Gibbs and Enzymes

Gibbs Free Energy G

The amount of energy capable of doing work during a reaction at constant temperature and pressure

Free energy ΔG

The amount of free energy released negative ΔG or absorbed positive ΔG during a reaction at constant temperature and pressure

Standard free energy change (ΔG°)

298 K reactants 1 M, products 1 M,

Biochemical standard free energy (ΔG°’)

298 K reactants 1 M, products 1 M, pH 7 H2O 55.5 M

Ground state

The free energy of an average molecule of S or P

Transition state

The point of highest free energy

Activation energy ΔG‡

The difference in free energy between the ground state and transition state

Reaction rate

The rate of the reaction is defined by the energy barrier that must be overcome to convert S to P

Reaction equilibrium ΔG°’ less than zero

Free energy of the product is less than free energy of the substrate. Under standard conditions the reaction is spontaneous. At equilibrium the concentration of product is greater than substrate

Relationship between ΔG°’ and Keq

ΔG°’ = RT ln (K*eq) (with T in Kelvin and R = gas constant)

Relationship between k and ΔG‡

Effect of enzymes on reaction rate

Enzymes increase rate by decreasing ΔG‡

Enzyme reaction scheme

Definitions in enzyme reaction scheme

E equals enzyme. S equals substrate. P equals product. ES and EP equal intermediates of E plus S and E plus P

Effect of enzymes on activation energy

ΔG‡ required to form ES and EP is much lower than the uncatalyzed reaction

How enzymes reduce ΔG‡

Enzymes reduce Delta G double dagger by stabilizing the transition state

Binding energy (ΔGB)

Derived from the enzyme substrate interaction

Role of binding energy

Multiple weak interactions between the active site and substrate stabilize the transition state and reduce ΔG‡

Enzyme catalytic mechanisms

Acid base catalysis. Covalent catalysis. Metal ion catalysis

Acid base catalysis

Transferring a proton to the substrate stabilizes charged intermediates formed during reactions

Covalent catalysis

A transient covalent bond forms between enzyme and substrate creating a new reaction path with lower free energy

Metal ion catalysis

Ionic interactions with a metal ion contribute binding energy or mediate oxidation reduction reactions