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Biochimica Generale
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- Enzymology:
- Catalysis: Enzymes accelerate chemical reactions without being consumed, reaching equilibrium faster and lowering activation energy.
- Properties: Stereospecificity, regulation (inhibitors, inducers), importance in metabolism. Activity measured in 'catal' or 'U/UI'.
- Binding site: Organized space recognizing substrates; catalytic triad (Ser, His, Asp) in serine proteases (Chymotrypsin, Trypsin, Elastase).
- Cofactors: Essential molecules for enzyme function, derived from diet (metallic ions like Zn+2, Cu+2; organic coenzymes like Cobalamin (Vit B12), Thiamine pyrophosphate (Vit B1), Pyridoxal phosphate (Vit B6), FAD/FMN (Vit B2), NAD(P) (Vit B3), Coenzyme A (Vit B5)).
- Inhibitors: Reversible (competitive, non-competitive, mixed) or irreversible (covalent binding). Discussed in context of antibacterials (Penicillins targeting transpeptidases, Clavulanic acid inhibiting beta-lactamases), antivirals (Tamiflu targeting neuraminidase), and respiratory poisons (organophosphates targeting cholinesterase).
- NSAIDs: Inhibit cyclooxygenases (Cox1/Cox2) involved in prostaglandin/thromboxane synthesis, impacting inflammation, gastroprotection, renal flow, and coagulation. Aspirin is an irreversible Cox inhibitor.
- Enzyme Kinetics: Michaelis-Menten constants (Km for affinity, kcat for turnover number/catalytic power). Km varies with substrate; kcat is max conversion rate when saturated. Regulatory enzymes often show sigmoidal kinetics.
- Classification: EC system with 6 main classes (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases, and Translocases).
- Diagnostic Enzymes: Used for diagnosing congenital diseases, non-genetic conditions, and prognosis. Measured in blood, urine, saliva. Examples include ALT, AST, LDH, CK, ALP, AMY, LIP, TRY, GGT, PchE. Isoenzymes help localize damage (e.g., CK-MB for heart, LDH1-5 for various tissues).
- Metabolism:
- General: Interconnected catabolic (degradative, energy-producing) and anabolic (biosynthetic, energy-consuming) pathways.
- Energy currency: ATP and reducing equivalents (NADPH). Exergonic processes release energy, endergonic require it.
- Glycolysis: Universal, cytosolic pathway (glucose to 2 pyruvate, net 2 ATP, 2 NADH). Operates aerobically or anaerobically (producing lactate, regenerating NAD+ via LDH). Irreversible steps (Hexokinase, PFK1, Pyruvate Kinase) are control points.
- Krebs Cycle (TCA Cycle): Mitochondrial, aerobic pathway. Oxidizes Acetyl-CoA to CO2, producing NADH, FADH2, GTP. Central for energy production from carbs, fatty acids, amino acids. Anaplerotic reactions feed intermediates.
- Oxidative Phosphorylation: Final energy pathway in mitochondria. Electrons from NADH/FADH2 flow through ETC (Complexes I-IV) to O2, pumping protons to create an electrochemical gradient. ATP Synthase (Complex V) uses this gradient to produce ATP. Yield: 2.5 ATP/NADH, 1.5 ATP/FADH2.
- Beta-oxidation: Mitochondrial pathway. Degrades fatty acids (Acyl-CoA) into Acetyl-CoA, NADH, FADH2, releasing significant energy. Requires carnitine shuttle to transport Acyl-CoA into mitochondria. Malonyl-CoA inhibits CAT1 (rate-limiting step).
- Pentose Phosphate Pathway: Cytosolic. Produces NADPH (for reductive biosyntheses and antioxidant defense) and pentose sugars (for nucleotide synthesis). Crucial for red blood cell integrity (G6PDH deficiency leads to hemolytic anemia).
- Ketogenesis: Liver pathway during fasting/energy deficit. Acetyl-CoA converted to ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone). Ketone bodies serve as alternative fuel for peripheral tissues (e.g., brain, muscle) but cause ketoacidosis if uncontrolled. Liver cannot utilize ketone bodies (lacks thiophorase).
- Lipogenesis: Synthesis of fatty acids and triglycerides, mainly in liver and adipose tissue, from excess glucose and amino acids. Requires Acetyl-CoA (transferred from mitochondria to cytoplasm via citrate shuttle) and NADPH. Glycerol-3-phosphate is derived from glucose.
- Amino Acid Catabolism: Amino acids degraded for energy or other molecules. Transamination (transaminases, PLP-dependent) forms alpha-ketoacids. Oxidative deamination (glutamate dehydrogenase) releases ammonia. Urea cycle detoxifies ammonia to urea in the liver. Bilancio dell'azoto (nitrogen balance) must be maintained.
- Regulation & Signaling:
- Allosteric Control: Ligand-mediated changes in enzyme conformation (R/T states) influencing activity. Examples: Aspartate transcarbamoylase in pyrimidine synthesis.
- Covalent Modification: Reversible phosphorylation/dephosphorylation (kinases add phosphate, phosphatases remove it) of enzymes, typically on Ser, Thr, or Tyr residues. Regulated by specific sequences.
- Proteolytic Cleavage: Irreversible activation of zymogens (inactive precursors) by peptide bond cleavage. Example: pancreatic digestive enzymes (trypsinogen to trypsin).
- Signal Transduction Systems: Ligands (hormones, local mediators, neurotransmitters) bind to receptors (GPCRs, enzyme-linked receptors, ligand-gated ion channels). Signal amplified and transduced via second messengers (cAMP, PIP3, DAG, Ca2+).
- Glucose Homeostasis: Insulin (hyperglycemia) and glucagon (hypoglycemia) regulate glucose metabolism in liver, muscle, adipose tissue. Insulin promotes glycogen synthesis, glucose uptake (GLUT4), protein synthesis. Glucagon promotes glycogenolysis and gluconeogenesis.
- Cori Cycle: Metabolic cooperation between muscle/RBCs (anaerobic glycolysis producing lactate) and liver (gluconeogenesis from lactate). Liver provides glucose, muscle/RBCs provide lactate for regeneration.
- Alanine-Glucose Cycle: Muscle-liver cycle. Muscle produces alanine from pyruvate (using amino acids from protein catabolism) which the liver uses for gluconeogenesis, and then returns glucose to the muscle. Avoids lactate accumulation and acidification.
- Transport Systems:
- Membrane Transport: Molecules cross membranes based on size, polarity, hydrophobicity. Simple diffusion for small nonpolar, facilitated diffusion (transporters for glucose via GLUT proteins), active transport (primary ATP-dependent pumps for ions, secondary co-transporters).
- Ion Gradients: Maintained by ATP-dependent pumps (Na+/K+ ATPase) and channels, essential for membrane potential and signaling (Ca2+ as second messenger).
- Shuttle Systems: For transferring molecules across impermeable membranes (e.g., carnitine shuttle for acyl-CoA into mitochondria, malate-aspartate and glycerol-3-phosphate shuttles for cytoplasmic NADH to mitochondria, citrate shuttle for Acetyl-CoA from mitochondria to cytoplasm).