قناة سنة ثانية بشري (دفعة 53)

Classification of Amino Acids 1. Chemical Classification a. By polarity of side chain • Nonpolar (hydrophobic): Valine, leucine, isoleucine, methionine, etc. • Do not form H-bonds; cluster inside proteins in aqueous solution. • Methionine contains sulfur; cysteine forms disulfide bonds (-S-S-) but is polar. • Polar uncharged (hydrophilic): Serine, threonine, tyrosine, asparagine, glutamine • Form H-bonds; interact with carbohydrates and phosphates. • Acidic (negatively charged): Aspartate, glutamate • Basic (positively charged): Lysine, arginine (strongly basic), histidine (weakly basic, depends on environment) b. Aromatic, heterocyclic, aliphatic: • Aromatic: Phenylalanine, tyrosine • Heterocyclic: Tryptophan (indole), histidine (imidazole), hydroxyproline • Aliphatic: Remaining amino acids without rings c. Special groups: • Sulfur-containing: Methionine, cysteine • Hydroxyl-containing: Serine, threonine, tyrosine ⸻ 2. Nutritional Classification • Essential (cannot synthesize): PVTIMHALL (Phenylalanine, Valine, Threonine, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine, Tryptophan) • Semi-essential: Arginine, histidine (enough for adults, not children) • Non-essential: Synthesized in the body, e.g., tyrosine from phenylalanine, cysteine from methionine ⸻ 3. Metabolic Classification • Ketogenic: Produce ketone bodies (leucine, lysine): • Glucogenic: Produce glucose (most amino acids) • Mixed (glucogenic & ketogenic): Phenylalanine, tyrosine, tryptophan, isoleucine ⸻ 4. Non-standard Amino Acids: • Non-protein amino acids: Ornithine, citrulline, homoserine, GABA, histamine, DOPA, etc. • D-amino acids: Found in bacterial walls (D-glutamate, D-alanine) • Modified amino acids (post-translational): Hydroxyproline, phosphoserine, N-formyl methionine, methylated residues, desmosine

Chemistry of Amino Acids and Proteins Biomedical Importance • Amino acids are small molecules with an amino (-NH₂) and carboxyl (-COOH) group attached to the α-carbon. • Proline contains an imino group instead of an amino group. • Functions of amino acids in the body: 1. Structural, hormonal, and catalytic roles as part of proteins. 2. Neuroendocrine functions: L-α-amino acids serve as hormones, neurotransmitters, or neuromodulators. 3. Intracellular roles: nerve transmission, regulation, phosphorylation. 4. Nutritional: Humans cannot synthesize 10 essential amino acids; diet must provide them. 5. Metabolic roles: • Ornithine, citrulline, argininosuccinate → urea synthesis • Tyrosine → thyroid hormones • Glutamate → neurotransmitters • Serine, glycine → lipids and bile salts • Amino acids participate in purine, pyrimidine, and porphyrin biosynthesis

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16. Ketone Bodies • Acetoacetate • β-hydroxybutyrate • Acetone Clinical • Produced during fasting & diabetes • Serve as alternative energy source 17. Eicosanoids Derived from arachidonic acid: • Prostaglandins • Prostacyclins • Thromboxanes • Leukotrienes Functions • Vasodilation/constriction • Platelet aggregation • Inflammation • Smooth muscle contraction

14. Cholesterol • Animal origin only • Synthesized in liver • Normal level: 150–240 mg/dL Functions • Cell membrane structure • Steroid hormones • Vitamin D • Bile acids & salts Clinical Significance • High cholesterol → Atherosclerosis & CAD 15. Bile Acids & Bile Salts • Derived from cholesterol • Aid fat digestion • Absorption of fat-soluble vitamins • Enterohepatic circulation

11. Compound Lipids A. Phospholipids • Contain phosphoric acid • Types: • Glycerophospholipids • Sphingophospholipids Phosphatidyl Inositol (PI) • Reservoir of arachidonic acid • Forms PIP₂ • Second messenger system: • PIP₂ → DAG + IP₃ • IP₃ releases Ca²⁺ • DAG activates protein kinase C Lecithin (Phosphatidylcholine) Functions • Major membrane phospholipid • Prevents fatty liver • Surfactant in lungs • Choline storage Clinical • Deficiency → Respiratory Distress Syndrome (RDS) 12. Sphingolipids • Based on sphingosine • Ceramide = sphingosine + fatty acid Types • Sphingomyelin • Cerebrosides • Gangliosides Diseases • Niemann-Pick disease (↓ sphingomyelinase) • Tay-Sachs disease (↓ hexosaminidase) 13. Lipoproteins • Transport lipids in plasma • Types: • Chylomicrons • VLDL • LDL • HDL

10. Branched Chain Fatty Acids • Example: Phytanic acid • Refsum’s disease: • Defect in α-oxidation • Accumulation of phytanic acid • Neurological symptoms • Treated by dietary restriction