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MBBS BIOCHEMISTRY - LONG ANSWER QUESTIONS

Complete Study Guide

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Q1. Structure and Functions of Mitochondria + Mitochondrial Disorders

STRUCTURE OF MITOCHONDRIA

Structural Components

┌─────────────────────────────────────────────────────────────┐
│                   MITOCHONDRION                              │
│                                                             │
│  Outer Membrane ─────────────────────────────────────────  │
│  Intermembrane Space ────────────────────────────────────  │
│  Inner Membrane ─────────────────────────────────────────  │
│     │                                                        │
│     ├── Cristae (infoldings) ◄── ATP synthase (F0F1)        │
│     └── Matrix ◄── Krebs cycle enzymes, mtDNA, ribosomes   │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Mitochondrial DNA (mtDNA)

• Circular, double-stranded DNA (16,569 base pairs)
• Encodes 13 proteins (all components of ETC/ATP synthase), 22 tRNAs, 2 rRNAs
• Maternally inherited (no recombination)
• Higher mutation rate than nuclear DNA (lacks histones, exposed to ROS)

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FUNCTIONS OF MITOCHONDRIA

         FUNCTIONS OF MITOCHONDRIA
                    │
    ┌───────────────┼───────────────────┐
    │               │                   │
 Energy        Metabolism          Regulation
    │               │                   │
 ATP synthesis  Beta-oxidation     Apoptosis
 ETC (I-IV)    Krebs cycle        Ca²⁺ homeostasis
 OXPHOS        Urea cycle (partial) Thermogenesis (UCP1)
               Steroidogenesis     ROS production
               Gluconeogenesis     Iron-sulfur cluster synthesis
               Heme synthesis

1. ATP Production (Oxidative Phosphorylation): The main function. NADH/FADH₂ feed electrons into the ETC → proton gradient → ATP synthase produces ATP. 1 glucose → ~30-32 ATP.
2. Beta-Oxidation of Fatty Acids: Fatty acyl-CoA → Acetyl-CoA → enters Krebs cycle. Carnitine shuttle transports fatty acids into matrix.
3. Krebs Cycle (TCA Cycle): Acetyl-CoA is oxidized → CO₂ + NADH + FADH₂.
4. Calcium Homeostasis: Mitochondria act as calcium buffers; regulate intracellular [Ca²⁺] signaling.
5. Apoptosis: Release of cytochrome c from intermembrane space → activates caspase cascade → programmed cell death.
6. Thermogenesis: Brown adipose tissue - uncoupling protein (UCP1) dissipates proton gradient as heat.
7. Steroidogenesis: Cholesterol → Pregnenolone conversion occurs in inner mitochondrial membrane.
8. Urea Cycle: Carbamoyl phosphate synthesis (CPS-I) occurs in matrix.
9. Heme Synthesis: First and last steps occur in mitochondria (ALA synthase).

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MITOCHONDRIAL DISORDERS

Classification

Mitochondrial Disorders
│
├── Due to mtDNA mutations (maternally inherited)
│   ├── Point mutations → MELAS, MERRF, NARP, LHON
│   └── Deletions → KSS, Pearson syndrome
│
└── Due to nuclear DNA mutations (Mendelian inheritance)
    ├── AD/AR/X-linked
    └── Examples: Alpers, GRACILE, POLG mutations

Key Clinical Features Table

Common Lab Findings in Mitochondrial Disorders

• Elevated blood/CSF lactate
• Elevated lactate:pyruvate ratio (>20)
• Ragged red fibers on muscle biopsy (Gomori trichrome stain) - due to subsarcolemmal mitochondrial proliferation
• Abnormal respiratory chain enzyme activity
• MRI: bilateral basal ganglia/cortical signal changes

Exam Tip: Ragged red fibers = hallmark of mitochondrial myopathies. Maternal inheritance = suspect mtDNA mutation. "Ragged red" on Gomori trichrome stain is a classic MBBS exam point.

