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262
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Basic Concepts in Biochemistry
Take a generic chemical reaction:
A
1
B
∆
P
1
Q
What will happen if we mix A, B, P, and Q together? There’s some
gray area here in that the answer depends somewhat on what we mean
by
happen
. First, it depends on direction. A more appropriate way to ask
the question is, Will the reaction happen in the direction written, that is,
left to right? Second, it depends on the actual concentrations of A, B, P,
and Q that you start with. Third, it really depends on the relationship
between the initial concentrations of A, B, P, and Q and the equilibrium
concentrations that will exist when the reaction finally comes to equilib-
rium. Finally, when A, B, P, and Q are mixed, they will take off toward
the equilibrium position, whatever that is, but thermodynamics doesn’t
tell you how long it might take for the reaction to actually get to equi-
librium. The
How fast
? is kinetics. So the real answer is that when we
mix A, B, P, and Q, the reaction will happen in the direction that takes
you to equilibrium. When the reaction is actually at equilibrium, the con-
centrations of A, B, P, and Q will be equal to their equilibrium concen-
trations.
The equilibrium constant for a reaction is just the ratio of the prod-
ucts to the reactants at equilibrium:
K
eq
5
}
[
[
A
P]
]
e
e
q
q
[
[
Q
B
]
]
e
e
q
q
}
If the initial ratio of products to reactants,
}
[
[
A
P]
]
[
[
Q
B
]
]
}
is different from the equilibrium ratio, the chemical reaction will proceed
until the real product/reactant ratio equals the equilibrium product/sub-
strate ratio, and then it stops at equilibrium.
If ([P][Q]/[A][B])
,
([P]
eq
[Q]
eq
/[A]
eq
[B]
eq
), the reaction goes in the
direction that increases P and Q and decreases A and B so that the prod-
uct/substrate ratio increases to the equilibrium value. This is the same as
saying that if the concentration of products is lower than their equilib-
rium values, the reaction goes in the direction that makes more products,
or to the right.
If ([P][Q]/[A][B])
.
([P]
eq
[Q]
eq
/[A]
eq
[B]
eq
), the reaction goes in the
direction that decreases P and Q and increases A and B so that the product/