Page 131 - Color Atlas of Biochemistry
P. 131
122 Metabolism
ATP In standard conditions, the change in free
enthalpy ∆G 0 (see p. 18) that occurs in the
The nucleotide coenzyme adenosine hydrolysis of phosphoric acid anhydride
triphosphate (ATP) is the most important bonds amounts to –30 to –35 kJ mol –1 at
form of chemical energy in all cells. Cleavage pH 7. The particular anhydride bond of ATP
of ATP is strongly exergonic. The energy this that is cleaved only has a minor influence on
provides (∆G; see p. 16) is used to drive ender- ∆G 0 (1–2). Even the hydrolysis of diphos-
gonic processes (such as biosynthesis and phate (also known as pyrophosphate; 4)still
–1
movementand transportprocesses) through yields more than –30 kJ mol . By contrast,
energetic coupling (see p. 124). The other nu- cleavage of the ester bond between ribose and
cleoside triphosphate coenzymes (GTP,CTP,and phosphate only provides –9 kJ mol –1 (3).
UTP) have similar chemical properties to ATP, In the cell, the ∆G of ATP hydrolysis is sub-
but they are used for different tasks in metab- stantially larger, because the concentrations
olism (see p. 110). of ATP, ADP and P i are much lower than in
standard conditions and there is an excess of
ATPoverADP (see p. 18). ThepHvalue and
A. ATP: structure 2+
Mg concentration also affect the value of ∆G.
In ATP, a chain of three phosphate residues is The physiological energy yield of ATP hydrol-
linked to the 5 -OH group of the nucleoside ysis to ADP and anorganic phosphate (P i )is
–1
adenosine(seep. 80).These phosphateresi- probably around –50 kJ mol .
dues are termed α, β,and γ.The α phosphate
is bound to ribose by a phosphoric acid ester
C. Types of ATP formation
bond. The linkages between the three phos-
phate residues, on the other hand, involve Only a few compounds contain phosphate
much more unstable phosphoric acid anhy- residues with a group transfer potential (see
dride bonds. Theactivecoenzyme is in fact p. 18) that is high enough to transfer them to
generally a complex of ATP with an Mg 2+ ADP and thus allow ATP synthesis. Processes
ion, which is coordinatively bound to the α that raise anorganic phosphate to this type of
4–
and β phosphates (Mg 2+ ATP ). However, high potential are called substrate level phos-
the term “ATP” is usually used for the sake phorylations (see p. 124). Reactions of this
of simplicity. type take place in glycolysis (see p. 150) and
in thetricarboxylic acid cycle(seep. 136).
Another “energy-rich” phosphate compound
B. Hydrolysis energies
is creatine phosphate, which is formed from
The formula for phosphate residues shown in ATP inmuscle and canregenerate ATP as
Fig. A, with single and double bonds, is not an needed (see p. 336).
accurate representation of the actual charge Most cellular ATP does not arise in the way
distribution. In ATP, the oxygen atoms of all described above (i. e., by transfer of phosphate
three phosphate residues have similarly residues from organic molecules to ADP), but
strong negative charges (orange), while the rather by oxidative phosphorylation.This
phosphorus atoms represent centers of posi- process takes place in mitochondria (or as
tive charge. One of the reasons for the insta- light-driven phosphorylation in chloroplasts)
bility of phosphoric anhydride bonds is the and is energetically coupled to a proton gra-
+
repulsion between these negatively charged dient over a membrane. These H gradients
oxygen atoms, which is partly relieved by are established by electron transport chains
cleavage of a phosphate residue. In addition, and are used by the enzyme ATP synthase as a
thefreephosphate anion formedbyhydroly- source of energy for direct linking of anor-
sis of ATP is better hydrated and more strongly ganic phosphate to ADP. In contrast to sub-
resonance-stabilized than the corresponding strate level phosphorylation, oxidative phos-
residue in ATP. This also contributes to the phorylation requires the presence of oxygen
strongly exergonic character of ATP hydroly- (i. e., aerobic conditions).
sis.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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