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Q2. Structure and Functions of Lysosomes + Lysosomal Storage Disorders

STRUCTURE OF LYSOSOMES

┌─────────────────────────────────────────────────┐
│              LYSOSOME                           │
│                                                  │
│  Single lipid bilayer membrane                   │
│  Interior pH = 4.5-5.0 (acidic)                 │
│                                                  │
│  Contains:                                       │
│  • ~60+ hydrolytic enzymes                      │
│  • Acid phosphatase (marker enzyme)              │
│  • Proteases, lipases, nucleases                 │
│  • Glycosidases, sulfatases                      │
│                                                  │
│  Membrane has:                                   │
│  • V-type H⁺-ATPase (maintains acidic pH)       │
│  • LAMP-1, LAMP-2 (lysosome-associated          │
│    membrane proteins - protect membrane)         │
│  • Mannose-6-phosphate receptors                 │
└─────────────────────────────────────────────────┘

Types of Lysosomes

Targeting Enzymes to Lysosomes

• Lysosomal enzymes synthesized on RER → tagged with mannose-6-phosphate (M6P) in Golgi
• M6P receptors in trans-Golgi network recognize and sort enzymes → transport to lysosomes
• This is key to understanding I-cell disease (see below)

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FUNCTIONS OF LYSOSOMES

         FUNCTIONS OF LYSOSOMES
                  │
    ┌─────────────┼──────────────┐
    │             │              │
Intracellular  Recycling    Defense
 digestion        │              │
    │         Autophagy    Phagocytosis
Heterophagy    (worn-out    of bacteria
(endocytosis)  organelles)  and pathogens
    │
Extracellular
 digestion
(bone resorption
 via osteoclasts)

1. Intracellular digestion: Break down ingested material via endocytosis/phagocytosis
2. Autophagy: Degrade worn-out organelles and proteins (cellular housekeeping)
3. Bone resorption: Osteoclasts use lysosomes to dissolve bone matrix
4. Sperm penetration of ovum: Acrosome = modified lysosome; contains hydrolases to penetrate zona pellucida
5. Thyroid hormone release: Thyroglobulin hydrolysis in follicle cells
6. Antigen processing: Break down foreign proteins for MHC II presentation
7. Programmed cell death: Release of hydrolases → apoptosis/necrosis

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LYSOSOMAL STORAGE DISORDERS (LSDs)

Pathogenesis

Normal:  Substrate → [Lysosomal enzyme] → Products (cleared)

LSD:     Substrate → [ABSENT enzyme] → Substrate ACCUMULATES
                                         in lysosomes → cell death

Classification and Major Disorders

• Defect: N-acetylglucosamine phosphotransferase (in Golgi)
• Result: Lysosomal enzymes lack M6P tag → secreted extracellularly instead of reaching lysosomes
• Features: Severe Hurler-like phenotype, high serum lysosomal enzymes, inclusions in fibroblasts (hence "I-cell")

Exam Mnemonic: "FANCY CATS GO HOME" - Fabry, Niemann-Pick, Krabbe, Gaucher, Tay-Sachs = all lipid storage; Cherry-red spot: Tay-Sachs, Niemann-Pick; No cherry-red = Gaucher

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Q3. Fluid Mosaic Model of Biological Membranes

INTRODUCTION

FLUID MOSAIC MODEL - DETAILED DESCRIPTION

The "Fluid" Component

• Undergo lateral diffusion (rapid, same leaflet)
• Undergo flip-flop (transverse diffusion) (very rare, requires enzyme flippase)
• Rotate about their long axis
• Flex (bend)

The "Mosaic" Component

EXTRACELLULAR SPACE
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  [Glycoprotein]      [Peripheral]    [Glycolipid]
        │                protein
        ▼                   │
~═══╤══╤═══════╤═══╤══════╤═══╤═════╤═══╤═══╤══~  ← Outer leaflet
    │  │       │Integral│  │  Channel│  │   │
    │  │       │protein │  │  protein│  │   │
~═══╧══╧═══════╧═══╧══════╧═══╧═════╧═══╧═══╧══~  ← Inner leaflet
                   │                         │
           [Peripheral protein]         [Cytoskeleton]
                   │                    anchored protein
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
CYTOPLASM

Types of Membrane Proteins

Lipid Components

Asymmetry of Membrane (Important!)

