Gluconeogenesis and glycolysis are not identical
pathways running in opposite directions, although they
do share several steps (Fig. 14–16); seven of the ten enzymatic
reactions of gluconeogenesis are the reverse of
glycolytic reactions. However, three reactions of glycolysis
are essentially irreversible in vivo and cannot be
used in gluconeogenesis: the conversion of glucose to
glucose 6-phosphate by hexokinase, the phosphorylation
of fructose 6-phosphate to fructose 1,6-bisphosphate
by phosphofructokinase-1, and the conversion of
phosphoenolpyruvate to pyruvate by pyruvate kinase
(Fig. 14–16). In cells, these three reactions are characterized