Basic Biochemical Thermodynamics
Metabolism deals with the interconversion of macromolecules (intermediates)
that are linked in a series, constituting a metabolic pathway.
Because all intermediates (A, B, C, D, E, etc.)
along the pathway are interconvertable
with each other, it is possible for an
to be established.
At equilibrium, the
concentrations of all intermediates is always the same, regardless of whether
A and B are said to be interconvertible, meaning that this reaction is reversible:
be converted to B, and B can be converted to A.
At equilibrium, A is converted to B
the same rate
that B is converted to A.
Thus, at equilibrium, there is no net change in
the relative concentrations of A and B.
Their ratio ([B]/[A]) is always constant, the value
of which is called
is determined by the structures of the reactants
In the reaction A
is dimensionless; it has no units associated
The ratio of [B]/[A] is referred to as the
mass action ratio (MAR).
is thus the
MAR at equilibrium
Relationship Between Free Energy and K
A reaction (such as A
B) is said to be in its lowest [free] energy state at
This is why no net change in the concentrations of A or B is observed at
If a change in the concentration of A or B is apparent over time, by
definition this means that energy is either being liberated (if the reaction is proceeding
toward equilibrium - ie. the MAR is approaching K
) or that energy is being consumed
by the reaction (if MAR is being displaced away from K
The liberated or consumed
energy corresponds to energy that is available, or “free”, to do chemical work and is
termed the Gibbs
free energy of reaction
The units for
G are kcal/mol or kJ/mol.
As a reaction approaches equilibrium from some starting initial MAR (MAR
absolute value for
G continually decreases until equilibrium is reached, at which point
G = 0.
A special situation is recognized for a reaction that is displaced away from
equilibrium, such that the
initial MAR is held at
1 M B/1 M A
This situation (in which the
ratio of products/reactants equals 1 M/1 M, respectively) is referred to as