Essential Cell Biology 5th edition

142 CHAPTER 4 Protein Structure and Function
Figure 4−34 Enzymes convert substrates
to products while remaining unchanged
themselves. Each enzyme has a site to
which substrate molecules bind, forming
an enzyme–substrate complex. There, a
covalent bond making and/or breaking
reaction occurs, generating an enzyme–
product complex. The product is then
released, allowing the enzyme to bind
additional substrate molecules and repeat
the reaction. An enzyme thus serves as
a catalyst, and it usually forms or breaks
a single covalent bond in a substrate
molecule.
enzyme
substratebinding
site
molecule A
(substrate)
enzyme–
substrate
complex
CATALYSIS
enzyme–
product
complex
enzyme
molecule B
(product)
doing this over and over again without themselves being changed
(Figure 4−34). Thus, enzymes act as catalysts that permit cells to make or
break covalent bonds at will. This catalysis of organized sets of chemical
reactions by enzymes creates and maintains all cell components, making
ECB5 04.34
life possible.
Enzymes can be grouped into functional classes based on the chemical
reactions they catalyze (Table 4−1). Each type of enzyme is highly specific,
catalyzing only a single type of reaction. Thus, hexokinase adds a
phosphate group to D-glucose but not to its optical isomer L-glucose; the
blood-clotting enzyme thrombin cuts one type of blood-clotting protein
between a particular arginine and its adjacent glycine and nowhere else.
As discussed in detail in Chapter 3, enzymes often work in sets, with the
product of one enzyme becoming the substrate for the next. The result
is an elaborate network of metabolic pathways that provides the cell with
energy and generates the many large and small molecules that the cell
needs.
TABLE 4–1 SOME COMMON FUNCTIONAL CLASSES OF ENZYMES
Enzymes Greatly Accelerate the Speed of Chemical
Reactions
The affinities of enzymes for their substrates, and the rates at which they
convert bound substrate to product, vary widely from one enzyme to
another. Both values can be determined experimentally by mixing purified
enzymes and substrates together in a test tube. At a low concentration
Enzyme Class
Hydrolase
Nuclease
Protease
Ligase
Isomerase
Polymerase
Kinase
Phosphatase
Oxido-reductase
ATPase
Biochemical Function
General term for enzymes that catalyze a hydrolytic cleavage reaction
Breaks down nucleic acids by hydrolyzing bonds between nucleotides
Breaks down proteins by hydrolyzing peptide bonds between amino acids
Joins two molecules together; DNA ligase joins two DNA strands together end-to-end
Catalyzes the rearrangement of bonds within a single molecule
Catalyzes polymerization reactions such as the synthesis of DNA and RNA
Catalyzes the addition of phosphate groups to molecules. Protein kinases are an important group of
kinases that attach phosphate groups to proteins
Catalyzes the hydrolytic removal of a phosphate group from a molecule
General name for enzymes that catalyze reactions in which one molecule is oxidized while the other is
reduced. Enzymes of this type are often called oxidases, reductases, or dehydrogenases
Hydrolyzes ATP. Many proteins have an energy-harnessing ATPase activity as part of their function,
including motor proteins such as myosin (discussed in Chapter 17) and membrane transport proteins such
as the Na + pump (discussed in Chapter 12)
Enzyme names typically end in “-ase,” with the exception of some enzymes, such as pepsin, trypsin, thrombin, lysozyme, and so on,
which were discovered and named before the convention became generally accepted, at the end of the nineteenth century. The
name of an enzyme usually indicates the nature of the reaction catalyzed. For example, citrate synthase catalyzes the synthesis of
citrate by a reaction between acetyl CoA and oxaloacetate.