Bioenergetics The study 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 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°’) 25 o C, 1M each component, pH 7, H2O at 55.6M

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°’ = * 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

Glutamic acid + NH3 --> H2O + Glutamine  G°’=+3.4 kcal/mol Coupling endergonic and exergonic rxns + NH3  H2O + The Problem: Many biologically important reactions are endergonic H

ATP hydrolysis is a highly exergonic reaction Frequently coupled to otherwise endergonic reactions Coupling endergonic and exergonic rxns

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

Equilibrium vs steady state Cells are open systems, not closed systems –O2 enters, CO2 leaves –Allows maintenance of reactions at conditions far from equilibrium O2O2

Biological Catalysts

1)Req’d in small amounts 2)Not altered/consumed in rxn 3)No effect on thermodynamics of rxn a)Do not supply E b)Do not determine [product]/[reactant] ratio (Keq) c)Do accelerate rate of reaction (kinetics) 4)Highly specific for substrate/reactant 5)Very few side reactions (i.e. very “clean”) 6)Subject to regulation No relationship between  G and rate of a reaction (kinetics) Biological Catalysts Why might a favorable rxn NOT occur rapidly?

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

Overcoming the activation energy barrier (E A ) Catalyst shifts the dotted line to the left

How enzymes lower E A The curve peak is the transition state (TS) Enzymes bind more tightly to TS than to either reactants or products

How enzymes lower E A 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

How enzymes lower E A 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

Covalent intermediates

How enzymes lower E A 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

Induced fit

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) Enzyme kinetics: The Michaelis-Menten Equation

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

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

Reversible Enzyme inhibitors: noncompetitive 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 Example: anandamide (endogenous cannabinoid) –Inhibits 5-HT3 serotonin receptors that normally increase anxiety