic joint pain (later in life)

• Dark/black urine (classic sign)
• Ochronosis (pigmentation of cartilage, tendons, connective tissue)
• Arthritis (especially spine, large joints)
• Stains diapers of affected infants

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SHORT ANSWER 3 — What is Porphyria?

Normal: Glycine + Succinyl CoA → ALA → PBG → Porphyrins → Heme
                                                   ↑
                        ENZYME BLOCK HERE causes accumulation

1. Acute (Hepatic) Porphyrias → Accumulate ALA & PBG → Neuropsychiatric + abdominal pain
   • Most important: Acute Intermittent Porphyria (AIP)
2. Cutaneous Porphyrias → Accumulate porphyrins → Photosensitivity + skin lesions
   • Most common: Porphyria Cutanea Tarda (PCT)

• Abdominal pain (severe, colicky)
• Neuropsychiatric symptoms (anxiety, psychosis)
• Dark red/port-wine urine (on standing)

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SHORT ANSWER 4 — Jaundice: Definition and Types

Definition

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Types of Jaundice

                        JAUNDICE
                           │
          ┌────────────────┼────────────────┐
          ▼                ▼                ▼
   PRE-HEPATIC        HEPATIC           POST-HEPATIC
 (Hemolytic)       (Hepatocellular)    (Obstructive)

Neonatal Jaundice (Physiological Jaundice):

• Appears at day 2–3; resolves by day 10
• Cause: Immature UDP-glucuronyl transferase + rapid hemolysis of fetal RBCs
• Unconjugated bilirubin ↑ → can cross BBB → Kernicterus (brain damage) if severe

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SHORT ANSWER 5 — Normal Blood Urea Level & Conditions of Elevated Blood Urea

Normal Blood Urea Level:

Note: BUN × 2.14 = Serum Urea (approximately)

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Elevated Blood Urea = UREMIA / AZOTEMIA

1. Renal Failure (Kidney Disease)

Damaged kidneys → ↓ GFR → Cannot filter/excrete urea → Urea accumulates in blood
→ UREMIA (symptoms: nausea, vomiting, fatigue, confusion, "uremic frost")

• Pre-renal azotemia: Dehydration, heart failure → ↓ blood flow to kidneys
• Renal azotemia: Glomerulonephritis, pyelonephritis, diabetic nephropathy
• Post-renal azotemia: Obstruction (stones, prostate enlargement)

2. High Protein Intake / Increased Protein Catabolism

↑ Protein intake → ↑ Amino acid catabolism → ↑ NH₄⁺ → ↑ Urea synthesis → ↑ Blood urea
Also: Fever, burns, starvation, trauma → ↑ tissue protein breakdown → ↑ urea

Summary Table:

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📝 Quick Revision Summary Table

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Sources: Lippincott Illustrated Reviews: Biochemistry, 8th ed.; Basic Medical Biochemistry: A Clinical Approach, 6th ed.; Harper's Illustrated Biochemistry, 32nd ed.; Guyton & Hall Textbook of Medical Physiology; Henry's Clinical Diagnosis and Management by Laboratory Methods