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Amino Acid Derivatives & Nucleic Acid Derivatives

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PART 1: AMINO ACID DERIVATIVES

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1. Derivatives of Tyrosine

a) Catecholamines

• Norepinephrine (noradrenaline) - via dopamine-beta-hydroxylase
• Epinephrine (adrenaline) - via PNMT (phenylethanolamine-N-methyltransferase), which methylates norepinephrine using SAM as methyl donor

• Deficiency of dopamine in substantia nigra → Parkinson's disease
• Excess dopamine activity → linked to schizophrenia
• L-DOPA is used therapeutically in Parkinson's disease

b) Thyroid Hormones (T3 and T4)

c) Melanin

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2. Derivatives of Tryptophan

a) Serotonin (5-Hydroxytryptamine, 5-HT)

• Found in: enterochromaffin cells of the gut (90%), platelets, brain
• Functions: mood regulation, peristalsis, vasoconstriction, platelet aggregation
• Clinical: Decreased serotonin → depression; SSRI drugs block serotonin reuptake

b) Niacin (Vitamin B3)

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3. Derivatives of Histidine

a) Histamine

• Found in: mast cells, basophils, gastric enterochromaffin-like cells
• Functions: mediates allergic reactions, increases vascular permeability, stimulates gastric acid secretion (H2 receptors), acts as a neurotransmitter (H1 receptors)
• Clinical: Antihistamines (H1 blockers) for allergies; H2 blockers (e.g., ranitidine) for peptic ulcer disease

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4. Derivatives of Glutamate

a) GABA (γ-Aminobutyric Acid)

• Deficiency → seizures
• GABA-A receptor is the target of benzodiazepines and barbiturates

b) Glutathione

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5. Derivatives of Glycine

a) Creatine

• Creatinine (degradation product of creatine phosphate) is used as a marker of renal function.

b) Bile Salts

c) Heme (Porphyrins)

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6. Derivatives of Methionine

• Norepinephrine → epinephrine
• DNA methylation
• Phosphatidylcholine synthesis
• Creatine synthesis

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7. Derivatives of Arginine

• Nitric Oxide (NO): Arginine → citrulline + NO, catalyzed by NO synthase (NOS). NO is a potent vasodilator (activates guanylyl cyclase → cGMP). Nitroglycerin is a prodrug that releases NO.
• Polyamines (spermine, spermidine): From ornithine (via arginine); involved in cell growth and proliferation.
• Urea cycle: Arginine is cleaved by arginase to produce urea + ornithine.

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8. Derivatives of Serine

• Acetylcholine: Serine → ethanolamine → choline → acetylcholine (major excitatory neurotransmitter at the neuromuscular junction; requires choline acetyltransferase + acetyl-CoA)
• Phosphatidylserine / phosphatidylethanolamine: Membrane phospholipid components

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9. Cysteine Derivatives

• Taurine: Cysteine → hypotaurine → taurine. Conjugates bile acids; antioxidant; neurotransmitter modulator.
• Coenzyme A (CoA): Cysteine contributes the thiol (-SH) group essential for CoA function.

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PART 2: NUCLEIC ACID DERIVATIVES

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1. Structural Components

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2. Purine Nucleotide Derivatives

De Novo Synthesis

• First committed step: PRPP → 5-phosphoribosylamine (by PRPP amidotransferase, inhibited by AMP/GMP)
• The process builds up to IMP (inosine monophosphate), which is then converted to:
  • AMP (requires aspartate; uses GTP as energy)
  • GMP (requires glutamine; uses ATP as energy)

Purine Salvage Pathway

• HGPRT (hypoxanthine-guanine phosphoribosyltransferase): hypoxanthine → IMP; guanine → GMP
• APRT (adenine phosphoribosyltransferase): adenine → AMP
• HGPRT deficiency → Lesch-Nyhan syndrome (hyperuricemia, self-mutilation, gout, neurological deficits)

Purine Degradation & Uric Acid

• Uric acid is the end product of purine catabolism in humans
• Gout = uric acid crystal deposition in joints
• Allopurinol inhibits xanthine oxidase → reduces uric acid production (used in gout therapy)
• Rasburicase (recombinant uricase) converts uric acid to allantoin (used in tumor lysis syndrome)

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3. Pyrimidine Nucleotide Derivatives

