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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-


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