• Outer leaflet: Phosphatidylcholine (PC), sphingomyelin, glycolipids
• Inner leaflet: Phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidylinositol (PI)
• PS exposure on outer leaflet = signal for apoptosis and platelet activation

Lipid Rafts

• Microdomains enriched in cholesterol + sphingolipids
• More ordered (gel phase) than surrounding membrane
• Concentrated signaling proteins (GPI-anchored proteins, Src kinases, receptors)
• Role in signal transduction, endocytosis, pathogen entry

LABELLED DIAGRAM

╔═══════════════════════════════════════════════════════════════════╗
║              FLUID MOSAIC MODEL OF CELL MEMBRANE                  ║
╠═══════════════════════════════════════════════════════════════════╣
║                                                                    ║
║  EXTRACELLULAR FACE                                                ║
║  ┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄   ║
║                                                                    ║
║   [Carbohydrate    [Glycoprotein]  [Glycolipid]                   ║
║    chains/Glycocalyx]      ↕                                       ║
║                            ║                                       ║
║ ~~~▓▓▓▓▓▓~~▓▓▓▓▓▓▓▓~~~▓▓▓▓╠╣▓▓▓▓▓~~▓▓▓▓▓▓~~~▓▓▓▓▓▓~~▓▓▓▓~~~~    ║
║ ~~~░░░░░░~~░░░░░░░░~~~░░░░╠╣░░░░░~~░░░░░░~~~░░░░░░~~░░░░~~~~    ║
║             ↑                  ↑                   ↑              ║
║        Phospholipid      Integral/Trans-      Phospholipid        ║
║         bilayer          membrane protein      bilayer            ║
║                          (channel/carrier)                        ║
║   [Peripheral   [Lipid        [Cholesterol]    [Peripheral]       ║
║    protein]    anchored                         protein]          ║
║                protein]                                           ║
║  ┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄   ║
║  CYTOPLASMIC FACE                                                  ║
║                                                                    ║
╚═══════════════════════════════════════════════════════════════════╝
  Key: ▓▓ = Polar head (hydrophilic)  ░░ = Fatty acid tails (hydrophobic)

MODIFICATIONS AND LIMITATIONS

• Membrane rafts and caveolae add complexity beyond the original model
• Membrane proteins are NOT freely mobile - cytoskeletal anchoring limits movement (fence and picket model)
• Membrane microdomains exist with specialized composition

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Q4. Classification of Membrane Transport Mechanisms + Description of Any Two

CLASSIFICATION

MEMBRANE TRANSPORT
│
├── PASSIVE TRANSPORT (No energy required; down concentration gradient)
│   ├── Simple Diffusion
│   │   ├── Directly through lipid bilayer
│   │   └── Small nonpolar: O₂, CO₂, N₂, steroid hormones, ethanol
│   │
│   ├── Facilitated Diffusion
│   │   ├── Channel proteins (aquaporins, ion channels)
│   │   └── Carrier proteins (GLUT transporters)
│   │
│   └── Osmosis (water movement across semipermeable membrane)
│
├── ACTIVE TRANSPORT (Energy required; against concentration gradient)
│   ├── Primary Active Transport
│   │   ├── Directly uses ATP
│   │   └── Examples: Na⁺/K⁺-ATPase, Ca²⁺-ATPase, H⁺/K⁺-ATPase
│   │
│   └── Secondary Active Transport
│       ├── Uses ion gradient (established by primary transport)
│       ├── Symport: both move same direction
│       │   └── Example: SGLT1 (Na⁺ + glucose, intestine)
│       └── Antiport: move in opposite directions
│           └── Example: Na⁺/Ca²⁺ exchanger
│
└── VESICULAR TRANSPORT (Bulk transport)
    ├── Endocytosis
    │   ├── Phagocytosis (particles >0.5 μm)
    │   ├── Pinocytosis (fluid)
    │   └── Receptor-mediated endocytosis (clathrin-coated pits)
    │       Example: LDL-receptor pathway
    └── Exocytosis
        ├── Constitutive (continuous)
        └── Regulated (triggered by signal)
            Example: Insulin secretion by beta cells

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DESCRIPTION OF TWO TRANSPORT MECHANISMS

1. Na⁺/K⁺-ATPase (Primary Active Transport)

Step 1: 3 Na⁺ bind to intracellular sites (E1 conformation)
        ↓
Step 2: ATP hydrolysis → phosphorylation of aspartyl residue
        ↓
Step 3: Conformational change → E2 (phospho) form
        ↓
Step 4: 3 Na⁺ released EXTRACELLULARLY
        ↓
Step 5: 2 K⁺ bind extracellularly
        ↓
Step 6: Dephosphorylation → E1 form
        ↓
Step 7: 2 K⁺ released INTRACELLULARLY
        ↓
Step 8: Cycle repeats