• Carbamoyl phosphate synthase II (cytoplasm) is the rate-limiting step
• Carbamoyl phosphate + aspartate → UMP (through several steps) → UDP → UTP → CTP
• Thymidine (TMP): dUMP → dTMP by thymidylate synthase (requires N5,N10-methylene-THF)
  • Methotrexate and 5-fluorouracil (5-FU) inhibit this pathway → cancer chemotherapy

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4. Important Nucleotide Derivatives with Biological Functions

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5. Cyclic Nucleotides as Second Messengers

• cAMP: Formed from ATP by adenylyl cyclase; activates Protein Kinase A (PKA). Degraded by phosphodiesterases. Example: adrenaline → beta receptor → Gs → adenylyl cyclase → cAMP → PKA → glycogen breakdown.
• cGMP: Formed from GTP by guanylyl cyclase; activates Protein Kinase G (PKG). NO activates soluble guanylyl cyclase → cGMP → smooth muscle relaxation. Sildenafil inhibits PDE5 (breaks down cGMP) → vasodilation.

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6. Clinical Pharmacology: Drug Derivatives of Nucleic Acids

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Summary Comparison Table

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Amino Acid Derivatives & Nucleic Acid Derivatives

📚 Easy Study Notes for Students

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FIRST: WHAT IS A "DERIVATIVE"?

Definition: A derivative is a compound that is made FROM another compound. So when we say "amino acid derivative," it means a biological molecule that is produced using an amino acid as the starting material.

• Amino acid = Raw material (like wheat)
• Derivative = Final product (like bread, cake, biscuit - all made from wheat!)

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PART 1: AMINO ACID DERIVATIVES

Definition of Amino Acid Derivatives: Biologically active molecules that are synthesized in the body using amino acids as precursors (starting materials). They include neurotransmitters, hormones, pigments, and other important compounds.

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GROUP 1 - FROM TYROSINE

Tyrosine is the "mother amino acid" - it makes 3 very important things!

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A) Catecholamines (Fight or Flight Chemicals)

Tyrosine → L-DOPA → Dopamine → Norepinephrine → Epinephrine

• ❌ Low dopamine in brain → Parkinson's disease (tremors, slow movement)
• 💊 Treatment: Give L-DOPA (it crosses blood-brain barrier; dopamine cannot)
• ❌ Too much dopamine → Schizophrenia

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B) Thyroid Hormones (T3 and T4)

• Function: Control body's metabolism (how fast you burn energy), growth, brain development
• ❌ Low T3/T4 → Hypothyroidism (fatigue, weight gain, cold intolerance)
• ❌ High T3/T4 → Hyperthyroidism (weight loss, sweating, fast heart rate)

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C) Melanin (Skin Pigment)

Tyrosine → DOPA → Melanin (by enzyme Tyrosinase)

• Function: Gives color to skin, hair, and eyes; protects from UV rays
• ❌ No tyrosinase → Albinism (no pigment, white skin and hair)

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GROUP 2 - FROM TRYPTOPHAN

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A) Serotonin (The "Happiness Chemical")

Tryptophan → 5-HTP → Serotonin (5-HT)

• Where found: 90% in gut cells, rest in brain and platelets
• Functions:
  • Controls mood (happiness, well-being)
  • Controls sleep
  • Helps with gut movement (peristalsis)
  • Helps in blood clotting (via platelets)
• ❌ Low serotonin → Depression
• 💊 SSRIs (e.g., fluoxetine/Prozac) = antidepressants that increase serotonin by blocking its reuptake

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B) Melatonin (The "Sleep Hormone")

Serotonin → Melatonin (in Pineal Gland, at night)

• Function: Tells the body "it's nighttime - time to sleep!" Controls the sleep-wake cycle (circadian rhythm)
• More melatonin in dark → sleepy; less in light → awake

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C) Niacin (Vitamin B3)

• Tryptophan can be converted to niacin (vitamin B3) when dietary niacin is low
• ❌ Deficiency of both tryptophan AND niacin → Pellagra
• Pellagra symptoms = 3 Ds: Dermatitis + Diarrhea + Dementia (and a 4th D = Death if severe)

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GROUP 3 - FROM HISTIDINE

Histamine (The "Allergy Chemical")

Histidine → Histamine (by Histidine Decarboxylase enzyme)

• Where found: Mast cells, basophils, stomach lining
• Functions:
  • Causes allergic reactions (itching, redness, swelling)
  • Increases blood vessel permeability (fluids leak out → swelling)
  • Stimulates stomach acid secretion
• 💊 H1 blockers (antihistamines like cetirizine) → for allergies
• 💊 H2 blockers (ranitidine, famotidine) → reduce stomach acid → for ulcers

