Biochemical Energetics

Molecular Biochemistry I

http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/bioener.htm

Contents of this page:
Free energy changes & equilibrium constants
Energy coupling
"High energy" bonds
Thermodynamics vs kinetics
Introduction to oxidation/reduction

The free energy change (DG) of a reaction determines its spontaneity. The free energy change (DG), and its relation to equilibrium constant, are discussed on p. 57-59 of Biochemistry 3rd Edition by Voet & Voet. A reaction is spontaneous if DG is negative (if the free energy of the products is less than the free energy of the reactants).

DG = change in free energy, DG^o'= standard free energy change (with 1 M reactants and products, at pH 7), R = gas constant, T = absolute temperature.

Note that the standard free energy change (DG^o') of a reaction may be positive, for example, and the actual free energy change (DG) negative, depending on cellular concentrations of reactants and products. Many reactions for which DG^o' is positive are spontaneous because other reactions cause depletion of products or maintenance of high substrate concentrations.

At equilibrium, DGequals zero.Solving forDG^o'yields the relationship at left. K'eq, the ratio [C][D]/[A][B] at equilibrium, is called the equilibrium constant. An equilibrium constant greater than one(more products than reactants at equilibrium) indicates aspontaneous reaction (negative DG°').

The variation of equilibrium constant with DG^o' is shown in the table below.

Keq	DG^o' (kJ/mol)	Starting with 1 M reactants and products, the reaction:
10^4	– 23	proceeds forward (spontaneous)
10^2	– 11	proceeds forward (spontaneous)
10^0 = 1		is at equilibrium
10^–2	+ 11	proceeds in reverse
10^–4	+ 23	proceeds in reverse

Energy coupling is discussed on p. 59-60 & 566-567.

• A spontaneous reaction may drive a non-spontaneous reaction.
• Free energy changes of coupled reactions are additive.

Examples of different types of coupling:

A. Some enzyme-catalyzed reactions are interpretable as two coupled half-reactions, one spontaneous and the other non-spontaneous. At the enzyme active site, the coupled reaction is kinetically facilitated, while the individual half-reactions are prevented. The free energy changes of the half-reactions may be summed, to yield the free energy of the coupled reaction.

For example, in the reaction catalyzed by the Glycolysis enzyme Hexokinase, the two half-reactions are:

• ATP + H₂O « ADP + P_i.................. DG^o' = -31 kJoules/mol
• P_i + glucose « glucose-6-P + H₂O... DG^o' = +14 kJoules/mol

Coupled reaction: ATP + glucose « ADP + glucose-6-P.. DG^o' = -17 kJoules/mol
The structure of the enzyme active site, from which water is excluded, prevents the individual hydrolytic reactions, while favoring the coupled reaction.

B. Two separate enzyme-catalyzed reactions occurring in the same cellular compartment, one spontaneous and the other non-spontaneous, may be coupled by a common intermediate (reactant or product).

A hypothetical, but typical, example involving pyrophosphate:

• enzyme 1: A + ATP « B + AMP + PP_i....DG^o' = +15 kJ/mol
• enzyme 2: PP_i + H₂O « 2 P_i....................DG^o' = –33 kJ/mol

Overall: A + ATP + H₂O « B + AMP + 2P_i... DG^o' = –18 kJ/mol

Pyrophosphate (PP_i) is often the product of a reaction that needs a driving force. Its spontaneous hydrolysis, catalyzed by Pyrophosphatase enzyme, drives the reaction for which PP_i is a product. For an example of such a reaction, see the discussion of cAMP formation below.

C. Ion transport may be coupled to a chemical reaction, e.g., hydrolysis or synthesis of ATP. In the diagram at right and below, water is not shown. It should be recalled that the ATP hydrolysis/synthesis reaction is ATP + H2O «ADP + Pi. Equivalent to equation 20-3 on p. 727, the free energy change (electrochemical potential difference) associated with transport of an ion S across a membrane from side 1 to side 2 is represented below.

R = gas constant, T = temperature, Z = charge on the ion, F = Faraday constant, and DY = voltage across the membrane.

Since free energy changes are additive, the spontaneous direction for the coupled reaction will depend on the relative magnitudes of:

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