Biochemistry & Molecular Biology

Biochemistry & Molecular Biology for USMLE

Master the fundamentals of biochemistry and molecular biology with free flashcards and evidence-based learning strategies. This lesson covers metabolic pathways, enzyme kinetics, DNA replication, protein synthesis, and cell signaling—essential concepts for USMLE Step 1 success and clinical practice.

Welcome

Welcome to Biochemistry & Molecular Biology! 🧬 This discipline forms the molecular foundation of medicine, explaining how cells function at the chemical level. Understanding these concepts is crucial for interpreting laboratory values, recognizing metabolic disorders, and comprehending drug mechanisms. This lesson synthesizes high-yield topics frequently tested on USMLE Step 1, with emphasis on clinical correlations that will serve you throughout your medical career.

Core Concepts

1. Enzyme Kinetics and Inhibition 🧪

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy. Understanding enzyme kinetics is essential for pharmacology and understanding metabolic regulation.

Michaelis-Menten Kinetics

The Michaelis-Menten equation describes the relationship between substrate concentration and reaction velocity:

V = (Vmax × [S]) / (Km + [S])

Where:

• V = reaction velocity
• Vmax = maximum velocity at substrate saturation
• [S] = substrate concentration
• Km = Michaelis constant (substrate concentration at ½ Vmax)

💡 Key Insight: Km represents enzyme affinity for substrate. LOW Km = HIGH affinity (enzyme binds substrate tightly). HIGH Km = LOW affinity (enzyme binds substrate weakly).

Types of Enzyme Inhibition

ENZYME INHIBITION PATTERNS

┌─────────────────────────────────────────────────┐
│                                                 │
│  COMPETITIVE           NON-COMPETITIVE          │
│                                                 │
│    Substrate              Substrate             │
│       ↓                      ↓                  │
│     ┌───┐                  ┌───┐               │
│  I─→│ E │←─S             S─→│ E │←─I           │
│     └───┘                  └───┘                │
│                              │                  │
│  • Km increases              I (allosteric)     │
│  • Vmax unchanged                               │
│  • Overcome with ↑[S]      • Km unchanged       │
│                            • Vmax decreases     │
│                            • Cannot overcome    │
│                                                 │
│  UNCOMPETITIVE            MIXED                 │
│                                                 │
│      Substrate              Substrate           │
│         ↓                      ↓                │
│     S─→┌───┐              ┌───┐←─I             │
│        │ E │←─I        I─→│ E │←─S             │
│        └───┘              └───┘                 │
│                                                 │
│  • Binds ES complex       • Binds E or ES       │
│  • Km & Vmax decrease     • Both Km & Vmax      │
│  • Rare in biology          affected variably   │
└─────────────────────────────────────────────────┘

🧠 Mnemonic - COMPETITIVE: "Can Overcome with More substrate, Preserves Vmax, Elevates Km, Tries to Imitate substrate, Takes Its place in actiVe site, Example: Methotrexate (folic acid analog)"

🤔 Did you know? Aspirin is an irreversible inhibitor of COX enzymes, forming a covalent bond that permanently inactivates the enzyme. Platelets cannot synthesize new COX, explaining why aspirin's antiplatelet effect lasts 7-10 days (the lifespan of platelets).

2. Glycolysis and Gluconeogenesis ⚡

Glycolysis is the metabolic pathway that converts glucose to pyruvate, generating ATP and NADH. It occurs in the cytoplasm and doesn't require oxygen.

Glycolysis Overview

GLYCOLYSIS PATHWAY (10 steps)

  Glucose (6C)
      │ ← ATP (Hexokinase/Glucokinase)
      ↓
  G6P │ (branch to PPP/glycogen)
      ↓
  F6P │ ← ATP (PFK-1) ⭐ RATE-LIMITING
      ↓
  F-1,6-BP
      │
   ┌──┴──┐ (Aldolase splits)
   ↓     ↓
  DHAP  G3P
   │     │
   └──→ G3P (×2) ← NAD+ → NADH
         │
      1,3-BPG ← substrate-level phosphorylation
         │ → ATP (×2)
         ↓
      3PG
         ↓
      2PG
         ↓
      PEP ← H2O released
         │ → ATP (×2) (Pyruvate kinase)
         ↓
    PYRUVATE (3C) ×2

📊 NET YIELD per glucose:
   • 2 ATP (4 produced - 2 consumed)
   • 2 NADH
   • 2 Pyruvate

Key Regulatory Enzymes

💡 Clinical Pearl: Hexokinase (most tissues) has LOW Km (high affinity) and is saturated at normal glucose levels. Glucokinase (liver, pancreatic β-cells) has HIGH Km (low affinity) and acts as a "glucose sensor," with activity proportional to glucose concentration. This explains why the liver increases glucose uptake after meals.