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GROUP 4 - FROM GLUTAMATE

GABA (The "Brain's Brake Pedal")

GABA = Gamma-Aminobutyric Acid

Glutamate → GABA (by Glutamate Decarboxylase, needs Vitamin B6)

• Function: Main INHIBITORY neurotransmitter of the brain (calms the brain down)
• Think of GABA as the "brakes" of the brain
• ❌ Low GABA → Seizures / Epilepsy / Anxiety
• 💊 Benzodiazepines (diazepam) and Barbiturates enhance GABA action → calm brain → treat seizures and anxiety

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GROUP 5 - FROM GLYCINE

A) Creatine (Muscle Energy Store)

• Function: Stored in muscles as creatine phosphate → quickly regenerates ATP (energy) during intense exercise
• Creatine → breaks down to creatinine → excreted in urine
• 🩺 Serum creatinine = marker of kidney function (high creatinine = poor kidney function)

B) Heme (The Oxygen Carrier's Scaffold)

Glycine + Succinyl-CoA → ALA (delta-aminolevulinic acid) [Rate-limiting step of heme synthesis]

• Heme is the iron-containing part of hemoglobin
• ❌ Block in heme synthesis → Porphyrias (urine turns dark, abdominal pain, neurological symptoms)
• ❌ Lead poisoning inhibits ALA dehydratase → prevents heme synthesis → anemia

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GROUP 6 - FROM METHIONINE

SAM (S-Adenosylmethionine) - The "Methyl Donor"

Think of SAM as a "postman" who delivers methyl groups (-CH3) to different reactions in the body.

• SAM donates methyl groups to make:
  • Norepinephrine → Epinephrine
  • DNA methylation (gene regulation)
  • Creatine synthesis
  • Phosphatidylcholine (cell membrane component)

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GROUP 7 - FROM ARGININE

Nitric Oxide (NO) - The "Blood Vessel Relaxer"

Arginine → Citrulline + NO (by NO Synthase enzyme)

• Function: Relaxes blood vessel walls → blood vessels widen → vasodilation → lowers blood pressure
• 💊 Nitroglycerin (angina drug) → releases NO → dilates coronary vessels → relieves chest pain
• 💊 Sildenafil (Viagra) → prevents breakdown of cGMP (which NO activates) → vasodilation

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QUICK SUMMARY TABLE - AMINO ACID DERIVATIVES

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PART 2: NUCLEIC ACID DERIVATIVES

Definition of Nucleic Acid Derivatives: Molecules that are derived from or closely related to the building blocks of DNA and RNA (purines and pyrimidines). They include nucleosides, nucleotides, energy molecules, and signaling compounds.

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STEP 1 - UNDERSTAND THE BASIC BUILDING BLOCKS FIRST

Level 1 → Nitrogenous Base (just the base: Adenine, Guanine, Cytosine, Uracil, Thymine)
Level 2 → Nucleoside = Base + Sugar (ribose or deoxyribose)
Level 3 → Nucleotide = Nucleoside + Phosphate group(s)

Simple Memory Aid:

• "Side" in nucleoSIDE = sugar is on the SIDE (no phosphate yet)
• "tide" in nucleoTIDE = phosphaTIDE is added!

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STEP 2 - TWO TYPES OF BASES

Purines (Double ring - BIGGER)

• Adenine and Guanine
• Memory trick: "PURe As Gold" = Purines = Adenine + Guanine

Pyrimidines (Single ring - SMALLER)

• Cytosine, Uracil, Thymine
• Memory trick: "CUT the PY" = Pyrimidines = Cytosine + Uracil + Thymine

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STEP 3 - IMPORTANT NUCLEOTIDE DERIVATIVES & THEIR FUNCTIONS

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1. ATP (Adenosine Triphosphate) - "Energy Currency"

Definition: ATP is a nucleotide with 3 phosphate groups. It is the universal energy currency of all living cells.

• When ATP loses one phosphate → ADP + energy released
• When ATP loses two phosphates → AMP + more energy
• Every process in the body needs ATP: muscle contraction, nerve signals, biosynthesis

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2. cAMP (Cyclic AMP) - "Second Messenger"

Definition: cAMP is a cyclic form of AMP, made from ATP by the enzyme adenylyl cyclase. It acts as a second messenger to transmit hormone signals inside the cell.

