Biochemistry I

10-29-01

Source of all energy for cells is Free Energy (G)

• Can’t be heat energy because cells are isothermic (constant temperature)
• Allows us to predict the
• direction of chemical reactions because directly related to Keq
• the exact equilibrium position
• the amount of work they can perform at a constant temperature
• In heterotrophic cells, the source of energy is nutrients
• In autotrophic cells the source is solar radiation

Relationship between DG’^o and Keq

• For the reaction aA + bB à cC + dD, Keq = [C]^c[D]^d/[A]^a[B]^b
• DG’^o = -RT ln Keq is the standard free energy change
• standard state is pH 7, 55.5 M H₂0, 25 ^oC (298 K), reactanats and products are initially at 1 M concentration and gas at 1 atm.
• DG’^o is the difference between the free-energy content of the products and the free-energy content of the reactants under standard conditions

When Keq is	DG’^o is	Starting with 1 M components the rxn
> 1.0	Negative	Proceeds forward
1.0	Zero	Is at equilibrium
< 1.0	Positive	Proceeds in reverse

Actual Free-Energy Changes Depend on Reactant and Product Concentrations

• DG =DG’^o + RT ln ([C][D]/[A][B]) (constants actually prevailing in the system)
• DG is a function of actual reactant and product concentrations and the temperature prevailing during the reaction, which does not necessarily match the standard conditions described above
• can get a reaction to go forward even if it has a positive DG’^o, as long as the remaining terms are very negative (e.g. product concentration gets very low (removed quickly), that will produce a negative number in the second term)
• Remember the free energy change for a reaction is independent of the pathway by which the reaction occurs the free energy depends only on the nature and concentration of the initial reactants and the final products i.e. it doesn’t matter if an enzyme is catalyzing the reaction and reduces the activation energy, the free energy is still the same
• see overhead

Standard Free-Energy Changes are Additive

• The DG’^o values of sequential chemical reactions are additive
• AàB and BàC, the overall reaction AàC has a DG’^o_total=DG’^o₁ + DG’^o₂
• Explains why we couple reactions
• Glucose + P_i à glucose 6-phosphate + H₂O DG’^o = 13.8 kJ/mol
• ATP + H₂O à ADP + P_i DG’^o = -30.5 kJ/mol
• ATP + glucose à ADP + glucose-6-phosphate DG’^o = -16.7 kJ/mol
• Energy stored in the bonds of ATP is used to drive the synthesis of glucose-6-phosphate
• The pathway is different in the reaction in which there is phosphoryl group transfer from ATP to glucose, but the net result is the same as the sum of the two reactions
• Keq’s are multiplicative
• Keq1 = [glucose-6-phosphate]/[glucose][Pi] = 3.9 x 10 –3 M-1
• Keq2 = [ADP][Pi]/[ATP] = 2 X 10 5 M
• Keqtotal = [glucose-6-phosphate][ADP][Pi]/[glucose][Pi][ATP] = Keq1*Keq2 = 7.8 x 102
• Plug into standard free energy equation and get -16.5 kJ/mol

Chemical Basis of Large Free-Energy Change associated with ATP Hydrolysis

· Hydrolytic cleavage of terminal phosphoric acid anhydride, separate one of the three negatively charged phosphates and relives some fo the electrostatic repulsion in ATP

· Pi (HPO₄^2-) is stabilized by formation of several resonance forms not possible for ATP

· ADP immediately releases H+ into medium with very low [H+], so

· Since concentration of ATP hydrolysis products are far below concentration at equilibrium, the reaction is highly favored

· ATP is relatively stable at pH 7 because it has a high E_A

· Actual substrate is Mg ATP^2-

ATP Provides energy by group transfers, not by simple hydrolysis--overhead

• Often reactions involving ATP will be written as a single reaction showing hydrolysis to ADP and Piàall this would do is liberate heatàwon’t help the reaction along much
• This is not what is happening, usually you have activation of the compound (group transfer) which serves to couple the reactions
• A phosphate is usually covalently attached to the substrate temporarily (usually a SN2 nucleophilic substitution reaction)
• Therefore adding a phosphate increases the free energy of a compound and activates it for reaction (will make the reaction proceed downhill)
• Can get direct hydrolysis where the energy goes to changing the conformation of a protein, e.g. actin and myosin in muscle contraction
• Also can work in reverse, if we have a reaction which has a large negative free energy change, can use this energy to make ATP
• PEP + H20 à pyruvate + Pi (-61.9 kJ/mol)
• ADP + Pi à ATP + H2O (30.5 kJ/mol)
• PEP + ADP à pyruvate + ATP (-31.4 kJ/mol)
• Product of reaction depends on nucleophile and which P is attacked (see overhead)

Other Phosphorylated compounds and thioesters have large free energies of hydrolysis—

see overhead

• A high energy phosphate bond does not relate to the amount of energy necessarily stored within the bond (always need to input energy to break it)
• Real source of energy does not come from breaking of a specific bond, but instead from the products of the reaction having a a smaller free-energy content than the reactants
• Hydrolysis reactions with large, negative, standard free energy changes, the products are more stable than reactants for one of 4 reasons
• Bond strain in reactants due to electrostatic repulsion is relieved by separation
• Products are stabilized by ionization
• Products are stabilized by isomerization
• Products are stabilized by resonance
• Several examples in book—you can look at those yourselves

Thioesters and acetyl-coenzyme A

• Have large, negative standard free energies of hydrolysis
• Acetyl Co-A is an example
• Acyl group activated for
• Transacylation
• Condensation
• Oxidation-reduction rxns
• Thioesters undergo much less resonance stabilization as oxygen esters, so again the products are stabilized relative to the reactants—see overhead