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (lactate, amino acids, glycerol). It's essentially glycolysis in reverse, but uses different enzymes to bypass irreversible steps.

GLUCONEOGENESIS BYPASS ENZYMES

    GLUCOSE
       ↑
    G6Pase ⭐ (ER lumen, liver/kidney only)
       │
     G6P
       ↑
     F6P
       ↑
  F-1,6-BPase ⭐ (rate-limiting)
       │
   F-1,6-BP
       ↑
     DHAP + G3P
       ↑
     1,3-BPG
       ↑
      PEP
       ↑
  PEPCK ⭐ (uses GTP)
       │
  Oxaloacetate (in mitochondria)
       ↑
  Pyruvate carboxylase ⭐ (uses ATP, biotin)
       │
    PYRUVATE
       ↑
  (from lactate, alanine)

🧠 Remember: "Pathway Goes From Liver"
   P = PEPCK
   G = Glucose-6-Phosphatase
   F = Fructose-1,6-Bisphosphatase
   L = (requires these in) Liver

⚠️ Critical Concept: Gluconeogenesis requires 4 ATP + 2 GTP + 2 NADH to make one glucose (energy investment of ~6 ATP), while glycolysis generates only 2 ATP. This makes sense—reversing a spontaneous process requires energy input.

3. Citric Acid Cycle (TCA/Krebs Cycle) 🔄

The citric acid cycle is the central metabolic hub that oxidizes acetyl-CoA to CO₂, generating reduced coenzymes (NADH, FADH₂) for the electron transport chain.

CITRIC ACID CYCLE

        Acetyl-CoA (2C)
             │
             ↓ + Oxaloacetate (4C)
      CITRATE (6C) ← Citrate synthase
             │
             ↓
      Isocitrate
             │ ← NAD+ → NADH + CO₂
             ↓ (Isocitrate dehydrogenase) ⭐
      α-Ketoglutarate (5C)
             │ ← NAD+ → NADH + CO₂
             ↓ (α-Ketoglutarate dehydrogenase) ⭐
      Succinyl-CoA (4C)
             │ → GTP (substrate-level phosphorylation)
             ↓
      Succinate
             │ ← FAD → FADH₂
             ↓ (Succinate dehydrogenase)
      Fumarate
             │ + H₂O
             ↓
      Malate
             │ ← NAD+ → NADH
             ↓
      Oxaloacetate (regenerated)
             │
             └──→ (cycle continues)

📊 NET YIELD per Acetyl-CoA:
   • 3 NADH (→ 7.5 ATP)
   • 1 FADH₂ (→ 1.5 ATP)
   • 1 GTP (= 1 ATP)
   • 2 CO₂
   TOTAL: ~10 ATP per acetyl-CoA

🧠 Mnemonic - TCA Intermediates: "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate"

• Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

💡 Regulation: The cycle is activated when energy is needed (↑ ADP, ↑ Ca²⁺, ↑ NAD⁺) and inhibited when energy is abundant (↑ ATP, ↑ NADH). The three rate-limiting enzymes are:

1. Citrate synthase (inhibited by citrate, ATP, NADH)
2. Isocitrate dehydrogenase (activated by ADP, Ca²⁺; inhibited by ATP, NADH)
3. α-Ketoglutarate dehydrogenase (inhibited by succinyl-CoA, NADH)

4. Electron Transport Chain and Oxidative Phosphorylation 🔋

The electron transport chain (ETC) consists of four protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH₂ to O₂, pumping protons to create an electrochemical gradient.