Hormone (adrenaline) → binds receptor → activates adenylyl cyclase → ATP → cAMP → activates Protein Kinase A (PKA) → cellular response

• Example: Adrenaline signals liver to break down glycogen (stored sugar) → release glucose into blood - all through cAMP
• 💊 Theophylline (asthma drug) inhibits phosphodiesterase → prevents cAMP breakdown → bronchodilation

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3. cGMP (Cyclic GMP) - "Second Messenger for Vasodilation"

Definition: Cyclic form of GMP, made from GTP by guanylyl cyclase. Activated mainly by Nitric Oxide (NO). Causes smooth muscle relaxation.

• NO → activates guanylyl cyclase → GTP → cGMP → relaxes blood vessel smooth muscle
• 💊 Sildenafil (Viagra) blocks PDE5 (enzyme that breaks down cGMP) → more cGMP → prolonged vasodilation

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4. NAD+ / NADH - "Electron Carrier"

Definition: NAD+ (Nicotinamide Adenine Dinucleotide) is a coenzyme derived from AMP + nicotinamide. It carries electrons (as NADH) in cellular respiration.

• NAD+ accepts electrons → becomes NADH (like a rechargeable battery getting charged)
• NADH gives electrons to ETC (mitochondria) → makes ATP (battery discharges to power the cell)
• Used in: Glycolysis, TCA cycle, Beta-oxidation of fatty acids

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5. FAD / FADH2 - "Another Electron Carrier"

Definition: FAD (Flavin Adenine Dinucleotide) is a coenzyme derived from AMP + riboflavin (Vitamin B2). Carries electrons in the TCA cycle and fatty acid oxidation.

• Similar to NAD+ but carries 2 electrons (NAD+ carries 1)
• ❌ Riboflavin (B2) deficiency → FAD and FMN deficiency → problems with energy production

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6. CoA (Coenzyme A) - "Acyl Group Carrier"

Definition: CoA is derived from AMP + pantothenic acid (Vitamin B5) + cysteine. It activates fatty acids and acetyl groups for metabolism.

• Acetyl-CoA is the key molecule entering the TCA cycle
• Used in: Fatty acid synthesis, fatty acid oxidation, amino acid metabolism
• ❌ Pantothenate (B5) deficiency → CoA deficiency → metabolic problems

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7. GTP (Guanosine Triphosphate)

Definition: GTP is a nucleotide like ATP but with guanine instead of adenine. It is the energy source for protein synthesis and G-protein signaling.

• Used in: Protein synthesis (ribosome needs GTP), signal transduction (G-proteins use GTP)
• G-proteins are "molecular switches" in cell signaling - they are active when bound to GTP

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STEP 4 - PURINE DEGRADATION AND GOUT

What happens to purines when they are broken down?

AMP → IMP → Inosine → Hypoxanthine → Xanthine → URIC ACID
GMP → Guanosine → Guanine → Xanthine → URIC ACID

• Uric acid = the final waste product of purine breakdown in humans
• Normally excreted in urine
• ❌ If uric acid builds up → crystals deposit in joints → GOUT (very painful joint inflammation, especially big toe)

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STEP 5 - LESCH-NYHAN SYNDROME (Important Exam Topic!)

Definition: A genetic disorder caused by deficiency of the enzyme HGPRT (Hypoxanthine-Guanine PhosphoRibosylTransferase), which is needed for the purine salvage pathway.

• Purines cannot be recycled (salvaged)
• Purines are all degraded → very high uric acid (hyperuricemia)
• Accumulation in the brain → neurological damage

• Severe gout
• Self-mutilation (the child bites their own fingers and lips - classic finding!)
• Intellectual disability
• Cerebral palsy-like movements
• X-linked recessive (affects mainly boys)

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STEP 6 - NUCLEOTIDE ANALOGUES AS DRUGS

Many drugs are designed to look like nucleotides but act as "fake" building blocks that block DNA synthesis.

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QUICK SUMMARY TABLE - NUCLEIC ACID DERIVATIVES

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FINAL COMPARISON: AMINO ACID vs. NUCLEIC ACID DERIVATIVES

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Golden Rule for Exams:

• Amino acid derivatives = mostly SIGNALING molecules (tell cells what to do)
• Nucleic acid derivatives = mostly ENERGY & INFORMATION molecules (power the cell and carry genetic info)