MBBS Biochemistry Short Answers (Vasudevan-Based)

1. ELISA (Enzyme-Linked Immunosorbent Assay) — Tumour & Immune Functions

Simple Cycle/Steps:

Coat plate with ANTIGEN (or antibody)
         ↓
Add patient SERUM (antibody binds)
         ↓
Wash off unbound material
         ↓
Add ENZYME-LINKED secondary antibody
         ↓
Wash again
         ↓
Add SUBSTRATE → Enzyme converts it → COLOUR CHANGE
         ↓
Read ABSORBANCE (OD) → quantify

Types:

Applications:

• Detection of tumour markers (AFP for hepatocellular carcinoma, PSA for prostate cancer, CEA for colorectal cancer, CA-125 for ovarian cancer)
• Monitoring cancer recurrence

• HIV antibody detection (screening test)
• Detection of autoantibodies (ANA in SLE, anti-dsDNA)
• Hepatitis B surface antigen (HBsAg) detection
• Quantification of immunoglobulins (IgE in allergy)
• TORCH infections during pregnancy

2. Xanthoma

Types & Associated Dyslipidemias:

Biochemical Basis:

• High circulating LDL/IDL/chylomicrons → macrophage uptake of oxidised LDL via scavenger receptors → foam cells → deposits in dermis and tendons

3. Allopurinol

Mechanism (Simple Cycle):

Hypoxanthine → [Xanthine Oxidase] → Xanthine → [Xanthine Oxidase] → URIC ACID

         ↑ ALLOPURINOL blocks both steps (competitive + suicide inhibition)
         ↓
Allopurinol itself is oxidised to ALLOXANTHINE (Oxypurinol)
         ↓
Alloxanthine binds IRREVERSIBLY to xanthine oxidase (Suicide/Mechanism-based inhibition)
         ↓
Hypoxanthine & Xanthine accumulate → More soluble → Excreted in urine
         ↓
Serum URATE ↓ → Gout prevented

Key Points:

• Suicide (mechanism-based) inhibitor - enzyme converts allopurinol to alloxanthine which then destroys the enzyme
• Half-life of allopurinol: 1-2 hours; alloxanthine: 15 hours (once-daily dosing)
• Caution: When given with azathioprine/6-mercaptopurine → reduce dose by 75% (xanthine oxidase normally inactivates these drugs)
• HLA-B*5801 screening recommended in Han Chinese/Korean/Thai patients (risk of Stevens-Johnson syndrome)

4. Translation — Drug Inhibition

Simple Summary Cycle:

DNA → mRNA (Transcription) → Translation on RIBOSOME
                                      ↓
          INITIATION → ELONGATION → TERMINATION → Protein

Drugs Inhibiting Translation (by site/step):

Memory Aid:

• "Buy AT 30" → Aminoglycosides, Tetracyclines block 30S
• "CELL at 50" → Chloramphenicol, Erythromycin, Lincosamides block 50S

5. Dietary Fibre

Types:

Biochemical Benefits / Functions:

SOLUBLE FIBRE                          INSOLUBLE FIBRE
   ↓                                        ↓
Forms gel in gut                     Increases stool bulk
   ↓                                        ↓
Slows glucose absorption             Reduces transit time
   ↓                                        ↓
↓ Post-prandial blood sugar          ↓ Constipation
   ↓                                        ↓
Binds bile acids → ↓ cholesterol     ↓ Risk of colon cancer
reabsorption → ↓ LDL
   ↓
Fermented by bacteria → SCFAs
(butyrate = colonocyte fuel)

RDA: 25-38 g/day

Clinical Benefits:

• Reduces risk of type 2 diabetes, cardiovascular disease, colon cancer, obesity, constipation, diverticular disease
• Prebiotics (inulin, FOS) selectively feed beneficial Bifidobacteria

6. Cell Cycle Stages

Cell Cycle Diagram:

              G0 (Quiescent)
              ↑
         ←←G1 PHASE←←←←←←←←←←←←←←←←←
        ↓    (Gap 1: RNA, protein synthesis; cell growth)       ↑
        ↓                                                        ↑
    RESTRICTION POINT (R point - point of no return)            ↑
        ↓     ← Rb phosphorylation by CDK4/6-Cyclin D           ↑
        ↓                                                     M PHASE
     S PHASE                                              (Mitosis: PMAT)
   (DNA synthesis:                                              ↑
   DNA amount 2n→4n)                                            ↑
        ↓                                                        ↑
     G2 PHASE ────────────────────────────────────────────→→→→→→
   (Gap 2: protein synthesis for mitosis; spindle formation)

Key Checkpoints:

Cyclin-CDK pairs (Vasudevan):

• G1 phase: Cyclin D - CDK4/6
• G1/S transition: Cyclin E - CDK2
• S phase: Cyclin A - CDK2
• G2/M transition: Cyclin B - CDK1 (MPF = Maturation Promoting Factor)

Duration (approx.):

• G1: longest (10-12 h variable)
• S: 6-8 h
• G2: 3-4 h
• M: shortest (1 h)

Exam Tips: For ELISA - remember it's based on Ag-Ab reaction with enzyme amplification. For allopurinol - suicide inhibitor is the classic exam point. For translation inhibition - 30S vs 50S distinction is heavily tested. For cell cycle - CDK-Cyclin pairs and checkpoints are high-yield. For xanthoma - palmar xanthoma pathognomonic for Type III hyperlipidaemia.