• Maintains resting membrane potential (-70 mV)
• Regulates cell volume (prevents osmotic swelling)
• Drives secondary active transport (maintains Na⁺ gradient)
• Accounts for ~25-40% of total ATP consumption

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2. Receptor-Mediated Endocytosis (Vesicular Transport)

1. LDL binds to LDL receptor on cell surface (in clathrin-coated pit)
         ↓
2. Pit invaginates and pinches off → clathrin-coated vesicle
         ↓
3. Clathrin coat shed → early endosome
         ↓
4. Endosome acidifies (pH 5-6) → LDL dissociates from receptor
         ↓
5. Receptor recycled to plasma membrane (receptor recycling vesicle)
         ↓
6. LDL → late endosome → lysosome
         ↓
7. ApoB-100 degraded → Cholesterol esters hydrolyzed → Free cholesterol
         ↓
8. Free cholesterol:
   • Inhibits HMG-CoA reductase (↓ new cholesterol synthesis)
   • Activates ACAT (↑ cholesterol esterification for storage)
   • Downregulates LDL receptor synthesis

Extracellular LDL
      │
      ▼
[LDL-R] on clathrin pit → Invagination → Coated vesicle
                                              │
                                              ▼ (clathrin shed)
                                         Endosome (pH 6)
                                         /          \
                              Receptor recycled    LDL → Lysosome
                              to surface           (hydrolysis)
                                                        │
                                                   Free cholesterol
                                                   │        │
                                            Inhibits    Stored as
                                           HMG-CoA R   cholesterol
                                                        ester

• Transferrin receptor - iron uptake
• Insulin receptor - receptor downregulation
• IgG - FcRn-mediated transport across placenta

• Familial Hypercholesterolemia: Mutations in LDL receptor gene → LDL not cleared → severe hypercholesterolemia, premature atherosclerosis, xanthomas, xanthelasma
• Homozygous FH: TC >600-700 mg/dL, MI in childhood

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Q5. Definition and Classification of Carbohydrates + Glycosaminoglycans

DEFINITION

CLASSIFICATION

CARBOHYDRATES
│
├── MONOSACCHARIDES (Cannot be hydrolyzed further)
│   ├── Trioses (C3): Glyceraldehyde, Dihydroxyacetone
│   ├── Tetroses (C4): Erythrose
│   ├── Pentoses (C5): Ribose, Deoxyribose, Xylulose, Arabinose
│   └── Hexoses (C6): Glucose, Fructose, Galactose, Mannose
│       (Most important in human metabolism)
│
├── DISACCHARIDES (2 monosaccharides + glycosidic bond)
│   ├── Sucrose = Glucose + Fructose (α1→β2) - Table sugar
│   ├── Lactose = Galactose + Glucose (β1→4) - Milk sugar
│   ├── Maltose = Glucose + Glucose (α1→4) - Malt sugar
│   └── Trehalose = Glucose + Glucose (α1→α1) - Mushrooms
│
├── OLIGOSACCHARIDES (3-10 monosaccharides)
│   ├── Raffinose (trisaccharide - in legumes)
│   └── Blood group antigens (ABO system)
│
└── POLYSACCHARIDES (>10 monosaccharides)
    ├── HOMOPOLYSACCHARIDES (one type of monosaccharide)
    │   ├── Starch (α1→4, α1→6 branches) - Plant storage
    │   │   ├── Amylose (unbranched)
    │   │   └── Amylopectin (branched)
    │   ├── Glycogen (α1→4, α1→6; more branches than starch) - Animal
    │   ├── Cellulose (β1→4 glucose) - Plant cell wall; NOT digested by humans
    │   ├── Chitin (β1→4 N-acetylglucosamine) - Exoskeleton
    │   └── Dextran (α1→6) - Blood volume expander
    │
    └── HETEROPOLYSACCHARIDES (more than one type)
        └── Glycosaminoglycans, Proteoglycans

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NOTE ON GLYCOSAMINOGLYCANS (GAGs)

Definition

1. An amino sugar (N-acetylglucosamine or N-acetylgalactosamine) - often sulfated
2. A uronic acid (glucuronic acid or iduronic acid) - or galactose

Properties

• Highly negatively charged (due to sulfate and carboxyl groups) → attract water → gel-like consistency
• Associated with core proteins → form proteoglycans

Classification of GAGs

Structure of Proteoglycans

Core protein
│
├── GAG chain 1 (via Ser residue)
├── GAG chain 2
├── GAG chain 3
└── ... (many GAG chains)

Large proteoglycans (e.g., Aggrecan):
Hyaluronic acid backbone
├── Link protein
├── Aggrecan (core protein + chondroitin sulfate chains)
├── Aggrecan
└── ...

