Page 31 - Color Atlas of Biochemistry

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22 Basics

Reaction kinetics doubles withanincrease intemperature of
10 °C.
Thechangein freeenthalpy ∆Gin a reaction
indicates whether or not the reaction can take
place spontaneously in given conditions and B. Reaction rate
how much work it can perform (see p.18). The velocity v of a chemical reaction is deter-
However, it does not tell us anything about mined experimentally by observing the
the rate of the reaction—i. e., its kinetics. change in the concentration of an educt or
product over time. In the example shown
(again a reaction of the A B type), 3 mmol
A. Activation energy
of the educt A is converted per second and
Most organic chemical reactions (with the 3 mmol of the product B is formed per second
exception of acid–base reactions) proceed in one liter of the solution. This corresponds
only very slowly, regardless of the value to a rate of
of ∆G. Thereason for theslow reaction rate
is that the molecules that react—the v =3mM s –1 =3 10 –3 mol L –1 s –1
educts—have to have a certain minimum en-
ergy before they can enter the reaction. This is
best understood with the help of an energy C. Reaction order
diagram (1) of the simplest possible reaction Reaction rates are influenced not only by the
A B. Theeduct A and theproduct B are each activation energy and the temperature, but
at a specific chemical potential (G e and G p , also by the concentrations of the reactants.
respectively). The change in the free enthalpy When thereis only oneeduct, A (1), v is
of the reaction, ∆G, corresponds to the differ- proportional to the concentration [A] of this
ence between these two potentials. To be substance, and a first-order reaction is in-
converted into B, A first has to overcome a volved. When two educts,A andB,react
potential energy barrier, the peak of which, with one another (2), it is a second order
G a ,lies well above G e . The potential difference reaction (shown on the right). In this case,
G a –G e is the activation energy E a of the re- the rate v is proportional to the product of
–1
action (in kJ mol ). the educt concentrations (12 mM 2 at the
2
2
ThefactthatAcan be convertedinto B at all top, 24 mM in the middle, and 36 mM at
is because the potential G e only represents the bottom). The proportionality factors k and
the average potential of all the molecules. k are the rate constants of the reaction. They
Individual molecules may occasionally reach are not dependent on the reaction concentra-
much higher potentials—e. g., due to collisions tions, but depend on the external conditions
with other molecules. When the increase in for the reaction, such as temperature.
energy thus gained is greater than E a ,these In B, only the kinetics of simple irreversible
molecules can overcome the barrier and be reactionsisshown. More complicated cases,
converted into B. The energy distribution for a such as reaction with three or more reversible
group of molecules of this type, as calculated steps, can usually be broken down into first-
from a simple model, is shown in (2)and (3). orderorsecond-orderpartial reactionsand
∆n/n is the fraction of molecules that have described using the corresponding equations
reached or exceeded energy E (in kJ per mol). (for an example, see the Michaelis–Menten
At 27 °C, for example, approximately 10% of reaction, p. 92).
–1
the molecules have energies > 6 kJ mol .
The typical activation energies of chemical
reactions aremuch higher. Thecourseof
the energy function at energies of around
50 kJ mol –1 is shownin(3). Statistically, at
9
27 °C only two out of 10 molecules reach this
energy. At 37 °C, the figure is already four.
This is the basis for the long-familiar “Q 10
law”—a rule of thumb that states that the
speed of biological processes approximately

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