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150 Metabolism
Glycolysis phorylation;see p. 124), and 1,3-bisphos-
phoglycerate is produced. This intermediate
contains a mixed acid–anhydride bond, the
A. Balance
phosphate part of which is at a high chemical
Glycolysis is a catabolic pathway in the cyto- potential.
plasm that is found in almost all organisms— [7] Catalyzed by phosphoglycerate kinase,
irrespective of whether they live aerobically this phosphate residue is transferred to ADP,
or anaerobically. The balance of glycolysis is producing 3-phosphoglycerate and ATP. The
simple: glucose is broken down into two mol- ATP balance is thus once again in equilibrium.
ecules of pyruvate, and in addition two mol- [8] As a result of shifting of the remaining
ecules of ATP and two of NADH+H + are phosphate residue within the molecule, the
formed. isomer 2-phosphoglycerate is formed.
In thepresenceofoxygen, pyruvate and [9] Elimination of water from 2-phospho-
NADH+H + reach the mitochondria, where glycerate produces the phosphate ester of the
they undergo further transformation (aerobic enol form of pyruvate—phosphoenolpyruvate
glycolysis; see p. 146). In anaerobic condi- (PEP). This reaction also raises the second
tions, fermentation products such as lactate phosphate residue to a high potential.
or ethanol have to be formed in the cytoplasm [10] In the last step, pyruvate kinase trans-
+
from pyruvate and NADH+H ,inorder to re- fers this residue to ADP. The remaining enol
+
generate NAD so that glycolysis can continue pyruvate is immediately rearranged into
(anaerobic glycolysis; see p. 146). In the anae- pyruvate,which is much more stable. Along
robic state, glycolysis is the only means of with step [7] and the thiokinase reaction in
obtaining ATP that animal cells have. the tricarboxylic acid cycle (see p. 136), the
pyruvate kinase reaction is one of the three
reactions in animal metabolism that are able
B. Reactions
to produce ATP independently of the respira-
Glycolysis involves ten individual steps, in- tory chain.
cluding three isomerizations and four phos- In glycolysis, two molecules of ATP are ini-
phate transfers. The only redox reaction takes tially used for activation ([1], [3]). Later, two
place in step [6]. ATPs are formed per C 3 fragment. Overall,
[1] Glucose, which is taken up by animal therefore, there is a small net gain of 2 mol
cells from the blood and other sources, is first ATP per mol of glucose.
phosphorylated to glucose 6-phosphate,with
ATP being consumed. The glucose 6-phos-
phate is not capableofleaving thecell. C. Energy profile
[2] In the next step, glucose 6-phosphate is The energy balance of metabolic pathways de-
isomerized into fructose 6-phosphate. pends not only on the standard changes in
0
[3] Using ATP again, another phosphoryla- enthalpy ∆G , but also on the concentrations
tion takes place, giving rise to fructose 1,6- of the metabolites (see p. 18). Fig. C shows the
bisphosphate. Phosphofructokinase is the actual enthalpy changes ∆Gfor the individual
most important key enzyme in glycolysis steps of glycolysis in erythrocytes.
(see p. 144). As can be seen, only three reactions ([1],
[4] Fructose 1,6-bisphosphate is broken [3], and [10]), are associated with large
down by aldolase into the C 3 compounds glyc- changes in free enthalpy. In these cases, the
eraldehyde 3-phosphate (also known as glyc- equilibrium lies well on the side of the prod-
eral 3-phosphate) and glycerone 3-phosphate ucts (see p. 18). All of the other steps are
(dihydroxyacetone 3-phosphate). freely reversible. Thesamesteps arealso fol-
[5] The latter two products are placed in lowed—in the reverse direction—in gluconeo-
fast equilibrium by triosephosphate isomerase. genesis (see p. 154), with the same enzymes
[6] Glyceraldehyde 3-phosphate is now being activated as in glucose degradation. The
oxidized by glyceraldehyde-3-phosphate de- non-reversible steps [1], [3], and [10] are by-
+
hydrogenase, with NADH+H being formed. passed in glucose biosynthesis (see p. 154).
In this reaction, inorganic phosphate is taken
up into the molecule (substrate-level phos-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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