ELECTRON TRANSPORT CHAIN

   INTERMEMBRANE SPACE (high [H⁺])
┌─────────────────────────────────────────┐
│  H⁺  H⁺  H⁺     H⁺  H⁺   H⁺  H⁺        │
│   ↑   ↑   ↑      ↑   ↑    ↑   ↑         │
├───┼───┼───┼──────┼───┼────┼───┼─────────┤
│  │ I │  │ III │ │ IV │  │ V │         │
│  │   │  │     │ │    │  │   │         │
│  e⁻→ UQ →Cyt c→ e⁻→ O₂  ATP←ADP+Pi    │
│  ↑       ↑             ↓               │
│ NADH   FADH₂         H₂O               │
│                                        │
│      MITOCHONDRIAL MATRIX (low [H⁺])   │
└─────────────────────────────────────────┘

Complex I:   NADH → UQ (pumps 4 H⁺)
Complex II:  FADH₂ → UQ (no H⁺ pumped)
Complex III: UQ → Cyt c (pumps 4 H⁺)
Complex IV:  Cyt c → O₂ (pumps 2 H⁺)
Complex V:   ATP synthase (uses H⁺ gradient)

💰 ATP YIELD:
   • NADH → 2.5 ATP
   • FADH₂ → 1.5 ATP
   • Complete glucose oxidation → ~30 ATP

🔧 Try this: Calculate ATP from complete glucose oxidation:

• Glycolysis: 2 ATP + 2 NADH (→ 5 ATP) = 7 ATP
• Pyruvate → Acetyl-CoA: 2 NADH (→ 5 ATP) = 5 ATP
• TCA cycle (×2): 6 NADH (→ 15 ATP) + 2 FADH₂ (→ 3 ATP) + 2 GTP = 20 ATP
• Total: ~32 ATP (accounting for transport costs)

ETC Inhibitors and Uncouplers

⚠️ Toxicology Pearl: Cyanide poisoning presents with cherry-red skin (tissues can't use O₂, so venous blood remains oxygenated), lactic acidosis, and almond breath odor. Treatment includes hydroxocobalamin or sodium thiosulfate.

5. DNA Replication 🧬

DNA replication is semiconservative—each new double helix contains one original strand and one newly synthesized strand.

DNA REPLICATION FORK

     5' ←─────────────── 3' (Leading strand)
          ↓ ↓ ↓ ↓ ↓ ↓
     3' ─→ CONTINUOUS synthesis

       REPLICATION FORK
            ╱  ╲
           ╱    ╲
       5'→╱      ╲←3' (Lagging strand)
         ╱        ╲
     3'←╱  ←─ ←─ ←─╲→5'
        Okazaki fragments
        (discontinuous)

🔧 KEY ENZYMES:

┌──────────────────┬─────────────────────────────┐
│  ENZYME          │  FUNCTION                   │
├──────────────────┼─────────────────────────────┤
│  Helicase        │  Unwinds DNA double helix   │
│  SSB proteins    │  Stabilize single strands   │
│  Primase         │  Synthesizes RNA primers    │
│  DNA Pol III     │  Main synthesis enzyme      │
│  DNA Pol I       │  Removes primers, fills gaps│
│  Ligase          │  Seals nicks in backbone    │
│  Topoisomerase   │  Relieves supercoiling      │
└──────────────────┴─────────────────────────────┘

💡 Key Concept: DNA polymerase can only add nucleotides to a 3'-OH group (5'→3' synthesis) and requires a primer. This explains why:

• Leading strand is continuous
• Lagging strand is discontinuous (Okazaki fragments)
• RNA primers are needed

🧠 Mnemonic - Replication Enzymes: "Help Stop Primer DNA Problems, Ligate To-fix"

• Helicase, SSB, Primase, DNA Pol III, DNA Pol I, Ligase, Topoisomerase

Replication Errors and Repair

🤔 Did you know? Telomerase is active in germ cells, stem cells, and ~90% of cancers, but not in most somatic cells. This is why normal cells have a limited replicative capacity (Hayflick limit), while cancer cells can divide indefinitely.

6. Transcription and RNA Processing 📝

Transcription synthesizes RNA from a DNA template using RNA polymerase.

Prokaryotic vs Eukaryotic Transcription

Eukaryotic RNA Processing

RNA PROCESSING STEPS

  PRIMARY TRANSCRIPT (pre-mRNA)
        │
        ├─→ 5' CAPPING (7-methylguanosine)
        │   (protects from degradation)
        │
        ├─→ SPLICING (removes introns)
        │   │
        │   └─→ Spliceosome (snRNPs)
        │       recognizes splice sites:
        │       • 5' splice: GU
        │       • 3' splice: AG
        │       • Branch point: A
        │
        └─→ 3' POLYADENYLATION
            (adds ~200 A residues)
            (increases stability)

  ↓

  MATURE mRNA
  │
  └─→ Exported to cytoplasm
      for translation

🧠 Remember: "CAP it, SPLICE it, Add poly-A"

💡 Alternative Splicing: A single gene can produce multiple protein isoforms by including or excluding different exons. This explains how ~20,000 human genes produce >100,000 proteins.