Functions of GAGs / Proteoglycans

1. Structural support: Major component of ECM
2. Lubrication: Hyaluronic acid in synovial fluid
3. Water retention: Negative charges attract water → turgidity of cartilage
4. Cell signaling: Heparan sulfate binds growth factors (FGF, VEGF)
5. Anticoagulation: Heparin activates antithrombin III
6. Filtration: Heparan sulfate in glomerular basement membrane (charge barrier)

Clinical Relevance

• Mucopolysaccharidoses: Enzyme defects in GAG degradation → accumulation
• Osteoarthritis: Breakdown of aggrecan in cartilage
• Marfan syndrome: Defective fibrillin in ECM
• Hyaluronic acid injections: Viscosupplementation in OA

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Q6. Classification of Amino Acids + Biologically Important Peptides

DEFINITION

CLASSIFICATION

A. Based on Nutritional Significance (Essential vs. Non-Essential)

B. Based on Metabolic Significance

AMINO ACIDS
│
├── GLUCOGENIC (Converted to glucose precursors)
│   Most amino acids are glucogenic
│   Examples: Alanine, Glycine, Serine, Glutamate, Aspartate, Valine,
│             Methionine, Histidine, Arginine, Proline, Threonine
│
├── KETOGENIC (Converted to acetyl-CoA/acetoacetyl-CoA only)
│   Leucine, Lysine (purely ketogenic - MNEMONICS: "LL")
│
└── BOTH GLUCOGENIC & KETOGENIC
    Isoleucine, Threonine, Phenylalanine, Tyrosine, Tryptophan
    Mnemonic: "I Try Phenyl Type"

C. Based on R-Group (Chemical Nature)

D. Based on Structure of R-Group

• Branched chain amino acids (BCAA): Valine, Leucine, Isoleucine - metabolized in muscle
• Sulfur-containing: Cysteine, Methionine
• Cyclic: Proline (imino acid), Phenylalanine, Tyrosine, Tryptophan, Histidine
• Hydroxy amino acids: Serine, Threonine, Tyrosine

---

FUNCTIONS OF BIOLOGICALLY IMPORTANT PEPTIDES

Definition of Peptides

Biologically Important Peptides

Exam Tip: Glutathione (γ-Glu-Cys-Gly) is a very common exam question. Its γ-peptide bond (Glu linked via γ-carboxyl to Cys) is unusual.

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Q7. Definition + Structural Organization of Proteins + Misfolded Protein Disorders

DEFINITION

STRUCTURAL ORGANIZATION OF PROTEINS

PRIMARY → SECONDARY → TERTIARY → QUATERNARY
(Sequence) (Local folding) (3D shape) (Multi-subunit assembly)

1. PRIMARY STRUCTURE

• Definition: Linear sequence of amino acids in a polypeptide chain, held together by covalent peptide bonds
• Determined by the gene
• Determines all higher order structures
• Example: Sickle cell hemoglobin - Glu→Val substitution at position 6 of β-chain
• Sequencing methods: Sanger's method, Edman degradation

H₂N - Ala - Gly - Ser - Leu - Trp - ... - COOH
       1     2     3     4     5
         ↑         ↑
      Peptide bonds (CO-NH)

2. SECONDARY STRUCTURE

• Definition: Regular, repeating local folding patterns due to hydrogen bonds between backbone C=O and N-H groups
• Two major types:

• Right-handed helix (Pauling & Corey, 1951)
• 3.6 amino acids per turn; pitch = 5.4 Å
• H-bond between residue i and i+4
• R-groups project outward
• Disrupted by: Proline (causes kink), Gly (too flexible)
• Examples: α-keratin (hair, nails), myoglobin

• H-bonds between adjacent strands (not within strand)
• Parallel (same N→C direction) or Antiparallel (opposite directions)
• Antiparallel is more stable
• Examples: β-keratin (silk, feathers), immunoglobulins

α-Helix:              β-Sheet (Antiparallel):
   /                  → → → →
  / (H-bonds within   ← ← ← ←
 /   coil)            → → → →

3. TERTIARY STRUCTURE

• Definition: Overall 3-dimensional folding of the entire polypeptide chain
• Held by multiple non-covalent interactions + disulfide bonds:

