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Download Outline05 Enzymes - Napa Valley College
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Biology 219 – Human Physiology Clemens Metabolism, Energy and Enzymes Ch. 4 A. Metabolism total of all chemical reactions in the body catabolic reactions - break down large molecules into smaller ones anabolic reactions - build larger molecules from smaller ones (synthesis) exergonic reactions release energy endergonic reactions require energy Major classes of metabolic reactions: 1. Hydrolysis and Dehydration (Condensation) hydrolysis - adds H2O to break bonds between monomer units, catabolic A–B + H2O → A–OH + H–B e.g., sucrose + H2O → glucose + fructose dehydration (condensation) - removes H2O to join monomers, anabolic A–OH + H–B → A–B + H2O e.g., peptide bond formation: amino acid1 + amino acid2 → dipeptide + H2O 2. Phosphorylation and Dephosphorylation phosphorylation – adds a phosphate group (Pi) C + Pi → C–P + H2O e.g., ATP synthesis: ADP + Pi + energy → ATP + H2O Transfer of phosphate from ATP: C + ATP → C–P + ADP; catalyzed by kinase enzymes dephosphorylation – removes a phosphate group C–P + H2O → C + Pi e.g., ATP hydrolysis: ATP + H2O → ADP + Pi + energy 3. Oxidation-Reduction (Redox) Reactions electron transfer reactions: Oxidation Is Loss, Reduction Is Gain of electrons - major energy source for cells: oxidation of sugars, fatty acids, and amino acids - redox reactions are coupled: one molecule is oxidized, another is reduced - redox reactions in cells often involve transfer of H atoms (not H+ ions) e.g., reduction of pyruvate to lactate: C=O + 2 H → H-C-OH - coenzymes are temporary carriers of H atoms and their electrons: NAD+ + 2 H → NADH + H+ oxidized reduced FAD + 2 H → FADH2 oxidized reduced e.g., pyruvate + NADH + H+ → lactate + NAD+ - oxygen (O2) is the final electron acceptor in cellular respiration: ½ O2+ 2 H → H2O B. Energy Metabolism - cells use chemical energy to do biological work: movement, synthesis, transport - energy is released in exergonic reactions that convert high-energy to lower-energy molecules e.g., oxidation of glucose C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (high energy) (lower energy) - ATP is the “energy currency” of cells energy captured from oxidation of substrates is used to make ATP (ADP + Pi → ATP) energy released from ATP hydrolysis powers energy-requiring processes Biology 219 – Human Physiology Clemens C. Enzymes biochemical catalysts – greatly speed up chemical reactions - most enzymes are proteins - increase reaction rate by lowering activation energy or provide an alternative chemical pathway for the reaction Rate Functional properties of enzymes: 1. Substrate specificity - substrate (reactant) binds reversibly to the active site of the enzyme - specific fit between substrate and active site (shape and charge/polarity) - induced fit: enzyme pulls on chemical bonds of the substrates 2. Sensitivity to temperature and pH - effects on tertiary structure of proteins 3. Saturation kinetics - reaction rate depends on substrate concentration (non-linear): saturation at low [S], reaction rate increases with increasing [S] at high [S], reaction rate reaches a maximum level - at the saturation point, all active sites are occupied - maximum rate is limited by number of available enzymes [Substrate] 4. Regulation a. covalent regulation - regulation of enzyme via covalent binding of a chemical group - usually involves addition of phosphate group which activates the enzyme E (inactive enzyme) + ATP → E~P (active enzyme) + ADP - protein kinase enzymes catalyze phosphorylation of other enzymes; important in cell signaling b. allosteric regulation - regulation by non-covalent binding of a modulator to a regulatory site on the enzyme - can be either allosteric activation or allosteric inhibition - reaction rate depends on concentration of the modulator as well as the substrate c. feedback inhibition - regulation of a multi-step metabolic pathway - a product of the reaction pathway acts as a modulator that inhibits an enzyme in an earlier step (via allosteric inhibition) to regulate formation of products enzyme 1 enzyme 2 A X → B → C more enzymes → → → Z Examples of enzymes: catalase 2 H2O2 → 2 H2O + O2 carbonic anhydrase H2O + CO2 H2CO3 H+ + HCO3hexokinase glucose + ATP → glucose-6-phosphate + ADP (first step in glycolysis) phosphofructokinase (PFK) - key regulatory enzyme in glycolysis; feedback inhibition by ATP Clinical applications: 1. abnormal enzyme levels may indicate disease (e.g., liver enzymes) 2. genetic deficiency of an enzyme → inborn errors of metabolism (e.g., PKU)