⚠️ Clinical Correlation: β-thalassemia often results from splicing mutations that cause aberrant intron retention or exon skipping, producing non-functional β-globin.

7. Translation and Protein Synthesis 🏭

Translation converts mRNA sequence into protein using ribosomes and tRNA.

TRANSLATION PROCESS

┌─────────────────────────────────────────────┐
│  INITIATION                                 │
│                                             │
│  5' Cap ─→ mRNA ←─ AUG (start codon)       │
│              │                              │
│         Met-tRNA ← eIF2·GTP                 │
│              │                              │
│         40S + 60S = 80S ribosome            │
└─────────────────────────────────────────────┘

┌─────────────────────────────────────────────┐
│  ELONGATION                                 │
│                                             │
│  Ribosome moves 5' → 3' along mRNA         │
│                                             │
│   A site  P site  E site                   │
│    │       │       │                        │
│  [new] → [peptide] → [exit]                │
│                                             │
│  • EF-Tu·GTP brings aa-tRNA to A site      │
│  • Peptidyl transferase forms peptide bond │
│  • EF-G·GTP translocates ribosome          │
└─────────────────────────────────────────────┘

┌─────────────────────────────────────────────┐
│  TERMINATION                                │
│                                             │
│  Stop codon (UAA, UAG, UGA)                 │
│              │                              │
│     Release factors (RF)                    │
│              │                              │
│     Polypeptide released                    │
│              │                              │
│     Ribosome dissociates                    │
└─────────────────────────────────────────────┘

🧠 Mnemonic - Stop Codons: "U Are Away, U Are Gone"

• UAA, UAG, UGA

Genetic Code Properties

• Universal: Same code in nearly all organisms
• Degenerate: Multiple codons for most amino acids (wobble base pairing)
• Unambiguous: Each codon specifies only one amino acid
• Non-overlapping: Read in consecutive triplets

💡 Wobble Hypothesis: The third position (3' end) of the codon can have non-Watson-Crick pairing with tRNA. This explains why:

• Leucine has 6 codons
• Methionine and tryptophan have only 1 codon each

Antibiotic Mechanisms Targeting Translation

🤔 Did you know? Mitochondria have their own 70S ribosomes (like bacteria), which is why chloramphenicol can cause bone marrow toxicity—it inhibits mitochondrial protein synthesis in eukaryotic cells.

8. Cell Signaling Pathways 📡

Cells communicate through signaling molecules (ligands) that bind to receptors, triggering intracellular cascades.

Major Receptor Types

G-Protein Coupled Receptor Signaling

GPCR SIGNALING CASCADE

   Ligand (hormone, NT)
        │
        ↓ (binds)
   ┌─────────┐
   │  GPCR   │ (7 transmembrane domains)
   └────┬────┘
        │ (activates)
        ↓
   G-protein (Gα, Gβ, Gγ)
        │
        ├─→ Gαs → ↑ Adenylyl cyclase → ↑ cAMP
        │                               ↓
        │                          PKA activation
        │                               ↓
        │                   Phosphorylates targets
        │
        ├─→ Gαi → ↓ Adenylyl cyclase → ↓ cAMP
        │
        ├─→ Gαq → ↑ Phospholipase C → ↑ IP₃ + DAG
        │                         ↓         ↓
        │                    ↑ Ca²⁺      PKC
        │
        └─→ Gα₁₂/₁₃ → Rho pathways

📊 Second Messengers:
   • cAMP
   • cGMP
   • IP₃
   • DAG
   • Ca²⁺

💡 Cholera vs Pertussis Toxins:

• Cholera toxin: Permanently activates Gαs (↑↑↑ cAMP) → massive Cl⁻/H₂O secretion → watery diarrhea
• Pertussis toxin: Inactivates Gαi (prevents inhibition) → ↑↑ cAMP → impaired immune response

Receptor Tyrosine Kinase (RTK) Signaling

RTK PATHWAY (e.g., Insulin Receptor)

    Insulin (ligand)
         │
         ↓ (binds)
    ┌─────────┐
    │   RTK   │ (dimerizes)
    └────┬────┘
         │ (autophosphorylation)
         ↓
   Tyrosine residues-PO₄
         │
         ├─→ RAS/MAPK pathway
         │   (cell growth/division)
         │
         ├─→ PI3K/AKT pathway
         │   (cell survival, glucose uptake)
         │
         └─→ JAK/STAT pathway
         (immune response, cell proliferation)

