- Isomerases: Enzymes in the isomerasegroup rearrange atoms within the
substrate rather than add or subtract anything from the reaction. Phospho-
glucoisomerase is an example of an isomerase because it converts glucose
6-phosphate into fructose 6-phosphate during the breakdown of glucose. - Ligases: These are anabolic reactions. These enzymes join molecules together
and use energy in the form of ATP. An example is DNA ligase to synthesis DNA. - Lyases: Enzymes in the lyases group split molecules without using water in
a catabolic reaction. For example, 1,6-biphosphate aldolase splits fructose
1,6 biphosphate into G-3P and DHAP during glycolysis. These are anabolic
reactions. - Oxidoreductases: Enzymes in the oxidoreductasesgroup oxidize (remove)
electrons or reduce (add) electrons to a substrate in both catabolic and ana-
bolic reactions. An example is lactic acid dehydrogenase, which oxidizes
pyruvate to form lactic acid during fermentation. - Transferases: Enzymes in the tranferases group transfer functional groups from
one molecule or another substrate in an anabolic reaction. A functional group
could be amino acids, a phosphate group, or an acetyl group. For example,
hexokinase transfers a phosphate group from ATP to glucose in the first step in
the breakdown of glucose during the process of gycolysis.
Brewing Up Protein
Most enzymes are proteins that can be inactive or active. An inactive enzymedoes
not act as a catalyst to increase the speed of a metabolic reaction. An active enzyme
is a catalyst. An inactive enzyme is composed of apoenzyme; when an apoenzyme
binds to its cofactor the enzyme becomes active and is called a holoenzyme.
Acofactoris a substance that is either an inorganic ion, such as iron, mag-
nesium, or zinc, or an organic molecule. Organic cofactors are called coen-
zymes. A coenzyme is a molecule that is required for metabolism. NAD, NADP,
and FAD are examples of coenzymes. Some vitamins are coenzyme precursors.The Magic of Enzymes: Enzyme Activities
All chemical reactions including those that occur in metabolism, need a boost of
energy to get started. The energy needed to begin a chemical reaction is called
activation energy. An enzyme catalyzes a reaction by lowering the activation
energy. Heat can lower the activation energy and set off a reaction. However, the(^90) CHAPTER 5 The Chemical Metabolism