4. QUATERNARY STRUCTURE

• Definition: Arrangement of two or more polypeptide chains (subunits) into a functional protein
• Found only in proteins with multiple subunits
• Held by same non-covalent interactions as tertiary structure + disulfide bonds

LEVELS OF PROTEIN STRUCTURE:
│
├── 1° Primary: -Ala-Gly-Ser-Pro-... (peptide bonds)
│
├── 2° Secondary: α-helix / β-sheet (H-bonds, backbone)
│
├── 3° Tertiary: 3D globular shape (all non-covalent + S-S)
│
└── 4° Quaternary: Multi-subunit assembly (non-covalent)

---

DISORDERS ASSOCIATED WITH MISFOLDED PROTEINS

Concept of Protein Misfolding

• Aggregate into insoluble fibrils
• Form amyloid fibrils (β-sheet rich cross-β structure)
• Cause gain-of-function toxicity or loss-of-function

Protein Conformational Diseases / Proteopathies

Amyloid Staining

• Congo red stain: Amyloid appears pink/red
• Apple-green birefringence under polarized light - pathognomonic
• Thioflavin T/S: Fluorescence gold standard

Key Exam Point: PrP diseases (prion diseases) are transmissible (infectious misfolded protein), unlike other conformational diseases. PrPᶜ → PrPˢᶜ is a template-directed misfolding.

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Q8. Functions of Plasma Proteins + Acute Phase Proteins

PLASMA PROTEINS - OVERVIEW

Electrophoretic Pattern

Serum Protein Electrophoresis (SPE):
                              Anode (+)
                                │
         ┌──────────────────────┤
         │ Albumin (4.0-5.5 g/dL)─── Major band (fastest)
         │ α1-globulins (0.1-0.4 g/dL)
         │ α2-globulins (0.5-1.0 g/dL)
         │ β-globulins (0.7-1.2 g/dL)
         │ γ-globulins (0.6-1.6 g/dL) ─── Slowest (immunoglobulins)
         └──────────────────────┤
                                │
                              Cathode (-)

---

FUNCTIONS OF INDIVIDUAL PLASMA PROTEINS

A. ALBUMIN (Most abundant; synthesized in liver; t½ = 20 days)

B. GLOBULINS

C. OTHER IMPORTANT PLASMA PROTEINS

---

ACUTE PHASE PROTEINS (APPs)

Definition

Mediators of APP Response

Positive Acute Phase Proteins (Increase in inflammation)

Negative Acute Phase Proteins (Decrease in inflammation)

Clinical Significance of APPs

Clinical Uses of CRP:
├── Diagnosis of bacterial vs viral infection (CRP >>10 mg/L → bacterial)
├── Monitoring disease activity (RA, IBD, SLE)
├── Post-operative monitoring (infection surveillance)
├── Risk stratification for CVD (high-sensitivity CRP, hsCRP)
│   Normal <1 mg/L; Moderate risk 1-3 mg/L; High risk >3 mg/L
└── Guide antibiotic therapy

ESR (Erythrocyte Sedimentation Rate):
├── Elevated in inflammation (fibrinogen causes rouleaux formation)
├── Less specific than CRP
└── Useful for: temporal arteritis, polymyalgia rheumatica

Exam Tip: CRP rises within 6 hours, peaks at 48 hours, normalizes rapidly when inflammation resolves → excellent marker for monitoring treatment response. ESR lags behind CRP.

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Q9. Structure of Immunoglobulins + Classification + Paraproteinemias

STRUCTURE OF IMMUNOGLOBULINS

Components

                    ANTIGEN BINDING SITE
                    /                   \
               VH VL                VH VL
                |   |                |   |
         ─────────────────────────────────────
         |     Fab (antigen-binding fragment) |
         |___________________________________|
                        |
                  HINGE REGION
                 (disulfide bonds)
                        |
         ─────────────────────────────────────
         |     Fc (crystallizable fragment)   |
         |     (effector functions)           |
         ─────────────────────────────────────

Heavy chains (H): 2 identical (γ, α, μ, δ, ε)
Light chains (L): 2 identical (κ or λ)
Linked by: Disulfide bonds (inter-chain)

Detailed Structure

Cleavage Products

• Papain: Cleaves above hinge → 2 Fab + 1 Fc
• Pepsin: Cleaves below hinge → 1 F(ab')₂ + pFc' (degraded)

