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Bioenergetics

Bioenergetics. The s tudy of energy transformations in living organisms. Review from Chemistry. Thermodynamics 1st Law: Conservation of Energy (E) Neither created nor destroyed, but can be transformed into different states 2nd Law: Events proceed from higher to lower E states

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Bioenergetics

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  1. Bioenergetics • The study of energy transformations in living organisms

  2. Review from Chemistry • Thermodynamics • 1st Law: Conservation of Energy (E) • Neither created nor destroyed, but can be transformed into different states • 2nd Law: Events proceed from higher to lower E states • Entropy (disorder) always increases • Universe = system + surroundings (E content of system) H = (useful free E) G + (E lost to disorder) TS • Gibbs Free Energy: G = H - TS • If G = negative, then rxn is exergonic, spontaneous • If G = positive, then rxn is endergonic, not spontaneous • Standard conditions (ΔG°’) • 25oC, 1M each component, pH 7, H2O at 55.6M

  3. Review from Chemistry A + B <--> C + D • Rate of reaction is directly proportional to concentration of reactants • At equilibrium, forward reaction = backward reaction k1[A][B] = k2[C][D] • Rearrange: k1/k2 = ([C][D])/([A][B]) = Keq • Relationship between ΔG°’ and K’eq is: G°’ = -2.303 * R * T * log K’eq If K’eq >1, G°’ is negative, rxn will go forward If K’eq <1, G°’ is positive, rxn will go backward

  4. Coupling endergonic and exergonicrxns • The Problem: Many biologically important reactions are endergonic Glutamicacid + NH3 --> H2O + Glutamine G°’=+3.4 kcal/mol + NH3  H2O + H

  5. Coupling endergonic and exergonicrxns • ATP hydrolysis is a highly exergonic reaction • Frequently coupled to otherwise endergonic reactions

  6. Coupling endergonic and exergonic rxns • Partial reactions: Glutamicacid + NH3 --> H2O + Glutamine G°’=+3.4 kcal/mol ATP --> ADP + Pi G°’=-7.3 kcal/mol ---------------------------------------------------------------------------------------- Glu + ATP + NH3 --> Gln + ADP + Pi G°’=-3.9 kcal/mol + ATP   + ADP + Pi + NH3 Glutamyl phosphate is the common intermediate

  7. Equilibrium vs steady state • Cells are open systems, not closed systems • O2 enters, CO2 leaves • Allows maintenance of reactions at conditions far from equilibrium O2

  8. Biological Catalysts

  9. Req’d in small amounts • Not altered/consumed in rxn • No effect on thermodynamics of rxn • Do not supply E • Do not determine [product]/[reactant] ratio (Keq) • Do accelerate rate of reaction (kinetics) • Highly specific for substrate/reactant • Very few side reactions (i.e. very “clean”) • Subject to regulation No relationship between G and rate of a reaction (kinetics) Biological Catalysts Why might a favorable rxnNOT occur rapidly?

  10. Overcoming the activation energy barrier (EA) • Bunsen burner: CH4 + 2O2 --> CO2 + 2H2O • The spark adds enough E to exceed EA (not a catalyst) • Metabolism ‘burning’ glucose • Enzyme lowers EA so that ambient fluctuations in E are sufficient

  11. Overcoming the activation energy barrier (EA) Catalyst shifts the dotted line to the left

  12. How enzymes lower EA • The curve peak is the transition state (TS) • Enzymes bind more tightly to TS than to either reactants or products

  13. How enzymes lower EA • Mechanism: form an Enzyme-Substrate (ES) complex at active site • Orient substrates properly for reaction to occur • Increase local concentration • Decrease potential for unwanted side reactions

  14. How enzymes lower EA • Mechanism: form an Enzyme-Substrate (ES) complex at active site • Enhance substrate reactivity • Enhance polarity of bonds via interaction with amino acid functional groups • Possibly form covalent bonded intermediates with amino acid side chains

  15. Covalent intermediates

  16. Covalent intermediates

  17. How enzymes lower EA • Mechanism: form an Enzyme-Substrate (ES) complex at active site • Induce bond strain • Alter bonding angles within substrate upon binding • Alter positions of atoms in enzyme too: Induced fit

  18. Induced fit

  19. Induced fit

  20. Enzyme kinetics: The Michaelis-Menten Equation S <--> P At low [S], velocity (rate)is slow, idle time on the enzyme At high [S], velocity (rate) is maximum (Vmax), enzyme is saturated V = Vmax [S]/([S] + Km) Km = [S] at Vmax/2 A low Km indicates high enzyme affinity for S (0.1mM is typical)

  21. Irreversible Enzyme Inhibitors • Form a covalent bond to an amino acid side chain of the enzyme active site • Example: penicillin • Inhibits Transpeptidase enzyme required for bacterial cell wall synthesis penicillin

  22. Reversible Enzyme inhibitors: competitive • Example: ritonavir • Inhibits HIV protease ability to process virus proteins to mature forms • Bind at active site • Steric block to substrate binding • Km increased • Vmax not affected (increase [S] can overcome)

  23. Reversible Enzyme inhibitors: noncompetitive • Example: anandamide (endogenous cannabinoid) • Inhibits 5-HT3 serotonin receptors that normally • increase anxiety • Do not bind at active site • Bind a distinct site and alter enzyme structure reducing catalysis • Km not affected • Vmax decreased, (increase [S] cannot overcome) Noncompetitive Competitive

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