⚠️ CLINICAL: Many oncogenes encode
   constitutively active RTKs or
   downstream signaling proteins
   (e.g., RAS mutations in 30% cancers)

🔧 Try this: Trace the insulin signaling cascade:

1. Insulin binds → RTK dimerization
2. Autophosphorylation of tyrosine residues
3. IRS-1 (insulin receptor substrate) binds
4. PI3K activation → PIP₂ → PIP₃
5. AKT activation
6. GLUT4 translocation to membrane
7. ↑ Glucose uptake into muscle/adipose tissue

Examples with Explanations

Example 1: Enzyme Kinetics Problem 🧮

Question: An enzyme has a Km of 10 mM and Vmax of 100 μmol/min. What is the reaction velocity when substrate concentration is 10 mM?

Solution:

Using the Michaelis-Menten equation:

V = (Vmax × [S]) / (Km + [S])

Answer: V = 50 μmol/min (exactly ½ Vmax, because [S] = Km)

💡 Key Insight: When [S] = Km, velocity is always ½ Vmax by definition. This is a quick check for exam questions.

Example 2: Glycolysis ATP Calculation 📊

Question: Calculate the net ATP yield from glycolysis when starting with fructose-6-phosphate (F6P) instead of glucose.

Solution:

Normal glycolysis from glucose:

• Investment phase: -2 ATP
• Payoff phase: +4 ATP
• Net: +2 ATP

Starting from F6P (bypasses first step):

• Investment phase: -1 ATP (only PFK-1 uses ATP)
• Payoff phase: +4 ATP (unchanged)
• Net: +3 ATP

Explanation: F6P enters after the first ATP-consuming step (hexokinase), saving one ATP. The downstream reactions remain identical, so you gain an extra ATP compared to starting from glucose.

🌍 Real-world application: Fructose from sucrose (table sugar) enters glycolysis at different points in liver vs muscle, affecting ATP yield and metabolic fate. Hepatic fructose metabolism bypasses PFK-1 regulation, contributing to fatty liver disease with excessive fructose consumption.

Example 3: DNA Replication Directionality 🧬

Question: During DNA replication, if the template strand is 3'-TACGGATCG-5', what is the sequence of the newly synthesized strand?

Solution:

Answer: 5'-ATGCCTAGC-3'

Explanation:

1. DNA polymerase synthesizes 5'→3'
2. Template is read 3'→5'
3. Base pairing rules: A-T, G-C
4. New strand is antiparallel and complementary

💡 Remember: The template strand runs opposite to the direction of synthesis. If confused, always mark the 5' and 3' ends first.

Example 4: Signal Transduction - cAMP Pathway 📡

Question: Epinephrine binds to β-adrenergic receptors on liver cells. Trace the pathway leading to glucose release.

Solution:

 SIGNAL TRANSDUCTION CASCADE

    Epinephrine (stress hormone)
          │
          ↓ (binds)
    β-adrenergic receptor (GPCR)
          │
          ↓ (activates)
    Gαs protein (GTP-bound form)
          │
          ↓ (stimulates)
    Adenylyl cyclase
          │
          ↓ (produces)
    cAMP (from ATP)
          │
          ↓ (activates)
    Protein Kinase A (PKA)
          │
          ├─→ Phosphorylates phosphorylase kinase
          │   │
          │   ↓ (activates)
          │   Glycogen phosphorylase
          │   │
          │   ↓
          │   Glycogen → Glucose-1-P → G6P → Glucose
          │
          └─→ Phosphorylates glycogen synthase
              (INACTIVATES - stops glycogen synthesis)

 Result: ↑ Blood glucose (fight-or-flight response)

Explanation: This cascade demonstrates signal amplification—one epinephrine molecule can activate many Gαs proteins, each activating multiple adenylyl cyclase molecules, producing thousands of cAMP molecules, activating many PKA enzymes, each phosphorylating multiple target proteins. The result is massive glucose mobilization from a single hormone signal.

🤔 Did you know? This is why epinephrine (adrenaline) injection works so quickly in anaphylaxis—the signal amplification allows rapid systemic effects from a single molecule trigger.

Common Mistakes

⚠️ Mistake 1: Confusing Km with Kd

• Km = substrate concentration at ½ Vmax (enzyme kinetics)
• Kd = dissociation constant (binding affinity)
• They're related but NOT identical. Km includes catalytic rate constants.