Domains of Heavy Chain

---

CLASSIFICATION AND FUNCTIONS OF IMMUNOGLOBULINS

Important Points

• Opsonins: IgG (best), IgM, complement C3b
• Complement activation: IgM > IgG1 > IgG3 > IgG2; IgA (alternate pathway in secretory form)
• Neonatal immunity: IgG (placenta) + IgA (breast milk)
• Valency: IgG = 2; IgM = 10 (pentamer); IgA (secretory) = 4 (dimer)

---

PARAPROTEINEMIAS

Definition

Types of M-proteins

• Intact immunoglobulin (IgG, IgA, IgM, IgD, IgE)
• Free light chains only (Bence Jones proteins - κ or λ)
• Heavy chains only (Heavy chain disease)

Classification of Paraproteinemias

PARAPROTEINEMIAS
│
├── MALIGNANT
│   ├── Multiple Myeloma (most common)
│   ├── Waldenström's Macroglobulinemia
│   ├── Primary Amyloidosis (AL)
│   └── Plasmacytoma
│
├── POTENTIALLY MALIGNANT
│   └── Monoclonal Gammopathy of Undetermined Significance (MGUS)
│
└── SECONDARY
    └── Associated with infections, autoimmune diseases

Multiple Myeloma

Waldenström's Macroglobulinemia

• Malignant lymphoplasmacytic lymphoma
• Produces IgM M-protein (high MW pentamer)
• Hyperviscosity syndrome: blurred vision, neurological symptoms, bleeding
• No bone lesions (differentiates from myeloma)
• Treatment: Plasmapheresis for hyperviscosity; ibrutinib, rituximab

MGUS (Monoclonal Gammopathy of Undetermined Significance)

• M-protein <3 g/dL, bone marrow plasma cells <10%, no CRAB criteria
• Risk of progression to myeloma: ~1%/year
• Requires monitoring but no treatment

Lab Diagnosis

1. Serum Protein Electrophoresis (SPE): M-spike (monoclonal band)
2. Immunofixation: Identifies class and type of M-protein
3. Serum Free Light Chain assay: κ/λ ratio
4. Urine protein electrophoresis: Bence Jones protein
5. Bone marrow biopsy: Plasma cell % and morphology

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Q10. Definition and Classification of Lipids + Functions and Clinical Significance of Phospholipids

DEFINITION OF LIPIDS

CLASSIFICATION OF LIPIDS

LIPIDS
│
├── SIMPLE LIPIDS (Esters of fatty acids)
│   ├── Fats and Oils (Triacylglycerols/Triglycerides)
│   │   Example: Glycerol + 3 Fatty acids
│   │   - Fats: Solid at room temp (saturated FAs)
│   │   - Oils: Liquid at room temp (unsaturated FAs)
│   │
│   └── Waxes (Long-chain alcohol + Long-chain FA)
│       Example: Beeswax (myricyl palmitate), lanolin
│
├── COMPOUND LIPIDS (Lipids + non-lipid group)
│   ├── Phospholipids (Lipid + Phosphoric acid + N-base)
│   │   ├── Glycerophospholipids
│   │   │   ├── Phosphatidylcholine (PC, Lecithin) - most abundant
│   │   │   ├── Phosphatidylethanolamine (PE, Cephalin)
│   │   │   ├── Phosphatidylserine (PS)
│   │   │   ├── Phosphatidylinositol (PI) - signaling
│   │   │   ├── Phosphatidylglycerol
│   │   │   └── Cardiolipin (diphosphatidylglycerol) - in mitochondria
│   │   │
│   │   └── Sphingomyelin (Sphingosine-based)
│   │       (Only phospholipid with no glycerol)
│   │
│   ├── Glycolipids (Lipid + Carbohydrate)
│   │   ├── Cerebrosides (galactocerebroside, glucocerebroside)
│   │   ├── Gangliosides (+ sialic acid; GM1, GM2, GM3)
│   │   └── Sulfatides
│   │
│   └── Lipoproteins (Lipid + Protein)
│       ├── Chylomicrons (TG-rich; from intestine)
│       ├── VLDL (TG-rich; from liver)
│       ├── IDL (intermediate)
│       ├── LDL (cholesterol-rich; "bad")
│       └── HDL (protein-rich; "good")
│
└── DERIVED LIPIDS (Products of hydrolysis)
    ├── Fatty Acids
    │   ├── Saturated (no double bonds): Palmitic (C16:0), Stearic (C18:0)
    │   ├── Monounsaturated: Oleic acid (C18:1 Δ9)
    │   └── Polyunsaturated (PUFA): Linoleic (ω-6), α-Linolenic (ω-3),
    │       Arachidonic (C20:4 ω-6, from linoleic)
    │
    ├── Cholesterol
    │   ├── Free cholesterol (membrane component)
    │   └── Cholesterol esters (storage form)
    │
    ├── Steroids (from cholesterol)
    │   ├── Steroid hormones (cortisol, aldosterone, sex hormones)
    │   ├── Vitamin D
    │   ├── Bile acids (cholic acid, chenodeoxycholic acid)
    │   └── Cardiac glycosides
    │
    └── Eicosanoids (from C20 PUFA, mainly arachidonic acid)
        ├── Prostaglandins (PG)
        ├── Thromboxanes (TXA₂)
        ├── Leukotrienes (LT)
        └── Lipoxins

