Page 133 - Color Atlas of Biochemistry
P. 133
124 Metabolism
Energetic coupling phorylations,” as they represent individual
steps within metabolic pathways.
Thecellstores chemicalenergy in the form of In the glyceraldehyde 3-phosphate dehy-
“energy-rich” metabolites. The most impor- drogenation reaction, a step involved in gly-
tant metabolite of this type is adenosine tri- colysis (1;see also C), thealdehydegroup in
phosphate (ATP), which drives a large number glyceraldehyde 3-phosphate is oxidized into a
of energy-dependent reactions via energetic carboxyl group. During the reaction, an anor-
coupling (see p. 16). ganic phosphate is also introduced into the
product, producing a mixedacidanhy-
dride—1,3-bisphosphoglycerate. Phosphopyr-
A. Energetic coupling
uvate hydratase (“enolase”, 2) catalyzes the
0
The change in free enthalpy ∆G during hy- elimination of water from 2-phosphoglycer-
drolysis (see p. 18) has been arbitrarily se- ate. In the enol phosphate formed (phosphoe-
lected as a measure of the group transfer nol pyruvate), the phosphate residue—in con-
potential of “energy-rich” compounds. How- trast to 2-phosphoglycerate—is at an ex-
ever, thisdoesnot mean that ATPisin fact tremely high potential (∆G 0 of hydrolysis:
–1
hydrolyzed in energetically coupled reactions. –62 kJ mol ). A third reaction of this type
If ATP hydrolysis and an endergonic process is the formation of succinyl phosphate, which
were simply allowed to run alongside each occurs in the tricarboxylic acid cycle as an
other, the hydrolysis would only produce individual step in the succinyl CoA ligase re-
heat, without influencing the endergonic action. Here again, anorganic phosphate is
process. For coupling, the two reactions have introduced into a mixed acid anhydride
to be linked in such a way that a common bond to be transferred from there to GDP.
intermediate arises. This connection is illus- Succinyl phosphate is only an intermediate
trated here using the example of the gluta- here, and is not released by the enzyme.
mine synthetase reaction. In the literature, the term “substrate level
Direct transfer of NH 3 to glutamate is en- phosphorylation” is used inconsistently.
0
–1
dergonic (∆G =+14 kJ mol ;see p. 18), Some authors use it to refer to reactions in
and can therefore not take place. In the cell, which anorganic phosphate is raised to a high
the reaction is divided into two exergonic potential, while others use it for the subse-
steps. First, the γ-phophate residue is trans- quent reactions, in which ATP or GTP is
ferred from ATP to glutamate. This gives rise formed from the energy-rich intermediates.
to an “energy-rich” mixedacidanhydride.In
the second step, the phosphate residue from C. Glyceraldehyde-3-phosphate
the intermediate is substituted by NH 3 ,and
glutamine and free phosphate are produced. dehydrogenase
Theenergybalance of thereaction asa whole The reaction catalyzed during glycolysis by
–1
0
(∆G =–17 kJ mol ) is the sum of the glyceraldehyde-3-phosphate dehydrogenase
changes in free enthalpy of direct glutamine (GADPH) is shown here in detail. Initially,
–1
synthesis (∆G 0 =14kJ mol )plus ATP hy- the SH group of a cysteine residue of the
0
–1
drolysis (∆G =–31 kJ mol ), although ATP enzyme is added to the carbonyl group of
has not been hydrolyzed at all. glyceraldehyde 3-phosphate (a). This inter-
+
mediate is oxidized by NAD into an “en-
ergy-rich” thioester (b). In the third step (c),
B. Substrate-level phosphorylation
anorganic phosphate displaces the thiol, and
As mentioned earlier (see p. 122), there are a the mixed anhydride 1,3-bisphosphoglycerate
few metabolites that transfer phosphate to arises. In this bond, the phosphate residue is
ADP in an exergonic reaction and can there- at ahighenoughpotential for it to be trans-
fore form ATP. In ATP synthesis, anorganic ferred to ADP in the next step (not shown; see
phosphate or phosphate bound in an ester- p. 150).
like fashion is transferred to bonds with a
high phosphate transfer potential. Reactions
of this type are termed “substrate-level phos-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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