⚠️ Mistake 2: Reversing DNA/RNA Polymerase Directionality

• ❌ WRONG: "Polymerase reads 5'→3'"
• ✅ CORRECT: "Polymerase reads template 3'→5', synthesizes 5'→3'"
• Mnemonic: Synthesis is always 5'→3' (adding to 3'-OH group)

⚠️ Mistake 3: Miscounting ATP in Glycolysis

• Don't forget the investment phase uses 2 ATP!
• Common error: reporting 4 ATP instead of net 2 ATP
• Remember: Each glucose yields TWO G3P molecules, so multiply payoff phase by 2

⚠️ Mistake 4: Mixing Up Glucokinase vs Hexokinase

⚠️ Mistake 5: Forgetting Wobble Base Pairing

• The genetic code is degenerate because the 3rd codon position (wobble position) allows non-standard pairing
• This is why most amino acids have multiple codons
• Clinically relevant: Some mutations at wobble positions are silent (don't change amino acid)

⚠️ Mistake 6: Confusing Transcription vs Translation Inhibitors

• Transcription inhibitors: Rifampin (RNA polymerase), Actinomycin D (intercalates DNA)
• Translation inhibitors: All the ribosome-targeting antibiotics
• Don't mix up prokaryotic (70S) vs eukaryotic (80S) ribosome targets!

⚠️ Mistake 7: Misunderstanding Competitive Inhibition

• ❌ WRONG: "Competitive inhibition decreases Vmax"
• ✅ CORRECT: "Competitive inhibition increases Km, Vmax unchanged"
• Can be overcome by increasing substrate concentration

Key Takeaways

✅ Enzyme kinetics: Km represents substrate affinity (LOW Km = HIGH affinity). Competitive inhibitors increase Km; non-competitive inhibitors decrease Vmax.

✅ Glycolysis: Occurs in cytoplasm, produces net 2 ATP + 2 NADH per glucose. Three irreversible steps regulated by hexokinase/glucokinase, PFK-1 (rate-limiting), and pyruvate kinase.

✅ Gluconeogenesis: Reverses glycolysis using 4 unique enzymes (pyruvate carboxylase, PEPCK, F-1,6-BPase, G6Pase). Requires 6 ATP equivalents per glucose.

✅ TCA cycle: Generates 3 NADH + 1 FADH₂ + 1 GTP per acetyl-CoA. Regulated by energy status (ATP/ADP, NADH/NAD⁺ ratios).

✅ Electron transport chain: Four protein complexes pump protons; Complex V (ATP synthase) uses gradient. NADH → 2.5 ATP, FADH₂ → 1.5 ATP. Complete glucose oxidation → ~30-32 ATP.

✅ DNA replication: Semiconservative, bidirectional, 5'→3' synthesis only. Leading strand continuous, lagging strand discontinuous (Okazaki fragments). Key enzymes: helicase, primase, DNA Pol III, DNA Pol I, ligase.

✅ Transcription: RNA polymerase synthesizes RNA 5'→3' from DNA template. Eukaryotic processing includes 5' cap, 3' poly-A tail, and splicing (removes introns).

✅ Translation: Ribosomes read mRNA 5'→3' in triplets (codons). Start codon AUG (Met), stop codons UAA/UAG/UGA. Aminoglycosides and tetracyclines target 30S; chloramphenicol and macrolides target 50S.

✅ Cell signaling: GPCRs activate G-proteins → second messengers (cAMP, IP₃, DAG). RTKs autophosphorylate → activate RAS/MAPK, PI3K/AKT pathways. Nuclear receptors directly regulate transcription.

💡 For USMLE Success: Focus on rate-limiting enzymes, regulatory mechanisms, clinical correlations (enzyme deficiencies, drug targets), and integration between pathways. Practice calculating ATP yields and tracing signaling cascades.

📚 Further Study

1. NCBI Bookshelf - Biochemistry (Berg, Tymoczko, Stryer): https://www.ncbi.nlm.nih.gov/books/NBK21154/ - Comprehensive free textbook covering all major biochemistry topics with clinical correlations

2. Khan Academy MCAT Biochemistry: https://www.khanacademy.org/test-prep/mcat/biomolecules - Video lectures and practice questions covering enzyme kinetics, metabolism, molecular biology with excellent visual explanations

3. PhysiologyWeb - Biochemistry Resources: https://www.physiologyweb.com/ - Detailed metabolic pathway diagrams, enzyme mechanisms, and interactive tutorials for visual learners preparing for board exams