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NOTE ON PHOSPHOLIPIDS: STRUCTURE, FUNCTIONS, AND CLINICAL SIGNIFICANCE

Structure of Glycerophospholipids

         Fatty acid 1 (sn-1 position: usually saturated)
         │
sn-1  O-C=O
sn-2  O-C=O      ← Glycerol backbone
         │
         Fatty acid 2 (sn-2 position: usually unsaturated)
         │
sn-3  O-PO₄-X  ← Phosphate + Head group (X)

X = Choline → Phosphatidylcholine (PC)
X = Ethanolamine → Phosphatidylethanolamine (PE)
X = Serine → Phosphatidylserine (PS)
X = Inositol → Phosphatidylinositol (PI)
X = Glycerol → Phosphatidylglycerol (PG)

Types and Special Features

FUNCTIONS OF PHOSPHOLIPIDS

• Form lipid bilayer of all biological membranes
• Amphipathic nature → self-assembly into bilayers
• Control membrane fluidity (composition of fatty acids)

• Dipalmitoylphosphatidylcholine (DPPC) = major component of lung surfactant (40-50%)
• Reduces alveolar surface tension; prevents alveolar collapse
• Synthesized by type II pneumocytes
• Deficiency in prematurity → Respiratory Distress Syndrome (RDS) of newborn
• L/S ratio (lecithin:sphingomyelin ratio) in amniotic fluid - measures fetal lung maturity
  • <2: lungs immature (risk of RDS)
  • ≥2: lungs mature

• PI(4,5)P₂ (PIP₂) → cleaved by Phospholipase C (PLC) → IP₃ (releases Ca²⁺ from ER) + DAG (activates Protein Kinase C)
• PIP₂ → PIP₃ (via PI3-kinase) → activates Akt/PKB → cell survival, growth

• Phosphatidylcholine and PE → Phospholipase A₂ (PLA₂) → Arachidonic acid → Prostaglandins, Thromboxanes, Leukotrienes (via COX and LOX pathways)
• PAF (platelet activating factor): potent inflammatory mediator

• Lecithin (phosphatidylcholine) in bile → emulsifies cholesterol → keeps cholesterol in solution (prevents gallstones)
• Bile: 80% bile salts + 16% lecithin + 4% cholesterol

• Phospholipid monolayer forms outer coat of lipoproteins (with apolipoproteins and cholesterol)

• Platelet membrane phospholipids (especially PS exposed on activated platelets) provide surface for coagulation factor assembly (prothrombinase complex)

• PS externalization on outer leaflet = "eat me" signal for macrophages → phagocytosis of apoptotic cells

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CLINICAL SIGNIFICANCE OF PHOSPHOLIPIDS

Exam Tip: L/S ratio for fetal lung maturity is a classic MBBS exam question. L/S ≥2 = mature lungs. Corticosteroids to mother in threatened preterm labor stimulate surfactant synthesis via glucocorticoid receptors on type II pneumocytes.

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QUICK REFERENCE SUMMARY TABLE

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References: Guyton & Hall Medical Physiology; Junqueira's Basic Histology; Robbins & Kumar Basic Pathology; Ganong's Review of Medical Physiology; Histology: A Text and Atlas (Pawlina); Basic Medical Biochemistry (Marks); Harrison's Principles of Internal Medicine 22E; Roitt's Essential Immunology