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Lyases catalyze the nonhydrolytic cleavage of their substrate
with the formation of a double bond. Examples are decarbox-
ylases, which remove carboxyl groups as carbon dioxide, and
dehydrases, which remove a molecule of water. The reverse
reactions are catalyzed by the same enzymes, but they are
difficult to demonstrate in some cases because of an unfavor-
able equilibrium. Thus, pyruvic decarboxylase, which requires
thiamine pyrophosphate and magnesium ions for activity,
catalyzes the chemical process shown in reaction (4).
Isomerases catalyze the interconversion of isomeric com-
pounds (that is, compounds with the same chemical formula,
but different structures). For example, triose phosphate isom-
erase catalyzes the chemical process shown in reaction (5).
Ligases/synthetases catalyze energy-consuming (endergon-
ic) syntheses coupled with the energy-releasing (exergonic)
hydrolysis of ATP. They allow the chemical energy stored in ATP
to drive reactions uphill. An example is glutamine synthetase;
this enzyme, which is abundant in brain tissue, catalyzes the
chemical process shown in reaction (6).
Enzyme (continued)
(4) (5)
(6)
