Page 33 - Color Atlas of Biochemistry

P. 33

24 Basics

Catalysis B. Catalysis of H 2 O 2 – breakdown by iodide
As a simple example of a catalyzed reaction,
Catalysts are substances that accelerate
chemical reactions without themselves being we can look at the disproportionation of hy-
drogen peroxide (H 2 O 2 )into oxygenand
consumed in the process. Since catalysts
emerge from the catalyzed reaction without water. In the uncatalyzed reaction (at the
top), an H 2 O 2 molecule initially decays into
being changed, even small amounts are usu-
ally suf cient to cause a powerful acceleration H 2 O and atomic oxygen (O), which then reacts
with a second H 2 O 2 molecule to form water
of the reaction. In the cell, enzymes (see p. 88)
generally serve as catalysts. A few chemical and molecular oxygen (O 2 ). The activation
changes are catalyzed by special RNA mole- energy E a required for this reaction is rela-
–1
tively high, at 75 kJ mol . In the presence of
cules, known as ribozymes (see p. 246).
–
iodide (I ) as a catalyst, the reaction takes a
different course (bottom). The intermediate
–
A. Catalysis: principle arising in this case is hypoiodide (OI ), which
Thereason for theslow rates of most reac- also forms H 2 Oand O 2 with another H 2 O 2
–
ion is released
molecule. In this step, the I
tionsinvolving organicsubstancesisthe high
activation energy (see p. 22) that the reacting and can once again take part in the reaction.
molecules have to reach before they can react. The lower activation energy of the reaction
–1
catalyzed
iodide
by
(E a =56kJ mol )
In aqueous solution, a large proportion of the
activation energy is required to remove the causes acceleration of the reaction by a factor
of 2000, as the reaction rate depends expo-
hydration shells surrounding the educts. Dur- –Ea/R T
ing the course of a reaction, resonance-stabi- nentially on E a (v ~e ).
Free metal ions such as iron (Fe) and plat-
lized structures (see p. 4) are often tempora- inum (Pt) are also effective catalysts for the
rily suspended; this also requires energy. The
highest point on the reaction coordinates cor- breakdown of H 2 O 2 . Catalase (see p. 284), an
enzyme that protects cells against the toxic
responds to an energetically unfavorable tran-
sition state of this type (1). effects of hydrogen peroxide(seep. 284), is
much more catalytically effective still. In the
A catalyst creates a new pathway for the
reaction (2). When all of the transition states enzyme-catalyzed disproportionation, H 2 O 2
arising have a lower activation energy than is bound to the enzyme’s heme group, where
it is quickly converted to atomic oxygen and
that of the uncatalyzed reaction, the reaction
will proceed more rapidly along the alterna- water, supported by amino acid residues of
the enzyme protein. The oxygen atom is tem-
tive pathway, even when the number of in- porarily bound to the central iron atom of the
termediates is greater. Since the starting
points and end points are the same in both heme group, and then transferred from there
to the second H 2 O 2 molecule. The activation
routes, the change in the enthalpy ∆Gof the
reaction is not influenced by the catalyst. Cat- energy of the enzyme-catalyzed reaction is
–1
only 23 kJ mol , which in comparison with
alysts—including enzymes—are in principle the uncatalyzed reaction leads to acceleration
not capable of altering the equilibrium state 9
of the catalyzed reaction. by a factor of 1.3 10 .
Catalase is one of the most ef cient en-
The often-heard statement that “a catalyst
reduces the activation energy of a reaction” is zymes there are. A single molecule can con-
8
vert up to 10 (a hundred million) H 2 O 2 mol-
not strictly correct, since a completely different ecules per second.
reaction takes place in the presence of a cata-
lyst than in uncatalyzed conditions. However,
its activation energy is lower than in the un-
catalyzed reaction.

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