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) TS

• Gibbs Free Energy: G = H - TS– 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

Review from ChemistryA + 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 forwardIf 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

O2

Biological Catalysts

1) Req’d in small amounts2) Not altered/consumed in rxn3) No effect on thermodynamics of rxn

a) Do not supply Eb) Do not determine [product]/[reactant]

ratio (Keq)c) Do accelerate rate of reaction (kinetics)

4) Highly specific for substrate/reactant5) 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 (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

Overcoming the activation energy barrier (EA)

Catalyst shifts the dotted lineto the left

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

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

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

Covalent intermediates

Covalent intermediates

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

Induced fit

Induced fit

S <--> PAt low [S], velocity (rate) is slow, idle time on the enzymeAt 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 synthesispenicillin

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)

NoncompetitiveCompetitive

• Example: anandamide (endogenous cannabinoid)– Inhibits 5-HT3 serotonin receptors that normally

increase anxiety